Fix FindMinSnapshotByGeneration() loop ref bug

Previously, a bug was introduced that added a `LIST` operation
on every replica sync which significantly increased the cost of
running Litestream against S3. This changes the behavior to only
issue the `LIST` operation when the generation has changed.

abs/replica_client.go

@@ -102,7 +102,7 @@ func (c *ReplicaClient) Generations(ctx context.Context) ([]string, error) {
 		internal.OperationTotalCounterVec.WithLabelValues(ReplicaClientType, "LIST").Inc()
 
 		resp, err := c.containerURL.ListBlobsHierarchySegment(ctx, marker, "/", azblob.ListBlobsSegmentOptions{
-			Prefix: path.Join(c.Path, "generations") + "/",
+			Prefix: litestream.GenerationsPath(c.Path) + "/",
 		})
 		if err != nil {
 			return nil, err
@@ -125,17 +125,18 @@ func (c *ReplicaClient) Generations(ctx context.Context) ([]string, error) {
 func (c *ReplicaClient) DeleteGeneration(ctx context.Context, generation string) error {
 	if err := c.Init(ctx); err != nil {
 		return err
-	} else if generation == "" {
-		return fmt.Errorf("generation required")
 	}
 
-	prefix := path.Join(c.Path, "generations", generation) + "/"
+	dir, err := litestream.GenerationPath(c.Path, generation)
+	if err != nil {
+		return fmt.Errorf("cannot determine generation path: %w", err)
+	}
 
 	var marker azblob.Marker
 	for marker.NotDone() {
 		internal.OperationTotalCounterVec.WithLabelValues(ReplicaClientType, "LIST").Inc()
 
-		resp, err := c.containerURL.ListBlobsFlatSegment(ctx, marker, azblob.ListBlobsSegmentOptions{Prefix: prefix})
+		resp, err := c.containerURL.ListBlobsFlatSegment(ctx, marker, azblob.ListBlobsSegmentOptions{Prefix: dir + "/"})
 		if err != nil {
 			return err
 		}
@@ -170,11 +171,12 @@ func (c *ReplicaClient) Snapshots(ctx context.Context, generation string) (lites
 func (c *ReplicaClient) WriteSnapshot(ctx context.Context, generation string, index int, rd io.Reader) (info litestream.SnapshotInfo, err error) {
 	if err := c.Init(ctx); err != nil {
 		return info, err
-	} else if generation == "" {
-		return info, fmt.Errorf("generation required")
 	}
 
-	key := path.Join(c.Path, "generations", generation, "snapshots", litestream.FormatIndex(index)+".snapshot.lz4")
+	key, err := litestream.SnapshotPath(c.Path, generation, index)
+	if err != nil {
+		return info, fmt.Errorf("cannot determine snapshot path: %w", err)
+	}
 	startTime := time.Now()
 
 	rc := internal.NewReadCounter(rd)
@@ -204,11 +206,12 @@ func (c *ReplicaClient) WriteSnapshot(ctx context.Context, generation string, in
 func (c *ReplicaClient) SnapshotReader(ctx context.Context, generation string, index int) (io.ReadCloser, error) {
 	if err := c.Init(ctx); err != nil {
 		return nil, err
-	} else if generation == "" {
-		return nil, fmt.Errorf("generation required")
 	}
 
-	key := path.Join(c.Path, "generations", generation, "snapshots", litestream.FormatIndex(index)+".snapshot.lz4")
+	key, err := litestream.SnapshotPath(c.Path, generation, index)
+	if err != nil {
+		return nil, fmt.Errorf("cannot determine snapshot path: %w", err)
+	}
 
 	blobURL := c.containerURL.NewBlobURL(key)
 	resp, err := blobURL.Download(ctx, 0, 0, azblob.BlobAccessConditions{}, false, azblob.ClientProvidedKeyOptions{})
@@ -228,11 +231,12 @@ func (c *ReplicaClient) SnapshotReader(ctx context.Context, generation string, i
 func (c *ReplicaClient) DeleteSnapshot(ctx context.Context, generation string, index int) error {
 	if err := c.Init(ctx); err != nil {
 		return err
-	} else if generation == "" {
-		return fmt.Errorf("generation required")
 	}
 
-	key := path.Join(c.Path, "generations", generation, "snapshots", litestream.FormatIndex(index)+".snapshot.lz4")
+	key, err := litestream.SnapshotPath(c.Path, generation, index)
+	if err != nil {
+		return fmt.Errorf("cannot determine snapshot path: %w", err)
+	}
 
 	internal.OperationTotalCounterVec.WithLabelValues(ReplicaClientType, "DELETE").Inc()
 
@@ -257,11 +261,12 @@ func (c *ReplicaClient) WALSegments(ctx context.Context, generation string) (lit
 func (c *ReplicaClient) WriteWALSegment(ctx context.Context, pos litestream.Pos, rd io.Reader) (info litestream.WALSegmentInfo, err error) {
 	if err := c.Init(ctx); err != nil {
 		return info, err
-	} else if pos.Generation == "" {
-		return info, fmt.Errorf("generation required")
 	}
 
-	key := path.Join(c.Path, "generations", pos.Generation, "wal", litestream.FormatIndex(pos.Index), litestream.FormatOffset(pos.Offset)+".wal.lz4")
+	key, err := litestream.WALSegmentPath(c.Path, pos.Generation, pos.Index, pos.Offset)
+	if err != nil {
+		return info, fmt.Errorf("cannot determine wal segment path: %w", err)
+	}
 	startTime := time.Now()
 
 	rc := internal.NewReadCounter(rd)
@@ -291,11 +296,12 @@ func (c *ReplicaClient) WriteWALSegment(ctx context.Context, pos litestream.Pos,
 func (c *ReplicaClient) WALSegmentReader(ctx context.Context, pos litestream.Pos) (io.ReadCloser, error) {
 	if err := c.Init(ctx); err != nil {
 		return nil, err
-	} else if pos.Generation == "" {
-		return nil, fmt.Errorf("generation required")
 	}
 
-	key := path.Join(c.Path, "generations", pos.Generation, "wal", litestream.FormatIndex(pos.Index), litestream.FormatOffset(pos.Offset)+".wal.lz4")
+	key, err := litestream.WALSegmentPath(c.Path, pos.Generation, pos.Index, pos.Offset)
+	if err != nil {
+		return nil, fmt.Errorf("cannot determine wal segment path: %w", err)
+	}
 
 	blobURL := c.containerURL.NewBlobURL(key)
 	resp, err := blobURL.Download(ctx, 0, 0, azblob.BlobAccessConditions{}, false, azblob.ClientProvidedKeyOptions{})
@@ -318,12 +324,11 @@ func (c *ReplicaClient) DeleteWALSegments(ctx context.Context, a []litestream.Po
 	}
 
 	for _, pos := range a {
-		if pos.Generation == "" {
-			return fmt.Errorf("generation required")
+		key, err := litestream.WALSegmentPath(c.Path, pos.Generation, pos.Index, pos.Offset)
+		if err != nil {
+			return fmt.Errorf("cannot determine wal segment path: %w", err)
 		}
 
-		key := path.Join(c.Path, "generations", pos.Generation, "wal", litestream.FormatIndex(pos.Index), litestream.FormatOffset(pos.Offset)+".wal.lz4")
-
 		internal.OperationTotalCounterVec.WithLabelValues(ReplicaClientType, "DELETE").Inc()
 
 		blobURL := c.containerURL.NewBlobURL(key)
@@ -367,24 +372,24 @@ func newSnapshotIterator(ctx context.Context, generation string, client *Replica
 func (itr *snapshotIterator) fetch() error {
 	defer close(itr.ch)
 
-	if itr.generation == "" {
-		return fmt.Errorf("generation required")
+	dir, err := litestream.SnapshotsPath(itr.client.Path, itr.generation)
+	if err != nil {
+		return fmt.Errorf("cannot determine snapshots path: %w", err)
 	}
 
-	prefix := path.Join(itr.client.Path, "generations", itr.generation) + "/"
-
 	var marker azblob.Marker
 	for marker.NotDone() {
 		internal.OperationTotalCounterVec.WithLabelValues(ReplicaClientType, "LIST").Inc()
 
-		resp, err := itr.client.containerURL.ListBlobsFlatSegment(itr.ctx, marker, azblob.ListBlobsSegmentOptions{Prefix: prefix})
+		resp, err := itr.client.containerURL.ListBlobsFlatSegment(itr.ctx, marker, azblob.ListBlobsSegmentOptions{Prefix: dir + "/"})
 		if err != nil {
 			return err
 		}
 		marker = resp.NextMarker
 
 		for _, item := range resp.Segment.BlobItems {
-			index, err := internal.ParseSnapshotPath(path.Base(item.Name))
+			key := path.Base(item.Name)
+			index, err := litestream.ParseSnapshotPath(key)
 			if err != nil {
 				continue
 			}
@@ -473,24 +478,24 @@ func newWALSegmentIterator(ctx context.Context, generation string, client *Repli
 func (itr *walSegmentIterator) fetch() error {
 	defer close(itr.ch)
 
-	if itr.generation == "" {
-		return fmt.Errorf("generation required")
+	dir, err := litestream.WALPath(itr.client.Path, itr.generation)
+	if err != nil {
+		return fmt.Errorf("cannot determine wal path: %w", err)
 	}
-	prefix := path.Join(itr.client.Path, "generations", itr.generation, "wal")
 
 	var marker azblob.Marker
 	for marker.NotDone() {
 		internal.OperationTotalCounterVec.WithLabelValues(ReplicaClientType, "LIST").Inc()
 
-		resp, err := itr.client.containerURL.ListBlobsFlatSegment(itr.ctx, marker, azblob.ListBlobsSegmentOptions{Prefix: prefix})
+		resp, err := itr.client.containerURL.ListBlobsFlatSegment(itr.ctx, marker, azblob.ListBlobsSegmentOptions{Prefix: dir + "/"})
 		if err != nil {
 			return err
 		}
 		marker = resp.NextMarker
 
 		for _, item := range resp.Segment.BlobItems {
-			key := strings.TrimPrefix(item.Name, prefix+"/")
-			index, offset, err := internal.ParseWALSegmentPath(key)
+			key := path.Base(item.Name)
+			index, offset, err := litestream.ParseWALSegmentPath(key)
 			if err != nil {
 				continue
 			}

cmd/litestream/main.go

@@ -9,7 +9,6 @@ import (
 	"log"
 	"net/url"
 	"os"
-	"os/signal"
 	"os/user"
 	"path"
 	"path/filepath"
@@ -87,8 +86,7 @@ func (m *Main) Run(ctx context.Context, args []string) (err error) {
 
 		// Setup signal handler.
 		ctx, cancel := context.WithCancel(ctx)
-		signalCh := make(chan os.Signal, 1)
-		signal.Notify(signalCh, notifySignals...)
+		signalCh := signalChan()
 
 		if err := c.Run(ctx); err != nil {
 			return err
@@ -96,8 +94,6 @@ func (m *Main) Run(ctx context.Context, args []string) (err error) {
 
 		// Wait for signal to stop program.
 		select {
-		case <-ctx.Done():
-			fmt.Println("context done, litestream shutting down")
 		case err = <-c.execCh:
 			cancel()
 			fmt.Println("subprocess exited, litestream shutting down")

cmd/litestream/main_notwindows.go

@@ -5,6 +5,7 @@ package main
 import (
 	"context"
 	"os"
+	"os/signal"
 	"syscall"
 )
 
@@ -18,4 +19,8 @@ func runWindowsService(ctx context.Context) error {
 	panic("cannot run windows service as unix process")
 }
 
-var notifySignals = []os.Signal{syscall.SIGINT, syscall.SIGTERM}
+func signalChan() <-chan os.Signal {
+	ch := make(chan os.Signal, 2)
+	signal.Notify(ch, syscall.SIGINT, syscall.SIGTERM)
+	return ch
+}

cmd/litestream/main_test.go

@@ -2,22 +2,16 @@ package main_test
 
 import (
 	"io/ioutil"
-	"log"
 	"os"
 	"path/filepath"
 	"testing"
 
-	"github.com/benbjohnson/litestream"
 	main "github.com/benbjohnson/litestream/cmd/litestream"
 	"github.com/benbjohnson/litestream/file"
 	"github.com/benbjohnson/litestream/gcs"
 	"github.com/benbjohnson/litestream/s3"
 )
 
-func init() {
-	litestream.LogFlags = log.Lmsgprefix | log.Ldate | log.Ltime | log.Lmicroseconds | log.LUTC | log.Lshortfile
-}
-
 func TestReadConfigFile(t *testing.T) {
 	// Ensure global AWS settings are propagated down to replica configurations.
 	t.Run("PropagateGlobalSettings", func(t *testing.T) {

cmd/litestream/main_windows.go

@@ -7,6 +7,7 @@ import (
 	"io"
 	"log"
 	"os"
+	"os/signal"
 
 	"golang.org/x/sys/windows"
 	"golang.org/x/sys/windows/svc"
@@ -104,4 +105,8 @@ func (w *eventlogWriter) Write(p []byte) (n int, err error) {
 	return 0, elog.Info(1, string(p))
 }
 
-var notifySignals = []os.Signal{os.Interrupt}
+func signalChan() <-chan os.Signal {
+	ch := make(chan os.Signal, 1)
+	signal.Notify(ch, os.Interrupt)
+	return ch
+}

cmd/litestream/replicate.go

@@ -42,6 +42,7 @@ func NewReplicateCommand() *ReplicateCommand {
 func (c *ReplicateCommand) ParseFlags(ctx context.Context, args []string) (err error) {
 	fs := flag.NewFlagSet("litestream-replicate", flag.ContinueOnError)
 	execFlag := fs.String("exec", "", "execute subcommand")
+	tracePath := fs.String("trace", "", "trace path")
 	configPath, noExpandEnv := registerConfigFlag(fs)
 	fs.Usage = c.Usage
 	if err := fs.Parse(args); err != nil {
@@ -79,6 +80,16 @@ func (c *ReplicateCommand) ParseFlags(ctx context.Context, args []string) (err e
 		c.Config.Exec = *execFlag
 	}
 
+	// Enable trace logging.
+	if *tracePath != "" {
+		f, err := os.Create(*tracePath)
+		if err != nil {
+			return err
+		}
+		defer f.Close()
+		litestream.Tracef = log.New(f, "", log.LstdFlags|log.Lmicroseconds|log.LUTC|log.Lshortfile).Printf
+	}
+
 	return nil
 }
 
@@ -204,5 +215,8 @@ Arguments:
 	-no-expand-env
 	    Disables environment variable expansion in configuration file.
 
+	-trace PATH
+	    Write verbose trace logging to PATH.
+
 `[1:], DefaultConfigPath())
 }

cmd/litestream/replicate_test.go

@@ -1,135 +0,0 @@
-package main_test
-
-import (
-	"context"
-	"database/sql"
-	"errors"
-	"fmt"
-	"hash/crc64"
-	"io"
-	"os"
-	"path/filepath"
-	"runtime"
-	"testing"
-	"time"
-
-	main "github.com/benbjohnson/litestream/cmd/litestream"
-	"golang.org/x/sync/errgroup"
-)
-
-func TestReplicateCommand(t *testing.T) {
-	if testing.Short() {
-		t.Skip("long running test, skipping")
-	} else if runtime.GOOS != "linux" {
-		t.Skip("must run system tests on Linux, skipping")
-	}
-
-	const writeTime = 10 * time.Second
-
-	dir := t.TempDir()
-	configPath := filepath.Join(dir, "litestream.yml")
-	dbPath := filepath.Join(dir, "db")
-	restorePath := filepath.Join(dir, "restored")
-	replicaPath := filepath.Join(dir, "replica")
-
-	if err := os.WriteFile(configPath, []byte(`
-dbs:
-  - path: `+dbPath+`
-    replicas:
-      - path: `+replicaPath+`
-`), 0666); err != nil {
-		t.Fatal(err)
-	}
-
-	// Generate data into SQLite database from separate goroutine.
-	g, ctx := errgroup.WithContext(context.Background())
-	mainctx, cancel := context.WithCancel(ctx)
-	g.Go(func() error {
-		defer cancel()
-
-		db, err := sql.Open("sqlite3", dbPath)
-		if err != nil {
-			return err
-		}
-		defer db.Close()
-
-		if _, err := db.ExecContext(ctx, `PRAGMA journal_mode = WAL`); err != nil {
-			return fmt.Errorf("cannot enable wal: %w", err)
-		} else if _, err := db.ExecContext(ctx, `PRAGMA synchronous = NORMAL`); err != nil {
-			return fmt.Errorf("cannot enable wal: %w", err)
-		} else if _, err := db.ExecContext(ctx, `CREATE TABLE t (id INTEGER PRIMARY KEY)`); err != nil {
-			return fmt.Errorf("cannot create table: %w", err)
-		}
-
-		ticker := time.NewTicker(1 * time.Millisecond)
-		defer ticker.Stop()
-		timer := time.NewTimer(writeTime)
-		defer timer.Stop()
-
-		for i := 0; ; i++ {
-			select {
-			case <-ctx.Done():
-				return ctx.Err()
-			case <-timer.C:
-				return nil
-			case <-ticker.C:
-				if _, err := db.ExecContext(ctx, `INSERT INTO t (id) VALUES (?);`, i); err != nil {
-					return fmt.Errorf("cannot insert: i=%d err=%w", i, err)
-				}
-			}
-		}
-	})
-
-	// Replicate database unless the context is canceled.
-	g.Go(func() error {
-		return main.NewMain().Run(mainctx, []string{"replicate", "-config", configPath})
-	})
-
-	if err := g.Wait(); err != nil {
-		t.Fatal(err)
-	}
-
-	// Checkpoint database.
-	mustCheckpoint(t, dbPath)
-	chksum0 := mustChecksum(t, dbPath)
-
-	// Restore to another path.
-	if err := main.NewMain().Run(context.Background(), []string{"restore", "-config", configPath, "-o", restorePath, dbPath}); err != nil && !errors.Is(err, context.Canceled) {
-		t.Fatal(err)
-	}
-
-	// Verify contents match.
-	if chksum1 := mustChecksum(t, restorePath); chksum0 != chksum1 {
-		t.Fatal("restore mismatch")
-	}
-}
-
-func mustCheckpoint(tb testing.TB, path string) {
-	tb.Helper()
-
-	db, err := sql.Open("sqlite3", path)
-	if err != nil {
-		tb.Fatal(err)
-	}
-	defer db.Close()
-
-	if _, err := db.Exec(`PRAGMA wal_checkpoint(TRUNCATE)`); err != nil {
-		tb.Fatal(err)
-	}
-}
-
-func mustChecksum(tb testing.TB, path string) uint64 {
-	tb.Helper()
-
-	f, err := os.Open(path)
-	if err != nil {
-		tb.Fatal(err)
-	}
-	defer f.Close()
-
-	h := crc64.New(crc64.MakeTable(crc64.ISO))
-	if _, err := io.Copy(h, f); err != nil {
-		tb.Fatal(err)
-	}
-	return h.Sum64()
-}

db_test.go

@@ -3,6 +3,7 @@ package litestream_test
 import (
 	"context"
 	"database/sql"
+	"io/ioutil"
 	"os"
 	"path/filepath"
 	"strings"
@@ -29,13 +30,13 @@ func TestDB_WALPath(t *testing.T) {
 func TestDB_MetaPath(t *testing.T) {
 	t.Run("Absolute", func(t *testing.T) {
 		db := litestream.NewDB("/tmp/db")
-		if got, want := db.MetaPath(), `/tmp/db-litestream`; got != want {
+		if got, want := db.MetaPath(), `/tmp/.db-litestream`; got != want {
 			t.Fatalf("MetaPath()=%v, want %v", got, want)
 		}
 	})
 	t.Run("Relative", func(t *testing.T) {
 		db := litestream.NewDB("db")
-		if got, want := db.MetaPath(), `db-litestream`; got != want {
+		if got, want := db.MetaPath(), `.db-litestream`; got != want {
 			t.Fatalf("MetaPath()=%v, want %v", got, want)
 		}
 	})
@@ -43,25 +44,32 @@ func TestDB_MetaPath(t *testing.T) {
 
 func TestDB_GenerationNamePath(t *testing.T) {
 	db := litestream.NewDB("/tmp/db")
-	if got, want := db.GenerationNamePath(), `/tmp/db-litestream/generation`; got != want {
+	if got, want := db.GenerationNamePath(), `/tmp/.db-litestream/generation`; got != want {
 		t.Fatalf("GenerationNamePath()=%v, want %v", got, want)
 	}
 }
 
 func TestDB_GenerationPath(t *testing.T) {
 	db := litestream.NewDB("/tmp/db")
-	if got, want := db.GenerationPath("xxxx"), `/tmp/db-litestream/generations/xxxx`; got != want {
+	if got, want := db.GenerationPath("xxxx"), `/tmp/.db-litestream/generations/xxxx`; got != want {
 		t.Fatalf("GenerationPath()=%v, want %v", got, want)
 	}
 }
 
 func TestDB_ShadowWALDir(t *testing.T) {
 	db := litestream.NewDB("/tmp/db")
-	if got, want := db.ShadowWALDir("xxxx"), `/tmp/db-litestream/generations/xxxx/wal`; got != want {
+	if got, want := db.ShadowWALDir("xxxx"), `/tmp/.db-litestream/generations/xxxx/wal`; got != want {
 		t.Fatalf("ShadowWALDir()=%v, want %v", got, want)
 	}
 }
 
+func TestDB_ShadowWALPath(t *testing.T) {
+	db := litestream.NewDB("/tmp/db")
+	if got, want := db.ShadowWALPath("xxxx", 1000), `/tmp/.db-litestream/generations/xxxx/wal/000003e8.wal`; got != want {
+		t.Fatalf("ShadowWALPath()=%v, want %v", got, want)
+	}
+}
+
 // Ensure we can check the last modified time of the real database and its WAL.
 func TestDB_UpdatedAt(t *testing.T) {
 	t.Run("ErrNotExist", func(t *testing.T) {
@@ -187,7 +195,9 @@ func TestDB_Sync(t *testing.T) {
 		}
 
 		// Ensure position now available.
-		if pos := db.Pos(); pos.Generation == "" {
+		if pos, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if pos.Generation == "" {
 			t.Fatal("expected generation")
 		} else if got, want := pos.Index, 0; got != want {
 			t.Fatalf("pos.Index=%v, want %v", got, want)
@@ -211,7 +221,10 @@ func TestDB_Sync(t *testing.T) {
 			t.Fatal(err)
 		}
 
-		pos0 := db.Pos()
+		pos0, err := db.Pos()
+		if err != nil {
+			t.Fatal(err)
+		}
 
 		// Insert into table.
 		if _, err := sqldb.Exec(`INSERT INTO foo (bar) VALUES ('baz');`); err != nil {
@@ -221,7 +234,9 @@ func TestDB_Sync(t *testing.T) {
 		// Sync to ensure position moves forward one page.
 		if err := db.Sync(context.Background()); err != nil {
 			t.Fatal(err)
-		} else if pos1 := db.Pos(); pos0.Generation != pos1.Generation {
+		} else if pos1, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if pos0.Generation != pos1.Generation {
 			t.Fatal("expected the same generation")
 		} else if got, want := pos1.Index, pos0.Index; got != want {
 			t.Fatalf("Index=%v, want %v", got, want)
@@ -241,7 +256,10 @@ func TestDB_Sync(t *testing.T) {
 		}
 
 		// Obtain initial position.
-		pos0 := db.Pos()
+		pos0, err := db.Pos()
+		if err != nil {
+			t.Fatal(err)
+		}
 
 		// Checkpoint & fully close which should close WAL file.
 		if err := db.Checkpoint(context.Background(), litestream.CheckpointModeTruncate); err != nil {
@@ -267,7 +285,9 @@ func TestDB_Sync(t *testing.T) {
 		}
 
 		// Obtain initial position.
-		if pos1 := db.Pos(); pos0.Generation == pos1.Generation {
+		if pos1, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if pos0.Generation == pos1.Generation {
 			t.Fatal("expected new generation after truncation")
 		}
 	})
@@ -288,7 +308,10 @@ func TestDB_Sync(t *testing.T) {
 		}
 
 		// Obtain initial position.
-		pos0 := db.Pos()
+		pos0, err := db.Pos()
+		if err != nil {
+			t.Fatal(err)
+		}
 
 		// Fully close which should close WAL file.
 		if err := db.Close(); err != nil {
@@ -321,13 +344,13 @@ func TestDB_Sync(t *testing.T) {
 		}
 
 		// Obtain initial position.
-		if pos1 := db.Pos(); pos0.Generation == pos1.Generation {
+		if pos1, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if pos0.Generation == pos1.Generation {
 			t.Fatal("expected new generation after truncation")
 		}
 	})
 
-	// TODO: Fix test to check for header mismatch
-	/*
 	// Ensure DB can handle a mismatched header-only and start new generation.
 	t.Run("WALHeaderMismatch", func(t *testing.T) {
 		db, sqldb := MustOpenDBs(t)
@@ -341,17 +364,19 @@ func TestDB_Sync(t *testing.T) {
 		}
 
 		// Grab initial position & close.
-			pos0 := db.Pos()
-			if err := db.Close(); err != nil {
+		pos0, err := db.Pos()
+		if err != nil {
+			t.Fatal(err)
+		} else if err := db.Close(); err != nil {
 			t.Fatal(err)
 		}
 
 		// Read existing file, update header checksum, and write back only header
 		// to simulate a header with a mismatched checksum.
 		shadowWALPath := db.ShadowWALPath(pos0.Generation, pos0.Index)
-			if buf, err := os.ReadFile(shadowWALPath); err != nil {
+		if buf, err := ioutil.ReadFile(shadowWALPath); err != nil {
 			t.Fatal(err)
-			} else if err := os.WriteFile(shadowWALPath, append(buf[:litestream.WALHeaderSize-8], 0, 0, 0, 0, 0, 0, 0, 0), 0600); err != nil {
+		} else if err := ioutil.WriteFile(shadowWALPath, append(buf[:litestream.WALHeaderSize-8], 0, 0, 0, 0, 0, 0, 0, 0), 0600); err != nil {
 			t.Fatal(err)
 		}
 
@@ -369,10 +394,98 @@ func TestDB_Sync(t *testing.T) {
 			t.Fatal("expected new generation")
 		}
 	})
-	*/
 
-	// TODO: Fix test for segmented shadow WAL.
-	/*
+	// Ensure DB can handle partial shadow WAL header write.
+	t.Run("PartialShadowWALHeader", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+
+		// Execute a query to force a write to the WAL and then sync.
+		if _, err := sqldb.Exec(`CREATE TABLE foo (bar TEXT);`); err != nil {
+			t.Fatal(err)
+		} else if err := db.Sync(context.Background()); err != nil {
+			t.Fatal(err)
+		}
+
+		pos0, err := db.Pos()
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		// Close & truncate shadow WAL to simulate a partial header write.
+		if err := db.Close(); err != nil {
+			t.Fatal(err)
+		} else if err := os.Truncate(db.ShadowWALPath(pos0.Generation, pos0.Index), litestream.WALHeaderSize-1); err != nil {
+			t.Fatal(err)
+		}
+
+		// Reopen managed database & ensure sync will still work.
+		db = MustOpenDBAt(t, db.Path())
+		defer MustCloseDB(t, db)
+		if err := db.Sync(context.Background()); err != nil {
+			t.Fatal(err)
+		}
+
+		// Verify a new generation was started.
+		if pos1, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if pos0.Generation == pos1.Generation {
+			t.Fatal("expected new generation")
+		}
+	})
+
+	// Ensure DB can handle partial shadow WAL writes.
+	t.Run("PartialShadowWALFrame", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+
+		// Execute a query to force a write to the WAL and then sync.
+		if _, err := sqldb.Exec(`CREATE TABLE foo (bar TEXT);`); err != nil {
+			t.Fatal(err)
+		} else if err := db.Sync(context.Background()); err != nil {
+			t.Fatal(err)
+		}
+
+		pos0, err := db.Pos()
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		// Obtain current shadow WAL size.
+		fi, err := os.Stat(db.ShadowWALPath(pos0.Generation, pos0.Index))
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		// Close & truncate shadow WAL to simulate a partial frame write.
+		if err := db.Close(); err != nil {
+			t.Fatal(err)
+		} else if err := os.Truncate(db.ShadowWALPath(pos0.Generation, pos0.Index), fi.Size()-1); err != nil {
+			t.Fatal(err)
+		}
+
+		// Reopen managed database & ensure sync will still work.
+		db = MustOpenDBAt(t, db.Path())
+		defer MustCloseDB(t, db)
+		if err := db.Sync(context.Background()); err != nil {
+			t.Fatal(err)
+		}
+
+		// Verify same generation is kept.
+		if pos1, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if got, want := pos1, pos0; got != want {
+			t.Fatalf("Pos()=%s want %s", got, want)
+		}
+
+		// Ensure shadow WAL has recovered.
+		if fi0, err := os.Stat(db.ShadowWALPath(pos0.Generation, pos0.Index)); err != nil {
+			t.Fatal(err)
+		} else if got, want := fi0.Size(), fi.Size(); got != want {
+			t.Fatalf("Size()=%v, want %v", got, want)
+		}
+	})
+
 	// Ensure DB can handle a generation directory with a missing shadow WAL.
 	t.Run("NoShadowWAL", func(t *testing.T) {
 		db, sqldb := MustOpenDBs(t)
@@ -385,7 +498,10 @@ func TestDB_Sync(t *testing.T) {
 			t.Fatal(err)
 		}
 
-			pos0 := db.Pos()
+		pos0, err := db.Pos()
+		if err != nil {
+			t.Fatal(err)
+		}
 
 		// Close & delete shadow WAL to simulate dir created but not WAL.
 		if err := db.Close(); err != nil {
@@ -412,7 +528,6 @@ func TestDB_Sync(t *testing.T) {
 			t.Fatalf("Offset=%v want %v", got, want)
 		}
 	})
-	*/
 
 	// Ensure DB checkpoints after minimum number of pages.
 	t.Run("MinCheckpointPageN", func(t *testing.T) {
@@ -439,7 +554,9 @@ func TestDB_Sync(t *testing.T) {
 		}
 
 		// Ensure position is now on the second index.
-		if got, want := db.Pos().Index, 1; got != want {
+		if pos, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if got, want := pos.Index, 1; got != want {
 			t.Fatalf("Index=%v, want %v", got, want)
 		}
 	})
@@ -467,7 +584,9 @@ func TestDB_Sync(t *testing.T) {
 		}
 
 		// Ensure position is now on the second index.
-		if got, want := db.Pos().Index, 1; got != want {
+		if pos, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if got, want := pos.Index, 1; got != want {
 			t.Fatalf("Index=%v, want %v", got, want)
 		}
 	})

file/replica_client.go

@@ -8,7 +8,6 @@ import (
 	"os"
 	"path/filepath"
 	"sort"
-	"strings"
 
 	"github.com/benbjohnson/litestream"
 	"github.com/benbjohnson/litestream/internal"
@@ -85,7 +84,7 @@ func (c *ReplicaClient) SnapshotPath(generation string, index int) (string, erro
 	if err != nil {
 		return "", err
 	}
-	return filepath.Join(dir, litestream.FormatIndex(index)+".snapshot.lz4"), nil
+	return filepath.Join(dir, litestream.FormatSnapshotPath(index)), nil
 }
 
 // WALDir returns the path to a generation's WAL directory
@@ -103,7 +102,7 @@ func (c *ReplicaClient) WALSegmentPath(generation string, index int, offset int6
 	if err != nil {
 		return "", err
 	}
-	return filepath.Join(dir, litestream.FormatIndex(index), fmt.Sprintf("%08x.wal.lz4", offset)), nil
+	return filepath.Join(dir, litestream.FormatWALSegmentPath(index, offset)), nil
 }
 
 // Generations returns a list of available generation names.
@@ -149,7 +148,7 @@ func (c *ReplicaClient) DeleteGeneration(ctx context.Context, generation string)
 func (c *ReplicaClient) Snapshots(ctx context.Context, generation string) (litestream.SnapshotIterator, error) {
 	dir, err := c.SnapshotsDir(generation)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("cannot determine snapshots path: %w", err)
 	}
 
 	f, err := os.Open(dir)
@@ -169,7 +168,7 @@ func (c *ReplicaClient) Snapshots(ctx context.Context, generation string) (lites
 	infos := make([]litestream.SnapshotInfo, 0, len(fis))
 	for _, fi := range fis {
 		// Parse index from filename.
-		index, err := internal.ParseSnapshotPath(filepath.Base(fi.Name()))
+		index, err := litestream.ParseSnapshotPath(fi.Name())
 		if err != nil {
 			continue
 		}
@@ -191,7 +190,7 @@ func (c *ReplicaClient) Snapshots(ctx context.Context, generation string) (lites
 func (c *ReplicaClient) WriteSnapshot(ctx context.Context, generation string, index int, rd io.Reader) (info litestream.SnapshotInfo, err error) {
 	filename, err := c.SnapshotPath(generation, index)
 	if err != nil {
-		return info, err
+		return info, fmt.Errorf("cannot determine snapshot path: %w", err)
 	}
 
 	var fileInfo, dirInfo os.FileInfo
@@ -244,7 +243,7 @@ func (c *ReplicaClient) WriteSnapshot(ctx context.Context, generation string, in
 func (c *ReplicaClient) SnapshotReader(ctx context.Context, generation string, index int) (io.ReadCloser, error) {
 	filename, err := c.SnapshotPath(generation, index)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("cannot determine snapshot path: %w", err)
 	}
 	return os.Open(filename)
 }
@@ -265,7 +264,7 @@ func (c *ReplicaClient) DeleteSnapshot(ctx context.Context, generation string, i
 func (c *ReplicaClient) WALSegments(ctx context.Context, generation string) (litestream.WALSegmentIterator, error) {
 	dir, err := c.WALDir(generation)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("cannot determine wal path: %w", err)
 	}
 
 	f, err := os.Open(dir)
@@ -282,25 +281,33 @@ func (c *ReplicaClient) WALSegments(ctx context.Context, generation string) (lit
 	}
 
 	// Iterate over every file and convert to metadata.
-	indexes := make([]int, 0, len(fis))
+	infos := make([]litestream.WALSegmentInfo, 0, len(fis))
 	for _, fi := range fis {
-		index, err := litestream.ParseIndex(fi.Name())
-		if err != nil || !fi.IsDir() {
+		// Parse index from filename.
+		index, offset, err := litestream.ParseWALSegmentPath(fi.Name())
+		if err != nil {
 			continue
 		}
-		indexes = append(indexes, index)
+
+		infos = append(infos, litestream.WALSegmentInfo{
+			Generation: generation,
+			Index:      index,
+			Offset:     offset,
+			Size:       fi.Size(),
+			CreatedAt:  fi.ModTime().UTC(),
+		})
 	}
 
-	sort.Ints(indexes)
+	sort.Sort(litestream.WALSegmentInfoSlice(infos))
 
-	return newWALSegmentIterator(dir, generation, indexes), nil
+	return litestream.NewWALSegmentInfoSliceIterator(infos), nil
 }
 
 // WriteWALSegment writes LZ4 compressed data from rd into a file on disk.
 func (c *ReplicaClient) WriteWALSegment(ctx context.Context, pos litestream.Pos, rd io.Reader) (info litestream.WALSegmentInfo, err error) {
 	filename, err := c.WALSegmentPath(pos.Generation, pos.Index, pos.Offset)
 	if err != nil {
-		return info, err
+		return info, fmt.Errorf("cannot determine wal segment path: %w", err)
 	}
 
 	var fileInfo, dirInfo os.FileInfo
@@ -354,7 +361,7 @@ func (c *ReplicaClient) WriteWALSegment(ctx context.Context, pos litestream.Pos,
 func (c *ReplicaClient) WALSegmentReader(ctx context.Context, pos litestream.Pos) (io.ReadCloser, error) {
 	filename, err := c.WALSegmentPath(pos.Generation, pos.Index, pos.Offset)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("cannot determine wal segment path: %w", err)
 	}
 	return os.Open(filename)
 }
@@ -364,7 +371,7 @@ func (c *ReplicaClient) DeleteWALSegments(ctx context.Context, a []litestream.Po
 	for _, pos := range a {
 		filename, err := c.WALSegmentPath(pos.Generation, pos.Index, pos.Offset)
 		if err != nil {
-			return err
+			return fmt.Errorf("cannot determine wal segment path: %w", err)
 		}
 		if err := os.Remove(filename); err != nil && !os.IsNotExist(err) {
 			return err
@@ -372,100 +379,3 @@ func (c *ReplicaClient) DeleteWALSegments(ctx context.Context, a []litestream.Po
 	}
 	return nil
 }
-
-type walSegmentIterator struct {
-	dir        string
-	generation string
-	indexes    []int
-
-	infos []litestream.WALSegmentInfo
-	err   error
-}
-
-func newWALSegmentIterator(dir, generation string, indexes []int) *walSegmentIterator {
-	return &walSegmentIterator{
-		dir:        dir,
-		generation: generation,
-		indexes:    indexes,
-	}
-}
-
-func (itr *walSegmentIterator) Close() (err error) {
-	return itr.err
-}
-
-func (itr *walSegmentIterator) Next() bool {
-	// Exit if an error has already occurred.
-	if itr.err != nil {
-		return false
-	}
-
-	for {
-		// Move to the next segment in cache, if available.
-		if len(itr.infos) > 1 {
-			itr.infos = itr.infos[1:]
-			return true
-		}
-		itr.infos = itr.infos[:0] // otherwise clear infos
-
-		// If no indexes remain, stop iteration.
-		if len(itr.indexes) == 0 {
-			return false
-		}
-
-		// Read segments into a cache for the current index.
-		index := itr.indexes[0]
-		itr.indexes = itr.indexes[1:]
-		f, err := os.Open(filepath.Join(itr.dir, litestream.FormatIndex(index)))
-		if err != nil {
-			itr.err = err
-			return false
-		}
-		defer f.Close()
-
-		fis, err := f.Readdir(-1)
-		if err != nil {
-			itr.err = err
-			return false
-		} else if err := f.Close(); err != nil {
-			itr.err = err
-			return false
-		}
-
-		for _, fi := range fis {
-			filename := filepath.Base(fi.Name())
-			if fi.IsDir() {
-				continue
-			}
-
-			offset, err := litestream.ParseOffset(strings.TrimSuffix(filename, ".wal.lz4"))
-			if err != nil {
-				continue
-			}
-
-			itr.infos = append(itr.infos, litestream.WALSegmentInfo{
-				Generation: itr.generation,
-				Index:      index,
-				Offset:     offset,
-				Size:       fi.Size(),
-				CreatedAt:  fi.ModTime().UTC(),
-			})
-		}
-
-		// Ensure segments are sorted within index.
-		sort.Sort(litestream.WALSegmentInfoSlice(itr.infos))
-
-		if len(itr.infos) > 0 {
-			return true
-		}
-	}
-}
-
-func (itr *walSegmentIterator) Err() error { return itr.err }
-
-func (itr *walSegmentIterator) WALSegment() litestream.WALSegmentInfo {
-	if len(itr.infos) == 0 {
-		return litestream.WALSegmentInfo{}
-	}
-	return itr.infos[0]
-}

file/replica_client_test.go

@@ -118,7 +118,7 @@ func TestReplicaClient_WALSegmentPath(t *testing.T) {
 	t.Run("OK", func(t *testing.T) {
 		if got, err := file.NewReplicaClient("/foo").WALSegmentPath("0123456701234567", 1000, 1001); err != nil {
 			t.Fatal(err)
-		} else if want := "/foo/generations/0123456701234567/wal/000003e8/000003e9.wal.lz4"; got != want {
+		} else if want := "/foo/generations/0123456701234567/wal/000003e8_000003e9.wal.lz4"; got != want {
 			t.Fatalf("WALPath()=%v, want %v", got, want)
 		}
 	})
@@ -133,3 +133,91 @@ func TestReplicaClient_WALSegmentPath(t *testing.T) {
 		}
 	})
 }
+
+/*
+func TestReplica_Sync(t *testing.T) {
+	// Ensure replica can successfully sync after DB has sync'd.
+	t.Run("InitialSync", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+
+		r := litestream.NewReplica(db, "", file.NewReplicaClient(t.TempDir()))
+		r.MonitorEnabled = false
+		db.Replicas = []*litestream.Replica{r}
+
+		// Sync database & then sync replica.
+		if err := db.Sync(context.Background()); err != nil {
+			t.Fatal(err)
+		} else if err := r.Sync(context.Background()); err != nil {
+			t.Fatal(err)
+		}
+
+		// Ensure posistions match.
+		if want, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if got, err := r.Pos(context.Background()); err != nil {
+			t.Fatal(err)
+		} else if got != want {
+			t.Fatalf("Pos()=%v, want %v", got, want)
+		}
+	})
+
+	// Ensure replica can successfully sync multiple times.
+	t.Run("MultiSync", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+
+		r := litestream.NewReplica(db, "", file.NewReplicaClient(t.TempDir()))
+		r.MonitorEnabled = false
+		db.Replicas = []*litestream.Replica{r}
+
+		if _, err := sqldb.Exec(`CREATE TABLE foo (bar TEXT);`); err != nil {
+			t.Fatal(err)
+		}
+
+		// Write to the database multiple times and sync after each write.
+		for i, n := 0, db.MinCheckpointPageN*2; i < n; i++ {
+			if _, err := sqldb.Exec(`INSERT INTO foo (bar) VALUES ('baz')`); err != nil {
+				t.Fatal(err)
+			}
+
+			// Sync periodically.
+			if i%100 == 0 || i == n-1 {
+				if err := db.Sync(context.Background()); err != nil {
+					t.Fatal(err)
+				} else if err := r.Sync(context.Background()); err != nil {
+					t.Fatal(err)
+				}
+			}
+		}
+
+		// Ensure posistions match.
+		pos, err := db.Pos()
+		if err != nil {
+			t.Fatal(err)
+		} else if got, want := pos.Index, 2; got != want {
+			t.Fatalf("Index=%v, want %v", got, want)
+		}
+
+		if want, err := r.Pos(context.Background()); err != nil {
+			t.Fatal(err)
+		} else if got := pos; got != want {
+			t.Fatalf("Pos()=%v, want %v", got, want)
+		}
+	})
+
+	// Ensure replica returns an error if there is no generation available from the DB.
+	t.Run("ErrNoGeneration", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+
+		r := litestream.NewReplica(db, "", file.NewReplicaClient(t.TempDir()))
+		r.MonitorEnabled = false
+		db.Replicas = []*litestream.Replica{r}
+
+		if err := r.Sync(context.Background()); err == nil || err.Error() != `no generation, waiting for data` {
+			t.Fatal(err)
+		}
+	})
+}
+*/

gcs/replica_client.go

@@ -68,7 +68,7 @@ func (c *ReplicaClient) Generations(ctx context.Context) ([]string, error) {
 	// Construct query to only pull generation directory names.
 	query := &storage.Query{
 		Delimiter: "/",
-		Prefix:    path.Join(c.Path, "generations") + "/",
+		Prefix:    litestream.GenerationsPath(c.Path) + "/",
 	}
 
 	// Loop over results and only build list of generation-formatted names.
@@ -96,15 +96,16 @@ func (c *ReplicaClient) Generations(ctx context.Context) ([]string, error) {
 func (c *ReplicaClient) DeleteGeneration(ctx context.Context, generation string) error {
 	if err := c.Init(ctx); err != nil {
 		return err
-	} else if generation == "" {
-		return fmt.Errorf("generation required")
 	}
 
-	prefix := path.Join(c.Path, "generations", generation) + "/"
+	dir, err := litestream.GenerationPath(c.Path, generation)
+	if err != nil {
+		return fmt.Errorf("cannot determine generation path: %w", err)
+	}
 
 	// Iterate over every object in generation and delete it.
 	internal.OperationTotalCounterVec.WithLabelValues(ReplicaClientType, "LIST").Inc()
-	for it := c.bkt.Objects(ctx, &storage.Query{Prefix: prefix}); ; {
+	for it := c.bkt.Objects(ctx, &storage.Query{Prefix: dir + "/"}); ; {
 		attrs, err := it.Next()
 		if err == iterator.Done {
 			break
@@ -129,22 +130,24 @@ func (c *ReplicaClient) DeleteGeneration(ctx context.Context, generation string)
 func (c *ReplicaClient) Snapshots(ctx context.Context, generation string) (litestream.SnapshotIterator, error) {
 	if err := c.Init(ctx); err != nil {
 		return nil, err
-	} else if generation == "" {
-		return nil, fmt.Errorf("generation required")
 	}
-	prefix := path.Join(c.Path, "generations", generation) + "/"
-	return newSnapshotIterator(generation, c.bkt.Objects(ctx, &storage.Query{Prefix: prefix})), nil
+	dir, err := litestream.SnapshotsPath(c.Path, generation)
+	if err != nil {
+		return nil, fmt.Errorf("cannot determine snapshots path: %w", err)
+	}
+	return newSnapshotIterator(generation, c.bkt.Objects(ctx, &storage.Query{Prefix: dir + "/"})), nil
 }
 
 // WriteSnapshot writes LZ4 compressed data from rd to the object storage.
 func (c *ReplicaClient) WriteSnapshot(ctx context.Context, generation string, index int, rd io.Reader) (info litestream.SnapshotInfo, err error) {
 	if err := c.Init(ctx); err != nil {
 		return info, err
-	} else if generation == "" {
-		return info, fmt.Errorf("generation required")
 	}
 
-	key := path.Join(c.Path, "generations", generation, "snapshots", litestream.FormatIndex(index)+".snapshot.lz4")
+	key, err := litestream.SnapshotPath(c.Path, generation, index)
+	if err != nil {
+		return info, fmt.Errorf("cannot determine snapshot path: %w", err)
+	}
 	startTime := time.Now()
 
 	w := c.bkt.Object(key).NewWriter(ctx)
@@ -174,11 +177,12 @@ func (c *ReplicaClient) WriteSnapshot(ctx context.Context, generation string, in
 func (c *ReplicaClient) SnapshotReader(ctx context.Context, generation string, index int) (io.ReadCloser, error) {
 	if err := c.Init(ctx); err != nil {
 		return nil, err
-	} else if generation == "" {
-		return nil, fmt.Errorf("generation required")
 	}
 
-	key := path.Join(c.Path, "generations", generation, "snapshots", litestream.FormatIndex(index)+".snapshot.lz4")
+	key, err := litestream.SnapshotPath(c.Path, generation, index)
+	if err != nil {
+		return nil, fmt.Errorf("cannot determine snapshot path: %w", err)
+	}
 
 	r, err := c.bkt.Object(key).NewReader(ctx)
 	if isNotExists(err) {
@@ -197,11 +201,12 @@ func (c *ReplicaClient) SnapshotReader(ctx context.Context, generation string, i
 func (c *ReplicaClient) DeleteSnapshot(ctx context.Context, generation string, index int) error {
 	if err := c.Init(ctx); err != nil {
 		return err
-	} else if generation == "" {
-		return fmt.Errorf("generation required")
 	}
 
-	key := path.Join(c.Path, "generations", generation, "snapshots", litestream.FormatIndex(index), ".snapshot.lz4")
+	key, err := litestream.SnapshotPath(c.Path, generation, index)
+	if err != nil {
+		return fmt.Errorf("cannot determine snapshot path: %w", err)
+	}
 
 	if err := c.bkt.Object(key).Delete(ctx); err != nil && !isNotExists(err) {
 		return fmt.Errorf("cannot delete snapshot %q: %w", key, err)
@@ -215,22 +220,24 @@ func (c *ReplicaClient) DeleteSnapshot(ctx context.Context, generation string, i
 func (c *ReplicaClient) WALSegments(ctx context.Context, generation string) (litestream.WALSegmentIterator, error) {
 	if err := c.Init(ctx); err != nil {
 		return nil, err
-	} else if generation == "" {
-		return nil, fmt.Errorf("generation required")
 	}
-	prefix := path.Join(c.Path, "generations", generation, "wal") + "/"
-	return newWALSegmentIterator(generation, prefix, c.bkt.Objects(ctx, &storage.Query{Prefix: prefix})), nil
+	dir, err := litestream.WALPath(c.Path, generation)
+	if err != nil {
+		return nil, fmt.Errorf("cannot determine wal path: %w", err)
+	}
+	return newWALSegmentIterator(generation, c.bkt.Objects(ctx, &storage.Query{Prefix: dir + "/"})), nil
 }
 
 // WriteWALSegment writes LZ4 compressed data from rd into a file on disk.
 func (c *ReplicaClient) WriteWALSegment(ctx context.Context, pos litestream.Pos, rd io.Reader) (info litestream.WALSegmentInfo, err error) {
 	if err := c.Init(ctx); err != nil {
 		return info, err
-	} else if pos.Generation == "" {
-		return info, fmt.Errorf("generation required")
 	}
 
-	key := path.Join(c.Path, "generations", pos.Generation, "wal", litestream.FormatIndex(pos.Index), litestream.FormatOffset(pos.Offset)+".wal.lz4")
+	key, err := litestream.WALSegmentPath(c.Path, pos.Generation, pos.Index, pos.Offset)
+	if err != nil {
+		return info, fmt.Errorf("cannot determine wal segment path: %w", err)
+	}
 	startTime := time.Now()
 
 	w := c.bkt.Object(key).NewWriter(ctx)
@@ -260,11 +267,12 @@ func (c *ReplicaClient) WriteWALSegment(ctx context.Context, pos litestream.Pos,
 func (c *ReplicaClient) WALSegmentReader(ctx context.Context, pos litestream.Pos) (io.ReadCloser, error) {
 	if err := c.Init(ctx); err != nil {
 		return nil, err
-	} else if pos.Generation == "" {
-		return nil, fmt.Errorf("generation required")
 	}
 
-	key := path.Join(c.Path, "generations", pos.Generation, "wal", litestream.FormatIndex(pos.Index), litestream.FormatOffset(pos.Offset)+".wal.lz4")
+	key, err := litestream.WALSegmentPath(c.Path, pos.Generation, pos.Index, pos.Offset)
+	if err != nil {
+		return nil, fmt.Errorf("cannot determine wal segment path: %w", err)
+	}
 
 	r, err := c.bkt.Object(key).NewReader(ctx)
 	if isNotExists(err) {
@@ -286,11 +294,11 @@ func (c *ReplicaClient) DeleteWALSegments(ctx context.Context, a []litestream.Po
 	}
 
 	for _, pos := range a {
-		if pos.Generation == "" {
-			return fmt.Errorf("generation required")
+		key, err := litestream.WALSegmentPath(c.Path, pos.Generation, pos.Index, pos.Offset)
+		if err != nil {
+			return fmt.Errorf("cannot determine wal segment path: %w", err)
 		}
 
-		key := path.Join(c.Path, "generations", pos.Generation, "wal", litestream.FormatIndex(pos.Index), litestream.FormatOffset(pos.Offset)+".wal.lz4")
 		if err := c.bkt.Object(key).Delete(ctx); err != nil && !isNotExists(err) {
 			return fmt.Errorf("cannot delete wal segment %q: %w", key, err)
 		}
@@ -336,7 +344,7 @@ func (itr *snapshotIterator) Next() bool {
 		}
 
 		// Parse index, otherwise skip to the next object.
-		index, err := internal.ParseSnapshotPath(path.Base(attrs.Name))
+		index, err := litestream.ParseSnapshotPath(path.Base(attrs.Name))
 		if err != nil {
 			continue
 		}
@@ -358,17 +366,15 @@ func (itr *snapshotIterator) Snapshot() litestream.SnapshotInfo { return itr.inf
 
 type walSegmentIterator struct {
 	generation string
-	prefix     string
 
 	it   *storage.ObjectIterator
 	info litestream.WALSegmentInfo
 	err  error
 }
 
-func newWALSegmentIterator(generation, prefix string, it *storage.ObjectIterator) *walSegmentIterator {
+func newWALSegmentIterator(generation string, it *storage.ObjectIterator) *walSegmentIterator {
 	return &walSegmentIterator{
 		generation: generation,
-		prefix:     prefix,
 		it:         it,
 	}
 }
@@ -394,7 +400,7 @@ func (itr *walSegmentIterator) Next() bool {
 		}
 
 		// Parse index & offset, otherwise skip to the next object.
-		index, offset, err := internal.ParseWALSegmentPath(strings.TrimPrefix(attrs.Name, itr.prefix))
+		index, offset, err := litestream.ParseWALSegmentPath(path.Base(attrs.Name))
 		if err != nil {
 			continue
 		}

internal/internal.go

@@ -1,11 +1,8 @@
 package internal
 
 import (
-	"fmt"
 	"io"
 	"os"
-	"regexp"
-	"strconv"
 	"syscall"
 
 	"github.com/prometheus/client_golang/prometheus"
@@ -39,39 +36,6 @@ func (r *ReadCloser) Close() error {
 	return r.c.Close()
 }
 
-// MultiReadCloser is a logical concatenation of io.ReadCloser.
-// It works like io.MultiReader except all objects are closed when Close() is called.
-type MultiReadCloser struct {
-	mr      io.Reader
-	closers []io.Closer
-}
-
-// NewMultiReadCloser returns a new instance of MultiReadCloser.
-func NewMultiReadCloser(a []io.ReadCloser) *MultiReadCloser {
-	readers := make([]io.Reader, len(a))
-	closers := make([]io.Closer, len(a))
-	for i, rc := range a {
-		readers[i] = rc
-		closers[i] = rc
-	}
-	return &MultiReadCloser{mr: io.MultiReader(readers...), closers: closers}
-}
-
-// Read reads from the next available reader.
-func (mrc *MultiReadCloser) Read(p []byte) (n int, err error) {
-	return mrc.mr.Read(p)
-}
-
-// Close closes all underlying ReadClosers and returns first error encountered.
-func (mrc *MultiReadCloser) Close() (err error) {
-	for _, c := range mrc.closers {
-		if e := c.Close(); e != nil && err == nil {
-			err = e
-		}
-	}
-	return err
-}
-
 // ReadCounter wraps an io.Reader and counts the total number of bytes read.
 type ReadCounter struct {
 	r io.Reader
@@ -163,33 +127,6 @@ func MkdirAll(path string, fi os.FileInfo) error {
 	return nil
 }
 
-// ParseSnapshotPath parses the index from a snapshot filename. Used by path-based replicas.
-func ParseSnapshotPath(s string) (index int, err error) {
-	a := snapshotPathRegex.FindStringSubmatch(s)
-	if a == nil {
-		return 0, fmt.Errorf("invalid snapshot path")
-	}
-
-	i64, _ := strconv.ParseUint(a[1], 16, 64)
-	return int(i64), nil
-}
-
-var snapshotPathRegex = regexp.MustCompile(`^([0-9a-f]{8})\.snapshot\.lz4$`)
-
-// ParseWALSegmentPath parses the index/offset from a segment filename. Used by path-based replicas.
-func ParseWALSegmentPath(s string) (index int, offset int64, err error) {
-	a := walSegmentPathRegex.FindStringSubmatch(s)
-	if a == nil {
-		return 0, 0, fmt.Errorf("invalid wal segment path")
-	}
-
-	i64, _ := strconv.ParseUint(a[1], 16, 64)
-	off64, _ := strconv.ParseUint(a[2], 16, 64)
-	return int(i64), int64(off64), nil
-}
-
-var walSegmentPathRegex = regexp.MustCompile(`^([0-9a-f]{8})\/([0-9a-f]{8})\.wal\.lz4$`)
-
 // Shared replica metrics.
 var (
 	OperationTotalCounterVec = promauto.NewCounterVec(prometheus.CounterOpts{

internal/internal_test.go

@@ -1,61 +0,0 @@
-package internal_test
-
-import (
-	"fmt"
-	"reflect"
-	"testing"
-
-	"github.com/benbjohnson/litestream/internal"
-)
-
-func TestParseSnapshotPath(t *testing.T) {
-	for _, tt := range []struct {
-		s     string
-		index int
-		err   error
-	}{
-		{"00bc614e.snapshot.lz4", 12345678, nil},
-		{"xxxxxxxx.snapshot.lz4", 0, fmt.Errorf("invalid snapshot path")},
-		{"00bc614.snapshot.lz4", 0, fmt.Errorf("invalid snapshot path")},
-		{"00bc614e.snapshot.lz", 0, fmt.Errorf("invalid snapshot path")},
-		{"00bc614e.snapshot", 0, fmt.Errorf("invalid snapshot path")},
-		{"00bc614e", 0, fmt.Errorf("invalid snapshot path")},
-		{"", 0, fmt.Errorf("invalid snapshot path")},
-	} {
-		t.Run("", func(t *testing.T) {
-			index, err := internal.ParseSnapshotPath(tt.s)
-			if got, want := index, tt.index; got != want {
-				t.Errorf("index=%#v, want %#v", got, want)
-			} else if got, want := err, tt.err; !reflect.DeepEqual(got, want) {
-				t.Errorf("err=%#v, want %#v", got, want)
-			}
-		})
-	}
-}
-
-func TestParseWALSegmentPath(t *testing.T) {
-	for _, tt := range []struct {
-		s      string
-		index  int
-		offset int64
-		err    error
-	}{
-		{"00bc614e/000003e8.wal.lz4", 12345678, 1000, nil},
-		{"00000000/00000000.wal", 0, 0, fmt.Errorf("invalid wal segment path")},
-		{"00000000/00000000", 0, 0, fmt.Errorf("invalid wal segment path")},
-		{"00000000/", 0, 0, fmt.Errorf("invalid wal segment path")},
-		{"00000000", 0, 0, fmt.Errorf("invalid wal segment path")},
-		{"", 0, 0, fmt.Errorf("invalid wal segment path")},
-	} {
-		t.Run("", func(t *testing.T) {
-			index, offset, err := internal.ParseWALSegmentPath(tt.s)
-			if got, want := index, tt.index; got != want {
-				t.Errorf("index=%#v, want %#v", got, want)
-			} else if got, want := offset, tt.offset; got != want {
-				t.Errorf("offset=%#v, want %#v", got, want)
-			} else if got, want := err, tt.err; !reflect.DeepEqual(got, want) {
-				t.Errorf("err=%#v, want %#v", got, want)
-			}
-		})
-	}
-}

litestream.go

@@ -7,7 +7,9 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"path"
 	"path/filepath"
+	"regexp"
 	"strconv"
 	"strings"
 	"time"
@@ -40,14 +42,6 @@ var (
 	ErrChecksumMismatch = errors.New("invalid replica, checksum mismatch")
 )
 
-var (
-	// LogWriter is the destination writer for all logging.
-	LogWriter = os.Stderr
-
-	// LogFlags are the flags passed to log.New().
-	LogFlags = 0
-)
-
 // SnapshotIterator represents an iterator over a collection of snapshot metadata.
 type SnapshotIterator interface {
 	io.Closer
@@ -213,11 +207,13 @@ func FilterSnapshotsAfter(a []SnapshotInfo, t time.Time) []SnapshotInfo {
 // FindMinSnapshotByGeneration finds the snapshot with the lowest index in a generation.
 func FindMinSnapshotByGeneration(a []SnapshotInfo, generation string) *SnapshotInfo {
 	var min *SnapshotInfo
-	for _, snapshot := range a {
+	for i := range a {
+		snapshot := &a[i]
+
 		if snapshot.Generation != generation {
 			continue
 		} else if min == nil || snapshot.Index < min.Index {
-			min = &snapshot
+			min = snapshot
 		}
 	}
 	return min
@@ -299,26 +295,6 @@ func (p Pos) Truncate() Pos {
 	return Pos{Generation: p.Generation, Index: p.Index}
 }
 
-// ComparePos returns -1 if a is less than b, 1 if a is greater than b, and
-// returns 0 if a and b are equal. Only index & offset are compared.
-// Returns an error if generations are not equal.
-func ComparePos(a, b Pos) (int, error) {
-	if a.Generation != b.Generation {
-		return 0, fmt.Errorf("generation mismatch")
-	}
-
-	if a.Index < b.Index {
-		return -1, nil
-	} else if a.Index > b.Index {
-		return 1, nil
-	} else if a.Offset < b.Offset {
-		return -1, nil
-	} else if a.Offset > b.Offset {
-		return 1, nil
-	}
-	return 0, nil
-}
-
 // Checksum computes a running SQLite checksum over a byte slice.
 func Checksum(bo binary.ByteOrder, s0, s1 uint32, b []byte) (uint32, uint32) {
 	assert(len(b)%8 == 0, "misaligned checksum byte slice")
@@ -410,34 +386,134 @@ func IsGenerationName(s string) bool {
 	return true
 }
 
-// FormatIndex formats an index as an 8-character hex value.
-func FormatIndex(index int) string {
-	return fmt.Sprintf("%08x", index)
+// GenerationsPath returns the path to a generation root directory.
+func GenerationsPath(root string) string {
+	return path.Join(root, "generations")
 }
 
-// ParseIndex parses a hex-formatted index into an integer.
-func ParseIndex(s string) (int, error) {
-	v, err := strconv.ParseUint(s, 16, 32)
+// GenerationPath returns the path to a generation's root directory.
+func GenerationPath(root, generation string) (string, error) {
+	dir := GenerationsPath(root)
+	if generation == "" {
+		return "", fmt.Errorf("generation required")
+	}
+	return path.Join(dir, generation), nil
+}
+
+// SnapshotsPath returns the path to a generation's snapshot directory.
+func SnapshotsPath(root, generation string) (string, error) {
+	dir, err := GenerationPath(root, generation)
 	if err != nil {
-		return -1, fmt.Errorf("cannot parse index: %q", s)
+		return "", err
 	}
-	return int(v), nil
+	return path.Join(dir, "snapshots"), nil
 }
 
-// FormatOffset formats an offset as an 8-character hex value.
-func FormatOffset(offset int64) string {
-	return fmt.Sprintf("%08x", offset)
-}
-
-// ParseOffset parses a hex-formatted offset into an integer.
-func ParseOffset(s string) (int64, error) {
-	v, err := strconv.ParseInt(s, 16, 32)
+// SnapshotPath returns the path to an uncompressed snapshot file.
+func SnapshotPath(root, generation string, index int) (string, error) {
+	dir, err := SnapshotsPath(root, generation)
 	if err != nil {
-		return -1, fmt.Errorf("cannot parse index: %q", s)
+		return "", err
 	}
-	return v, nil
+	return path.Join(dir, FormatSnapshotPath(index)), nil
 }
 
+// WALPath returns the path to a generation's WAL directory
+func WALPath(root, generation string) (string, error) {
+	dir, err := GenerationPath(root, generation)
+	if err != nil {
+		return "", err
+	}
+	return path.Join(dir, "wal"), nil
+}
+
+// WALSegmentPath returns the path to a WAL segment file.
+func WALSegmentPath(root, generation string, index int, offset int64) (string, error) {
+	dir, err := WALPath(root, generation)
+	if err != nil {
+		return "", err
+	}
+	return path.Join(dir, FormatWALSegmentPath(index, offset)), nil
+}
+
+// IsSnapshotPath returns true if s is a path to a snapshot file.
+func IsSnapshotPath(s string) bool {
+	return snapshotPathRegex.MatchString(s)
+}
+
+// ParseSnapshotPath returns the index for the snapshot.
+// Returns an error if the path is not a valid snapshot path.
+func ParseSnapshotPath(s string) (index int, err error) {
+	s = filepath.Base(s)
+
+	a := snapshotPathRegex.FindStringSubmatch(s)
+	if a == nil {
+		return 0, fmt.Errorf("invalid snapshot path: %s", s)
+	}
+
+	i64, _ := strconv.ParseUint(a[1], 16, 64)
+	return int(i64), nil
+}
+
+// FormatSnapshotPath formats a snapshot filename with a given index.
+func FormatSnapshotPath(index int) string {
+	assert(index >= 0, "snapshot index must be non-negative")
+	return fmt.Sprintf("%08x%s", index, SnapshotExt)
+}
+
+var snapshotPathRegex = regexp.MustCompile(`^([0-9a-f]{8})\.snapshot\.lz4$`)
+
+// IsWALPath returns true if s is a path to a WAL file.
+func IsWALPath(s string) bool {
+	return walPathRegex.MatchString(s)
+}
+
+// ParseWALPath returns the index for the WAL file.
+// Returns an error if the path is not a valid WAL path.
+func ParseWALPath(s string) (index int, err error) {
+	s = filepath.Base(s)
+
+	a := walPathRegex.FindStringSubmatch(s)
+	if a == nil {
+		return 0, fmt.Errorf("invalid wal path: %s", s)
+	}
+
+	i64, _ := strconv.ParseUint(a[1], 16, 64)
+	return int(i64), nil
+}
+
+// FormatWALPath formats a WAL filename with a given index.
+func FormatWALPath(index int) string {
+	assert(index >= 0, "wal index must be non-negative")
+	return fmt.Sprintf("%08x%s", index, WALExt)
+}
+
+var walPathRegex = regexp.MustCompile(`^([0-9a-f]{8})\.wal$`)
+
+// ParseWALSegmentPath returns the index & offset for the WAL segment file.
+// Returns an error if the path is not a valid wal segment path.
+func ParseWALSegmentPath(s string) (index int, offset int64, err error) {
+	s = filepath.Base(s)
+
+	a := walSegmentPathRegex.FindStringSubmatch(s)
+	if a == nil {
+		return 0, 0, fmt.Errorf("invalid wal segment path: %s", s)
+	}
+
+	i64, _ := strconv.ParseUint(a[1], 16, 64)
+	off64, _ := strconv.ParseUint(a[2], 16, 64)
+	return int(i64), int64(off64), nil
+}
+
+// FormatWALSegmentPath formats a WAL segment filename with a given index & offset.
+func FormatWALSegmentPath(index int, offset int64) string {
+	assert(index >= 0, "wal index must be non-negative")
+	assert(offset >= 0, "wal offset must be non-negative")
+	return fmt.Sprintf("%08x_%08x%s", index, offset, WALSegmentExt)
+}
+
+var walSegmentPathRegex = regexp.MustCompile(`^([0-9a-f]{8})(?:_([0-9a-f]{8}))\.wal\.lz4$`)
+
 // isHexChar returns true if ch is a lowercase hex character.
 func isHexChar(ch rune) bool {
 	return (ch >= '0' && ch <= '9') || (ch >= 'a' && ch <= 'f')

litestream_test.go

@@ -40,6 +40,104 @@ func TestChecksum(t *testing.T) {
 	})
 }
 
+func TestGenerationsPath(t *testing.T) {
+	t.Run("OK", func(t *testing.T) {
+		if got, want := litestream.GenerationsPath("foo"), "foo/generations"; got != want {
+			t.Fatalf("GenerationsPath()=%v, want %v", got, want)
+		}
+	})
+	t.Run("NoPath", func(t *testing.T) {
+		if got, want := litestream.GenerationsPath(""), "generations"; got != want {
+			t.Fatalf("GenerationsPath()=%v, want %v", got, want)
+		}
+	})
+}
+
+func TestGenerationPath(t *testing.T) {
+	t.Run("OK", func(t *testing.T) {
+		if got, err := litestream.GenerationPath("foo", "0123456701234567"); err != nil {
+			t.Fatal(err)
+		} else if want := "foo/generations/0123456701234567"; got != want {
+			t.Fatalf("GenerationPath()=%v, want %v", got, want)
+		}
+	})
+	t.Run("ErrNoGeneration", func(t *testing.T) {
+		if _, err := litestream.GenerationPath("foo", ""); err == nil || err.Error() != `generation required` {
+			t.Fatalf("expected error: %v", err)
+		}
+	})
+}
+
+func TestSnapshotsPath(t *testing.T) {
+	t.Run("OK", func(t *testing.T) {
+		if got, err := litestream.SnapshotsPath("foo", "0123456701234567"); err != nil {
+			t.Fatal(err)
+		} else if want := "foo/generations/0123456701234567/snapshots"; got != want {
+			t.Fatalf("SnapshotsPath()=%v, want %v", got, want)
+		}
+	})
+	t.Run("ErrNoGeneration", func(t *testing.T) {
+		if _, err := litestream.SnapshotsPath("foo", ""); err == nil || err.Error() != `generation required` {
+			t.Fatalf("unexpected error: %v", err)
+		}
+	})
+}
+
+func TestSnapshotPath(t *testing.T) {
+	t.Run("OK", func(t *testing.T) {
+		if got, err := litestream.SnapshotPath("foo", "0123456701234567", 1000); err != nil {
+			t.Fatal(err)
+		} else if want := "foo/generations/0123456701234567/snapshots/000003e8.snapshot.lz4"; got != want {
+			t.Fatalf("SnapshotPath()=%v, want %v", got, want)
+		}
+	})
+	t.Run("ErrNoGeneration", func(t *testing.T) {
+		if _, err := litestream.SnapshotPath("foo", "", 1000); err == nil || err.Error() != `generation required` {
+			t.Fatalf("unexpected error: %v", err)
+		}
+	})
+}
+
+func TestWALPath(t *testing.T) {
+	t.Run("OK", func(t *testing.T) {
+		if got, err := litestream.WALPath("foo", "0123456701234567"); err != nil {
+			t.Fatal(err)
+		} else if want := "foo/generations/0123456701234567/wal"; got != want {
+			t.Fatalf("WALPath()=%v, want %v", got, want)
+		}
+	})
+	t.Run("ErrNoGeneration", func(t *testing.T) {
+		if _, err := litestream.WALPath("foo", ""); err == nil || err.Error() != `generation required` {
+			t.Fatalf("unexpected error: %v", err)
+		}
+	})
+}
+
+func TestWALSegmentPath(t *testing.T) {
+	t.Run("OK", func(t *testing.T) {
+		if got, err := litestream.WALSegmentPath("foo", "0123456701234567", 1000, 1001); err != nil {
+			t.Fatal(err)
+		} else if want := "foo/generations/0123456701234567/wal/000003e8_000003e9.wal.lz4"; got != want {
+			t.Fatalf("WALPath()=%v, want %v", got, want)
+		}
+	})
+	t.Run("ErrNoGeneration", func(t *testing.T) {
+		if _, err := litestream.WALSegmentPath("foo", "", 1000, 0); err == nil || err.Error() != `generation required` {
+			t.Fatalf("unexpected error: %v", err)
+		}
+	})
+}
+
+func TestFindMinSnapshotByGeneration(t *testing.T) {
+	infos := []litestream.SnapshotInfo{
+		{Generation: "29cf4bced74e92ab", Index: 0},
+		{Generation: "5dfeb4aa03232553", Index: 24},
+	}
+	if got, want := litestream.FindMinSnapshotByGeneration(infos, "29cf4bced74e92ab"), &infos[0]; got != want {
+		t.Fatalf("info=%#v, want %#v", got, want)
+	}
+}
+
 func MustDecodeHexString(s string) []byte {
 	b, err := hex.DecodeString(s)
 	if err != nil {

replica.go

@@ -2,6 +2,7 @@ package litestream
 
 import (
 	"context"
+	"encoding/binary"
 	"fmt"
 	"hash/crc64"
 	"io"
@@ -66,8 +67,6 @@ type Replica struct {
 	// If true, replica monitors database for changes automatically.
 	// Set to false if replica is being used synchronously (such as in tests).
 	MonitorEnabled bool
-
-	Logger *log.Logger
 }
 
 func NewReplica(db *DB, name string) *Replica {
@@ -82,12 +81,6 @@ func NewReplica(db *DB, name string) *Replica {
 		MonitorEnabled:         true,
 	}
 
-	prefix := fmt.Sprintf("%s: ", r.Name())
-	if db != nil {
-		prefix = fmt.Sprintf("%s(%s): ", db.Path(), r.Name())
-	}
-	r.Logger = log.New(LogWriter, prefix, LogFlags)
-
 	return r
 }
 
@@ -156,12 +149,19 @@ func (r *Replica) Sync(ctx context.Context) (err error) {
 	}()
 
 	// Find current position of database.
-	dpos := r.db.Pos()
-	if dpos.IsZero() {
+	dpos, err := r.db.Pos()
+	if err != nil {
+		return fmt.Errorf("cannot determine current generation: %w", err)
+	} else if dpos.IsZero() {
 		return fmt.Errorf("no generation, waiting for data")
 	}
 	generation := dpos.Generation
 
+	Tracef("%s(%s): replica sync: db.pos=%s", r.db.Path(), r.Name(), dpos)
+
+	// Create a new snapshot and update the current replica position if
+	// the generation on the database has changed.
+	if r.Pos().Generation != generation {
 		// Create snapshot if no snapshots exist for generation.
 		snapshotN, err := r.snapshotN(generation)
 		if err != nil {
@@ -172,151 +172,124 @@ func (r *Replica) Sync(ctx context.Context) (err error) {
 			} else if info.Generation != generation {
 				return fmt.Errorf("generation changed during snapshot, exiting sync")
 			}
-		snapshotN = 1
 		}
-	replicaSnapshotTotalGaugeVec.WithLabelValues(r.db.Path(), r.Name()).Set(float64(snapshotN))
 
-	// Determine position, if necessary.
-	if r.Pos().Generation != generation {
 		pos, err := r.calcPos(ctx, generation)
 		if err != nil {
 			return fmt.Errorf("cannot determine replica position: %s", err)
 		}
 
+		Tracef("%s(%s): replica sync: calc new pos: %s", r.db.Path(), r.Name(), pos)
 		r.mu.Lock()
 		r.pos = pos
 		r.mu.Unlock()
 	}
 
 	// Read all WAL files since the last position.
-	if err = r.syncWAL(ctx); err != nil {
+	for {
+		if err = r.syncWAL(ctx); err == io.EOF {
+			break
+		} else if err != nil {
 			return err
 		}
+	}
 
 	return nil
 }
 
 func (r *Replica) syncWAL(ctx context.Context) (err error) {
-	pos := r.Pos()
-
-	itr, err := r.db.WALSegments(ctx, pos.Generation)
-	if err != nil {
+	rd, err := r.db.ShadowWALReader(r.Pos())
+	if err == io.EOF {
 		return err
+	} else if err != nil {
+		return fmt.Errorf("replica wal reader: %w", err)
 	}
-	defer itr.Close()
-
-	// Group segments by index.
-	var segments [][]WALSegmentInfo
-	for itr.Next() {
-		info := itr.WALSegment()
-		if cmp, err := ComparePos(pos, info.Pos()); err != nil {
-			return fmt.Errorf("compare pos: %w", err)
-		} else if cmp == 1 {
-			continue // already processed, skip
-		}
-
-		// Start a new chunk if index has changed.
-		if len(segments) == 0 || segments[len(segments)-1][0].Index != info.Index {
-			segments = append(segments, []WALSegmentInfo{info})
-			continue
-		}
-
-		// Add segment to the end of the current index, if matching.
-		segments[len(segments)-1] = append(segments[len(segments)-1], info)
-	}
-
-	// Write out segments to replica by index so they can be combined.
-	for i := range segments {
-		if err := r.writeIndexSegments(ctx, segments[i]); err != nil {
-			return fmt.Errorf("write index segments: index=%d err=%w", segments[i][0].Index, err)
-		}
-	}
-
-	return nil
-}
-
-func (r *Replica) writeIndexSegments(ctx context.Context, segments []WALSegmentInfo) (err error) {
-	assert(len(segments) > 0, "segments required for replication")
-
-	// First segment position must be equal to last replica position or
-	// the start of the next index.
-	if pos := r.Pos(); pos != segments[0].Pos() {
-		nextIndexPos := pos.Truncate()
-		nextIndexPos.Index++
-		if nextIndexPos != segments[0].Pos() {
-			return fmt.Errorf("replica skipped position: replica=%s initial=%s", pos, segments[0].Pos())
-		}
-	}
-
-	pos := segments[0].Pos()
-	initialPos := pos
+	defer rd.Close()
 
 	// Copy shadow WAL to client write via io.Pipe().
 	pr, pw := io.Pipe()
 	defer func() { _ = pw.CloseWithError(err) }()
 
+	// Obtain initial position from shadow reader.
+	// It may have moved to the next index if previous position was at the end.
+	pos := rd.Pos()
+
 	// Copy through pipe into client from the starting position.
 	var g errgroup.Group
 	g.Go(func() error {
-		_, err := r.Client.WriteWALSegment(ctx, initialPos, pr)
+		_, err := r.Client.WriteWALSegment(ctx, pos, pr)
 		return err
 	})
 
 	// Wrap writer to LZ4 compress.
 	zw := lz4.NewWriter(pw)
 
-	// Write each segment out to the replica.
-	for _, info := range segments {
-		if err := func() error {
-			// Ensure segments are in order and no bytes are skipped.
-			if pos != info.Pos() {
-				return fmt.Errorf("non-contiguous segment: expected=%s current=%s", pos, info.Pos())
+	// Track total WAL bytes written to replica client.
+	walBytesCounter := replicaWALBytesCounterVec.WithLabelValues(r.db.Path(), r.Name())
+
+	// Copy header if at offset zero.
+	var psalt uint64 // previous salt value
+	if pos := rd.Pos(); pos.Offset == 0 {
+		buf := make([]byte, WALHeaderSize)
+		if _, err := io.ReadFull(rd, buf); err != nil {
+			return err
 		}
 
-			rc, err := r.db.WALSegmentReader(ctx, info.Pos())
+		psalt = binary.BigEndian.Uint64(buf[16:24])
+
+		n, err := zw.Write(buf)
 		if err != nil {
 			return err
 		}
-			defer rc.Close()
+		walBytesCounter.Add(float64(n))
+	}
 
-			n, err := io.Copy(zw, lz4.NewReader(rc))
+	// Copy frames.
+	for {
+		pos := rd.Pos()
+		assert(pos.Offset == frameAlign(pos.Offset, r.db.pageSize), "shadow wal reader not frame aligned")
+
+		buf := make([]byte, WALFrameHeaderSize+r.db.pageSize)
+		if _, err := io.ReadFull(rd, buf); err == io.EOF {
+			break
+		} else if err != nil {
+			return err
+		}
+
+		// Verify salt matches the previous frame/header read.
+		salt := binary.BigEndian.Uint64(buf[8:16])
+		if psalt != 0 && psalt != salt {
+			return fmt.Errorf("replica salt mismatch: %s", pos.String())
+		}
+		psalt = salt
+
+		n, err := zw.Write(buf)
 		if err != nil {
 			return err
-			} else if err := rc.Close(); err != nil {
-				return err
+		}
+		walBytesCounter.Add(float64(n))
 	}
 
-			// Track last position written.
-			pos = info.Pos()
-			pos.Offset += n
-
-			return nil
-		}(); err != nil {
-			return fmt.Errorf("wal segment: pos=%s err=%w", info.Pos(), err)
-		}
-	}
-
-	// Flush LZ4 writer, close pipe, and wait for write to finish.
+	// Flush LZ4 writer and close pipe.
 	if err := zw.Close(); err != nil {
 		return err
 	} else if err := pw.Close(); err != nil {
 		return err
-	} else if err := g.Wait(); err != nil {
-		return err
+	}
+
+	// Wait for client to finish write.
+	if err := g.Wait(); err != nil {
+		return fmt.Errorf("client write: %w", err)
 	}
 
 	// Save last replicated position.
 	r.mu.Lock()
-	r.pos = pos
+	r.pos = rd.Pos()
 	r.mu.Unlock()
 
-	replicaWALBytesCounterVec.WithLabelValues(r.db.Path(), r.Name()).Add(float64(pos.Offset - initialPos.Offset))
-
-	// Track total WAL bytes written to replica client.
-	replicaWALIndexGaugeVec.WithLabelValues(r.db.Path(), r.Name()).Set(float64(pos.Index))
-	replicaWALOffsetGaugeVec.WithLabelValues(r.db.Path(), r.Name()).Set(float64(pos.Offset))
-
-	r.Logger.Printf("wal segment written: %s sz=%d", initialPos, pos.Offset-initialPos.Offset)
+	// Track current position
+	replicaWALIndexGaugeVec.WithLabelValues(r.db.Path(), r.Name()).Set(float64(rd.Pos().Index))
+	replicaWALOffsetGaugeVec.WithLabelValues(r.db.Path(), r.Name()).Set(float64(rd.Pos().Offset))
 
 	return nil
 }
@@ -474,8 +447,10 @@ func (r *Replica) Snapshot(ctx context.Context) (info SnapshotInfo, err error) {
 	defer func() { _ = tx.Rollback() }()
 
 	// Obtain current position.
-	pos := r.db.Pos()
-	if pos.IsZero() {
+	pos, err := r.db.Pos()
+	if err != nil {
+		return info, fmt.Errorf("cannot determine db position: %w", err)
+	} else if pos.IsZero() {
 		return info, ErrNoGeneration
 	}
 
@@ -515,7 +490,7 @@ func (r *Replica) Snapshot(ctx context.Context) (info SnapshotInfo, err error) {
 		return info, err
 	}
 
-	log.Printf("snapshot written %s/%08x", pos.Generation, pos.Index)
+	log.Printf("%s(%s): snapshot written %s/%08x", r.db.Path(), r.Name(), pos.Generation, pos.Index)
 
 	return info, nil
 }
@@ -583,7 +558,7 @@ func (r *Replica) deleteSnapshotsBeforeIndex(ctx context.Context, generation str
 		if err := r.Client.DeleteSnapshot(ctx, info.Generation, info.Index); err != nil {
 			return fmt.Errorf("delete snapshot %s/%08x: %w", info.Generation, info.Index, err)
 		}
-		r.Logger.Printf("snapshot deleted %s/%08x", generation, index)
+		log.Printf("%s(%s): snapshot deleted %s/%08x", r.db.Path(), r.Name(), generation, index)
 	}
 
 	return itr.Close()
@@ -615,10 +590,7 @@ func (r *Replica) deleteWALSegmentsBeforeIndex(ctx context.Context, generation s
 	if err := r.Client.DeleteWALSegments(ctx, a); err != nil {
 		return fmt.Errorf("delete wal segments: %w", err)
 	}
-
-	for _, pos := range a {
-		r.Logger.Printf("wal segmented deleted: %s", pos)
-	}
+	log.Printf("%s(%s): wal segmented deleted before %s/%08x: n=%d", r.db.Path(), r.Name(), generation, index, len(a))
 
 	return nil
 }
@@ -655,7 +627,7 @@ func (r *Replica) monitor(ctx context.Context) {
 
 		// Synchronize the shadow wal into the replication directory.
 		if err := r.Sync(ctx); err != nil {
-			r.Logger.Printf("monitor error: %s", err)
+			log.Printf("%s(%s): monitor error: %s", r.db.Path(), r.Name(), err)
 			continue
 		}
 	}
@@ -683,7 +655,7 @@ func (r *Replica) retainer(ctx context.Context) {
 			return
 		case <-ticker.C:
 			if err := r.EnforceRetention(ctx); err != nil {
-				r.Logger.Printf("retainer error: %s", err)
+				log.Printf("%s(%s): retainer error: %s", r.db.Path(), r.Name(), err)
 				continue
 			}
 		}
@@ -705,7 +677,7 @@ func (r *Replica) snapshotter(ctx context.Context) {
 			return
 		case <-ticker.C:
 			if _, err := r.Snapshot(ctx); err != nil && err != ErrNoGeneration {
-				r.Logger.Printf("snapshotter error: %s", err)
+				log.Printf("%s(%s): snapshotter error: %s", r.db.Path(), r.Name(), err)
 				continue
 			}
 		}
@@ -733,7 +705,7 @@ func (r *Replica) validator(ctx context.Context) {
 			return
 		case <-ticker.C:
 			if err := r.Validate(ctx); err != nil {
-				r.Logger.Printf("validation error: %s", err)
+				log.Printf("%s(%s): validation error: %s", r.db.Path(), r.Name(), err)
 				continue
 			}
 		}
@@ -795,7 +767,7 @@ func (r *Replica) Validate(ctx context.Context) error {
 	if mismatch {
 		status = "mismatch"
 	}
-	r.Logger.Printf("validator: status=%s db=%016x replica=%016x pos=%s", status, chksum0, chksum1, pos)
+	log.Printf("%s(%s): validator: status=%s db=%016x replica=%016x pos=%s", db.Path(), r.Name(), status, chksum0, chksum1, pos)
 
 	// Validate checksums match.
 	if mismatch {
@@ -813,6 +785,8 @@ func (r *Replica) Validate(ctx context.Context) error {
 
 // waitForReplica blocks until replica reaches at least the given position.
 func (r *Replica) waitForReplica(ctx context.Context, pos Pos) error {
+	db := r.DB()
+
 	ticker := time.NewTicker(500 * time.Millisecond)
 	defer ticker.Stop()
 
@@ -835,7 +809,7 @@ func (r *Replica) waitForReplica(ctx context.Context, pos Pos) error {
 		// Obtain current position of replica, check if past target position.
 		curr := r.Pos()
 		if curr.IsZero() {
-			r.Logger.Printf("validator: no replica position available")
+			log.Printf("%s(%s): validator: no replica position available", db.Path(), r.Name())
 			continue
 		}
 
@@ -1332,13 +1306,6 @@ func (r *Replica) downloadWAL(ctx context.Context, generation string, index int,
 
 // Replica metrics.
 var (
-	replicaSnapshotTotalGaugeVec = promauto.NewGaugeVec(prometheus.GaugeOpts{
-		Namespace: "litestream",
-		Subsystem: "replica",
-		Name:      "snapshot_total",
-		Help:      "The current number of snapshots",
-	}, []string{"db", "name"})
-
 	replicaWALBytesCounterVec = promauto.NewCounterVec(prometheus.CounterOpts{
 		Namespace: "litestream",
 		Subsystem: "replica",

replica_client_test.go

@@ -177,7 +177,7 @@ func TestReplicaClient_Snapshots(t *testing.T) {
 		if err == nil {
 			err = itr.Close()
 		}
-		if err == nil || err.Error() != `generation required` {
+		if err == nil || err.Error() != `cannot determine snapshots path: generation required` {
 			t.Fatalf("unexpected error: %v", err)
 		}
 	})
@@ -204,7 +204,7 @@ func TestReplicaClient_WriteSnapshot(t *testing.T) {
 
 	RunWithReplicaClient(t, "ErrNoGeneration", func(t *testing.T, c litestream.ReplicaClient) {
 		t.Parallel()
-		if _, err := c.WriteSnapshot(context.Background(), "", 0, nil); err == nil || err.Error() != `generation required` {
+		if _, err := c.WriteSnapshot(context.Background(), "", 0, nil); err == nil || err.Error() != `cannot determine snapshot path: generation required` {
 			t.Fatalf("unexpected error: %v", err)
 		}
 	})
@@ -242,13 +242,13 @@ func TestReplicaClient_SnapshotReader(t *testing.T) {
 	RunWithReplicaClient(t, "ErrNoGeneration", func(t *testing.T, c litestream.ReplicaClient) {
 		t.Parallel()
 
-		if _, err := c.SnapshotReader(context.Background(), "", 1); err == nil || err.Error() != `generation required` {
+		if _, err := c.SnapshotReader(context.Background(), "", 1); err == nil || err.Error() != `cannot determine snapshot path: generation required` {
 			t.Fatalf("unexpected error: %v", err)
 		}
 	})
 }
 
-func TestReplicaClient_WALSegments(t *testing.T) {
+func TestReplicaClient_WALs(t *testing.T) {
 	RunWithReplicaClient(t, "OK", func(t *testing.T, c litestream.ReplicaClient) {
 		t.Parallel()
 
@@ -362,7 +362,7 @@ func TestReplicaClient_WALSegments(t *testing.T) {
 		if err == nil {
 			err = itr.Close()
 		}
-		if err == nil || err.Error() != `generation required` {
+		if err == nil || err.Error() != `cannot determine wal path: generation required` {
 			t.Fatalf("unexpected error: %v", err)
 		}
 	})
@@ -389,13 +389,13 @@ func TestReplicaClient_WriteWALSegment(t *testing.T) {
 
 	RunWithReplicaClient(t, "ErrNoGeneration", func(t *testing.T, c litestream.ReplicaClient) {
 		t.Parallel()
-		if _, err := c.WriteWALSegment(context.Background(), litestream.Pos{Generation: "", Index: 0, Offset: 0}, nil); err == nil || err.Error() != `generation required` {
+		if _, err := c.WriteWALSegment(context.Background(), litestream.Pos{Generation: "", Index: 0, Offset: 0}, nil); err == nil || err.Error() != `cannot determine wal segment path: generation required` {
 			t.Fatalf("unexpected error: %v", err)
 		}
 	})
 }
 
-func TestReplicaClient_WALSegmentReader(t *testing.T) {
+func TestReplicaClient_WALReader(t *testing.T) {
 
 	RunWithReplicaClient(t, "OK", func(t *testing.T, c litestream.ReplicaClient) {
 		t.Parallel()
@@ -451,7 +451,7 @@ func TestReplicaClient_DeleteWALSegments(t *testing.T) {
 
 	RunWithReplicaClient(t, "ErrNoGeneration", func(t *testing.T, c litestream.ReplicaClient) {
 		t.Parallel()
-		if err := c.DeleteWALSegments(context.Background(), []litestream.Pos{{}}); err == nil || err.Error() != `generation required` {
+		if err := c.DeleteWALSegments(context.Background(), []litestream.Pos{{}}); err == nil || err.Error() != `cannot determine wal segment path: generation required` {
 			t.Fatalf("unexpected error: %v", err)
 		}
 	})

replica_test.go

@@ -43,7 +43,10 @@ func TestReplica_Sync(t *testing.T) {
 	}
 
 	// Fetch current database position.
-	dpos := db.Pos()
+	dpos, err := db.Pos()
+	if err != nil {
+		t.Fatal(err)
+	}
 
 	c := file.NewReplicaClient(t.TempDir())
 	r := litestream.NewReplica(db, "")
@@ -66,11 +69,11 @@ func TestReplica_Sync(t *testing.T) {
 	// Verify WAL matches replica WAL.
 	if b0, err := os.ReadFile(db.Path() + "-wal"); err != nil {
 		t.Fatal(err)
-	} else if r0, err := c.WALSegmentReader(context.Background(), litestream.Pos{Generation: generations[0], Index: 0, Offset: 0}); err != nil {
+	} else if r, err := c.WALSegmentReader(context.Background(), litestream.Pos{Generation: generations[0], Index: 0, Offset: 0}); err != nil {
 		t.Fatal(err)
-	} else if b1, err := io.ReadAll(lz4.NewReader(r0)); err != nil {
+	} else if b1, err := io.ReadAll(lz4.NewReader(r)); err != nil {
 		t.Fatal(err)
-	} else if err := r0.Close(); err != nil {
+	} else if err := r.Close(); err != nil {
 		t.Fatal(err)
 	} else if !bytes.Equal(b0, b1) {
 		t.Fatalf("wal mismatch: len(%d), len(%d)", len(b0), len(b1))
@@ -95,8 +98,10 @@ func TestReplica_Snapshot(t *testing.T) {
 	}
 
 	// Fetch current database position & snapshot.
-	pos0 := db.Pos()
-	if info, err := r.Snapshot(context.Background()); err != nil {
+	pos0, err := db.Pos()
+	if err != nil {
+		t.Fatal(err)
+	} else if info, err := r.Snapshot(context.Background()); err != nil {
 		t.Fatal(err)
 	} else if got, want := info.Pos(), pos0.Truncate(); got != want {
 		t.Fatalf("pos=%s, want %s", got, want)
@@ -117,8 +122,10 @@ func TestReplica_Snapshot(t *testing.T) {
 	}
 
 	// Fetch current database position & snapshot.
-	pos1 := db.Pos()
-	if info, err := r.Snapshot(context.Background()); err != nil {
+	pos1, err := db.Pos()
+	if err != nil {
+		t.Fatal(err)
+	} else if info, err := r.Snapshot(context.Background()); err != nil {
 		t.Fatal(err)
 	} else if got, want := info.Pos(), pos1.Truncate(); got != want {
 		t.Fatalf("pos=%v, want %v", got, want)

s3/replica_client.go

@@ -10,7 +10,6 @@ import (
 	"os"
 	"path"
 	"regexp"
-	"strings"
 	"sync"
 	"time"
 
@@ -155,7 +154,7 @@ func (c *ReplicaClient) Generations(ctx context.Context) ([]string, error) {
 	var generations []string
 	if err := c.s3.ListObjectsPagesWithContext(ctx, &s3.ListObjectsInput{
 		Bucket:    aws.String(c.Bucket),
-		Prefix:    aws.String(path.Join(c.Path, "generations") + "/"),
+		Prefix:    aws.String(litestream.GenerationsPath(c.Path) + "/"),
 		Delimiter: aws.String("/"),
 	}, func(page *s3.ListObjectsOutput, lastPage bool) bool {
 		internal.OperationTotalCounterVec.WithLabelValues(ReplicaClientType, "LIST").Inc()
@@ -179,15 +178,18 @@ func (c *ReplicaClient) Generations(ctx context.Context) ([]string, error) {
 func (c *ReplicaClient) DeleteGeneration(ctx context.Context, generation string) error {
 	if err := c.Init(ctx); err != nil {
 		return err
-	} else if generation == "" {
-		return fmt.Errorf("generation required")
+	}
+
+	dir, err := litestream.GenerationPath(c.Path, generation)
+	if err != nil {
+		return fmt.Errorf("cannot determine generation path: %w", err)
 	}
 
 	// Collect all files for the generation.
 	var objIDs []*s3.ObjectIdentifier
 	if err := c.s3.ListObjectsPagesWithContext(ctx, &s3.ListObjectsInput{
 		Bucket: aws.String(c.Bucket),
-		Prefix: aws.String(path.Join(c.Path, "generations", generation)),
+		Prefix: aws.String(dir),
 	}, func(page *s3.ListObjectsOutput, lastPage bool) bool {
 		internal.OperationTotalCounterVec.WithLabelValues(ReplicaClientType, "LIST").Inc()
 
@@ -234,11 +236,12 @@ func (c *ReplicaClient) Snapshots(ctx context.Context, generation string) (lites
 func (c *ReplicaClient) WriteSnapshot(ctx context.Context, generation string, index int, rd io.Reader) (info litestream.SnapshotInfo, err error) {
 	if err := c.Init(ctx); err != nil {
 		return info, err
-	} else if generation == "" {
-		return info, fmt.Errorf("generation required")
 	}
 
-	key := path.Join(c.Path, "generations", generation, "snapshots", litestream.FormatIndex(index)+".snapshot.lz4")
+	key, err := litestream.SnapshotPath(c.Path, generation, index)
+	if err != nil {
+		return info, fmt.Errorf("cannot determine snapshot path: %w", err)
+	}
 	startTime := time.Now()
 
 	rc := internal.NewReadCounter(rd)
@@ -267,11 +270,12 @@ func (c *ReplicaClient) WriteSnapshot(ctx context.Context, generation string, in
 func (c *ReplicaClient) SnapshotReader(ctx context.Context, generation string, index int) (io.ReadCloser, error) {
 	if err := c.Init(ctx); err != nil {
 		return nil, err
-	} else if generation == "" {
-		return nil, fmt.Errorf("generation required")
 	}
 
-	key := path.Join(c.Path, "generations", generation, "snapshots", litestream.FormatIndex(index)+".snapshot.lz4")
+	key, err := litestream.SnapshotPath(c.Path, generation, index)
+	if err != nil {
+		return nil, fmt.Errorf("cannot determine snapshot path: %w", err)
+	}
 
 	out, err := c.s3.GetObjectWithContext(ctx, &s3.GetObjectInput{
 		Bucket: aws.String(c.Bucket),
@@ -292,11 +296,12 @@ func (c *ReplicaClient) SnapshotReader(ctx context.Context, generation string, i
 func (c *ReplicaClient) DeleteSnapshot(ctx context.Context, generation string, index int) error {
 	if err := c.Init(ctx); err != nil {
 		return err
-	} else if generation == "" {
-		return fmt.Errorf("generation required")
 	}
 
-	key := path.Join(c.Path, "generations", generation, "snapshots", litestream.FormatIndex(index)+".snapshot.lz4")
+	key, err := litestream.SnapshotPath(c.Path, generation, index)
+	if err != nil {
+		return fmt.Errorf("cannot determine snapshot path: %w", err)
+	}
 
 	if _, err := c.s3.DeleteObjectsWithContext(ctx, &s3.DeleteObjectsInput{
 		Bucket: aws.String(c.Bucket),
@@ -321,11 +326,12 @@ func (c *ReplicaClient) WALSegments(ctx context.Context, generation string) (lit
 func (c *ReplicaClient) WriteWALSegment(ctx context.Context, pos litestream.Pos, rd io.Reader) (info litestream.WALSegmentInfo, err error) {
 	if err := c.Init(ctx); err != nil {
 		return info, err
-	} else if pos.Generation == "" {
-		return info, fmt.Errorf("generation required")
 	}
 
-	key := path.Join(c.Path, "generations", pos.Generation, "wal", litestream.FormatIndex(pos.Index), litestream.FormatOffset(pos.Offset)+".wal.lz4")
+	key, err := litestream.WALSegmentPath(c.Path, pos.Generation, pos.Index, pos.Offset)
+	if err != nil {
+		return info, fmt.Errorf("cannot determine wal segment path: %w", err)
+	}
 	startTime := time.Now()
 
 	rc := internal.NewReadCounter(rd)
@@ -354,11 +360,12 @@ func (c *ReplicaClient) WriteWALSegment(ctx context.Context, pos litestream.Pos,
 func (c *ReplicaClient) WALSegmentReader(ctx context.Context, pos litestream.Pos) (io.ReadCloser, error) {
 	if err := c.Init(ctx); err != nil {
 		return nil, err
-	} else if pos.Generation == "" {
-		return nil, fmt.Errorf("generation required")
 	}
 
-	key := path.Join(c.Path, "generations", pos.Generation, "wal", litestream.FormatIndex(pos.Index), litestream.FormatOffset(pos.Offset)+".wal.lz4")
+	key, err := litestream.WALSegmentPath(c.Path, pos.Generation, pos.Index, pos.Offset)
+	if err != nil {
+		return nil, fmt.Errorf("cannot determine wal segment path: %w", err)
+	}
 
 	out, err := c.s3.GetObjectWithContext(ctx, &s3.GetObjectInput{
 		Bucket: aws.String(c.Bucket),
@@ -390,10 +397,10 @@ func (c *ReplicaClient) DeleteWALSegments(ctx context.Context, a []litestream.Po
 
 		// Generate a batch of object IDs for deleting the WAL segments.
 		for i, pos := range a[:n] {
-			if pos.Generation == "" {
-				return fmt.Errorf("generation required")
+			key, err := litestream.WALSegmentPath(c.Path, pos.Generation, pos.Index, pos.Offset)
+			if err != nil {
+				return fmt.Errorf("cannot determine wal segment path: %w", err)
 			}
-			key := path.Join(c.Path, "generations", pos.Generation, "wal", litestream.FormatIndex(pos.Index), litestream.FormatOffset(pos.Offset)+".wal.lz4")
 			objIDs[i] = &s3.ObjectIdentifier{Key: &key}
 		}
 
@@ -491,12 +498,11 @@ func newSnapshotIterator(ctx context.Context, client *ReplicaClient, generation
 func (itr *snapshotIterator) fetch() error {
 	defer close(itr.ch)
 
-	if itr.generation == "" {
-		return fmt.Errorf("generation required")
+	dir, err := litestream.SnapshotsPath(itr.client.Path, itr.generation)
+	if err != nil {
+		return fmt.Errorf("cannot determine snapshots path: %w", err)
 	}
 
-	dir := path.Join(itr.client.Path, "generations", itr.generation, "snapshots")
-
 	return itr.client.s3.ListObjectsPagesWithContext(itr.ctx, &s3.ListObjectsInput{
 		Bucket:    aws.String(itr.client.Bucket),
 		Prefix:    aws.String(dir + "/"),
@@ -505,7 +511,8 @@ func (itr *snapshotIterator) fetch() error {
 		internal.OperationTotalCounterVec.WithLabelValues(ReplicaClientType, "LIST").Inc()
 
 		for _, obj := range page.Contents {
-			index, err := internal.ParseSnapshotPath(path.Base(*obj.Key))
+			key := path.Base(*obj.Key)
+			index, err := litestream.ParseSnapshotPath(key)
 			if err != nil {
 				continue
 			}
@@ -594,20 +601,21 @@ func newWALSegmentIterator(ctx context.Context, client *ReplicaClient, generatio
 func (itr *walSegmentIterator) fetch() error {
 	defer close(itr.ch)
 
-	if itr.generation == "" {
-		return fmt.Errorf("generation required")
+	dir, err := litestream.WALPath(itr.client.Path, itr.generation)
+	if err != nil {
+		return fmt.Errorf("cannot determine wal path: %w", err)
 	}
 
-	prefix := path.Join(itr.client.Path, "generations", itr.generation, "wal") + "/"
-
 	return itr.client.s3.ListObjectsPagesWithContext(itr.ctx, &s3.ListObjectsInput{
 		Bucket:    aws.String(itr.client.Bucket),
-		Prefix: aws.String(prefix),
+		Prefix:    aws.String(dir + "/"),
+		Delimiter: aws.String("/"),
 	}, func(page *s3.ListObjectsOutput, lastPage bool) bool {
 		internal.OperationTotalCounterVec.WithLabelValues(ReplicaClientType, "LIST").Inc()
 
 		for _, obj := range page.Contents {
-			index, offset, err := internal.ParseWALSegmentPath(strings.TrimPrefix(*obj.Key, prefix))
+			key := path.Base(*obj.Key)
+			index, offset, err := litestream.ParseWALSegmentPath(key)
 			if err != nil {
 				continue
 			}

sftp/replica_client.go

@@ -9,7 +9,6 @@ import (
 	"os"
 	"path"
 	"sort"
-	"strings"
 	"sync"
 	"time"
 
@@ -122,7 +121,7 @@ func (c *ReplicaClient) Generations(ctx context.Context) (_ []string, err error)
 		return nil, err
 	}
 
-	fis, err := sftpClient.ReadDir(path.Join(c.Path, "generations"))
+	fis, err := sftpClient.ReadDir(litestream.GenerationsPath(c.Path))
 	if os.IsNotExist(err) {
 		return nil, nil
 	} else if err != nil {
@@ -154,11 +153,12 @@ func (c *ReplicaClient) DeleteGeneration(ctx context.Context, generation string)
 	sftpClient, err := c.Init(ctx)
 	if err != nil {
 		return err
-	} else if generation == "" {
-		return fmt.Errorf("generation required")
 	}
 
-	dir := path.Join(c.Path, "generations", generation)
+	dir, err := litestream.GenerationPath(c.Path, generation)
+	if err != nil {
+		return fmt.Errorf("cannot determine generation path: %w", err)
+	}
 
 	var dirs []string
 	walker := sftpClient.Walk(dir)
@@ -198,11 +198,12 @@ func (c *ReplicaClient) Snapshots(ctx context.Context, generation string) (_ lit
 	sftpClient, err := c.Init(ctx)
 	if err != nil {
 		return nil, err
-	} else if generation == "" {
-		return nil, fmt.Errorf("generation required")
 	}
 
-	dir := path.Join(c.Path, "generations", generation, "snapshots")
+	dir, err := litestream.SnapshotsPath(c.Path, generation)
+	if err != nil {
+		return nil, fmt.Errorf("cannot determine snapshots path: %w", err)
+	}
 
 	fis, err := sftpClient.ReadDir(dir)
 	if os.IsNotExist(err) {
@@ -215,7 +216,7 @@ func (c *ReplicaClient) Snapshots(ctx context.Context, generation string) (_ lit
 	infos := make([]litestream.SnapshotInfo, 0, len(fis))
 	for _, fi := range fis {
 		// Parse index from filename.
-		index, err := internal.ParseSnapshotPath(path.Base(fi.Name()))
+		index, err := litestream.ParseSnapshotPath(path.Base(fi.Name()))
 		if err != nil {
 			continue
 		}
@@ -240,11 +241,12 @@ func (c *ReplicaClient) WriteSnapshot(ctx context.Context, generation string, in
 	sftpClient, err := c.Init(ctx)
 	if err != nil {
 		return info, err
-	} else if generation == "" {
-		return info, fmt.Errorf("generation required")
 	}
 
-	filename := path.Join(c.Path, "generations", generation, "snapshots", litestream.FormatIndex(index)+".snapshot.lz4")
+	filename, err := litestream.SnapshotPath(c.Path, generation, index)
+	if err != nil {
+		return info, fmt.Errorf("cannot determine snapshot path: %w", err)
+	}
 	startTime := time.Now()
 
 	if err := sftpClient.MkdirAll(path.Dir(filename)); err != nil {
@@ -284,11 +286,12 @@ func (c *ReplicaClient) SnapshotReader(ctx context.Context, generation string, i
 	sftpClient, err := c.Init(ctx)
 	if err != nil {
 		return nil, err
-	} else if generation == "" {
-		return nil, fmt.Errorf("generation required")
 	}
 
-	filename := path.Join(c.Path, "generations", generation, "snapshots", litestream.FormatIndex(index)+".snapshot.lz4")
+	filename, err := litestream.SnapshotPath(c.Path, generation, index)
+	if err != nil {
+		return nil, fmt.Errorf("cannot determine snapshot path: %w", err)
+	}
 
 	f, err := sftpClient.Open(filename)
 	if err != nil {
@@ -307,11 +310,12 @@ func (c *ReplicaClient) DeleteSnapshot(ctx context.Context, generation string, i
 	sftpClient, err := c.Init(ctx)
 	if err != nil {
 		return err
-	} else if generation == "" {
-		return fmt.Errorf("generation required")
 	}
 
-	filename := path.Join(c.Path, "generations", generation, "snapshots", litestream.FormatIndex(index)+".snapshot.lz4")
+	filename, err := litestream.SnapshotPath(c.Path, generation, index)
+	if err != nil {
+		return fmt.Errorf("cannot determine snapshot path: %w", err)
+	}
 
 	if err := sftpClient.Remove(filename); err != nil && !os.IsNotExist(err) {
 		return fmt.Errorf("cannot delete snapshot %q: %w", filename, err)
@@ -328,11 +332,12 @@ func (c *ReplicaClient) WALSegments(ctx context.Context, generation string) (_ l
 	sftpClient, err := c.Init(ctx)
 	if err != nil {
 		return nil, err
-	} else if generation == "" {
-		return nil, fmt.Errorf("generation required")
 	}
 
-	dir := path.Join(c.Path, "generations", generation, "wal")
+	dir, err := litestream.WALPath(c.Path, generation)
+	if err != nil {
+		return nil, fmt.Errorf("cannot determine wal path: %w", err)
+	}
 
 	fis, err := sftpClient.ReadDir(dir)
 	if os.IsNotExist(err) {
@@ -342,18 +347,25 @@ func (c *ReplicaClient) WALSegments(ctx context.Context, generation string) (_ l
 	}
 
 	// Iterate over every file and convert to metadata.
-	indexes := make([]int, 0, len(fis))
+	infos := make([]litestream.WALSegmentInfo, 0, len(fis))
 	for _, fi := range fis {
-		index, err := litestream.ParseIndex(fi.Name())
-		if err != nil || !fi.IsDir() {
+		index, offset, err := litestream.ParseWALSegmentPath(path.Base(fi.Name()))
+		if err != nil {
 			continue
 		}
-		indexes = append(indexes, index)
+
+		infos = append(infos, litestream.WALSegmentInfo{
+			Generation: generation,
+			Index:      index,
+			Offset:     offset,
+			Size:       fi.Size(),
+			CreatedAt:  fi.ModTime().UTC(),
+		})
 	}
 
-	sort.Ints(indexes)
+	sort.Sort(litestream.WALSegmentInfoSlice(infos))
 
-	return newWALSegmentIterator(ctx, c, dir, generation, indexes), nil
+	return litestream.NewWALSegmentInfoSliceIterator(infos), nil
 }
 
 // WriteWALSegment writes LZ4 compressed data from rd into a file on disk.
@@ -363,11 +375,12 @@ func (c *ReplicaClient) WriteWALSegment(ctx context.Context, pos litestream.Pos,
 	sftpClient, err := c.Init(ctx)
 	if err != nil {
 		return info, err
-	} else if pos.Generation == "" {
-		return info, fmt.Errorf("generation required")
 	}
 
-	filename := path.Join(c.Path, "generations", pos.Generation, "wal", litestream.FormatIndex(pos.Index), litestream.FormatOffset(pos.Offset)+".wal.lz4")
+	filename, err := litestream.WALSegmentPath(c.Path, pos.Generation, pos.Index, pos.Offset)
+	if err != nil {
+		return info, fmt.Errorf("cannot determine wal segment path: %w", err)
+	}
 	startTime := time.Now()
 
 	if err := sftpClient.MkdirAll(path.Dir(filename)); err != nil {
@@ -407,11 +420,12 @@ func (c *ReplicaClient) WALSegmentReader(ctx context.Context, pos litestream.Pos
 	sftpClient, err := c.Init(ctx)
 	if err != nil {
 		return nil, err
-	} else if pos.Generation == "" {
-		return nil, fmt.Errorf("generation required")
 	}
 
-	filename := path.Join(c.Path, "generations", pos.Generation, "wal", litestream.FormatIndex(pos.Index), litestream.FormatOffset(pos.Offset)+".wal.lz4")
+	filename, err := litestream.WALSegmentPath(c.Path, pos.Generation, pos.Index, pos.Offset)
+	if err != nil {
+		return nil, fmt.Errorf("cannot determine wal segment path: %w", err)
+	}
 
 	f, err := sftpClient.Open(filename)
 	if err != nil {
@@ -433,12 +447,11 @@ func (c *ReplicaClient) DeleteWALSegments(ctx context.Context, a []litestream.Po
 	}
 
 	for _, pos := range a {
-		if pos.Generation == "" {
-			return fmt.Errorf("generation required")
+		filename, err := litestream.WALSegmentPath(c.Path, pos.Generation, pos.Index, pos.Offset)
+		if err != nil {
+			return fmt.Errorf("cannot determine wal segment path: %w", err)
 		}
 
-		filename := path.Join(c.Path, "generations", pos.Generation, "wal", litestream.FormatIndex(pos.Index), litestream.FormatOffset(pos.Offset)+".wal.lz4")
-
 		if err := sftpClient.Remove(filename); err != nil && !os.IsNotExist(err) {
 			return fmt.Errorf("cannot delete wal segment %q: %w", filename, err)
 		}
@@ -457,7 +470,7 @@ func (c *ReplicaClient) Cleanup(ctx context.Context) (err error) {
 		return err
 	}
 
-	if err := sftpClient.RemoveDirectory(path.Join(c.Path, "generations")); err != nil && !os.IsNotExist(err) {
+	if err := sftpClient.RemoveDirectory(litestream.GenerationsPath(c.Path)); err != nil && !os.IsNotExist(err) {
 		return fmt.Errorf("cannot delete generations path: %w", err)
 	} else if err := sftpClient.RemoveDirectory(c.Path); err != nil && !os.IsNotExist(err) {
 		return fmt.Errorf("cannot delete path: %w", err)
@@ -480,101 +493,3 @@ func (c *ReplicaClient) resetOnConnError(err error) {
 		c.sshClient = nil
 	}
 }
-
-type walSegmentIterator struct {
-	ctx        context.Context
-	client     *ReplicaClient
-	dir        string
-	generation string
-	indexes    []int
-
-	infos []litestream.WALSegmentInfo
-	err   error
-}
-
-func newWALSegmentIterator(ctx context.Context, client *ReplicaClient, dir, generation string, indexes []int) *walSegmentIterator {
-	return &walSegmentIterator{
-		ctx:        ctx,
-		client:     client,
-		dir:        dir,
-		generation: generation,
-		indexes:    indexes,
-	}
-}
-
-func (itr *walSegmentIterator) Close() (err error) {
-	return itr.err
-}
-
-func (itr *walSegmentIterator) Next() bool {
-	sftpClient, err := itr.client.Init(itr.ctx)
-	if err != nil {
-		itr.err = err
-		return false
-	}
-
-	// Exit if an error has already occurred.
-	if itr.err != nil {
-		return false
-	}
-
-	for {
-		// Move to the next segment in cache, if available.
-		if len(itr.infos) > 1 {
-			itr.infos = itr.infos[1:]
-			return true
-		}
-		itr.infos = itr.infos[:0] // otherwise clear infos
-
-		// Move to the next index unless this is the first time initializing.
-		if itr.infos != nil && len(itr.indexes) > 0 {
-			itr.indexes = itr.indexes[1:]
-		}
-
-		// If no indexes remain, stop iteration.
-		if len(itr.indexes) == 0 {
-			return false
-		}
-
-		// Read segments into a cache for the current index.
-		index := itr.indexes[0]
-		fis, err := sftpClient.ReadDir(path.Join(itr.dir, litestream.FormatIndex(index)))
-		if err != nil {
-			itr.err = err
-			return false
-		}
-
-		for _, fi := range fis {
-			filename := path.Base(fi.Name())
-			if fi.IsDir() {
-				continue
-			}
-
-			offset, err := litestream.ParseOffset(strings.TrimSuffix(filename, ".wal.lz4"))
-			if err != nil {
-				continue
-			}
-
-			itr.infos = append(itr.infos, litestream.WALSegmentInfo{
-				Generation: itr.generation,
-				Index:      index,
-				Offset:     offset,
-				Size:       fi.Size(),
-				CreatedAt:  fi.ModTime().UTC(),
-			})
-		}
-
-		if len(itr.infos) > 0 {
-			return true
-		}
-	}
-}
-
-func (itr *walSegmentIterator) Err() error { return itr.err }
-
-func (itr *walSegmentIterator) WALSegment() litestream.WALSegmentInfo {
-	if len(itr.infos) == 0 {
-		return litestream.WALSegmentInfo{}
-	}
-	return itr.infos[0]
-}