Fix restore -if-replica-exists with multiple replicas

cmd/litestream/restore.go

@@ -111,8 +111,16 @@ func (c *RestoreCommand) Run(ctx context.Context, args []string) (err error) {
 
 	// Build replica from either a URL or config.
 	r, err := c.loadReplica(ctx, config, pathOrURL)
-	if err != nil {
+	if err == litestream.ErrNoGeneration {
-		return err
+		// Return an error if no replicas can be loaded to restore from.
+		// If optional flag set, return success. Useful for automated recovery.
+		if c.ifReplicaExists {
+			fmt.Fprintln(c.stdout, "no replicas have generations to restore from, skipping")
+			return nil
+		}
+		return fmt.Errorf("no replicas have generations to restore from")
+	} else if err != nil {
+		return fmt.Errorf("cannot determine latest replica: %w", err)
 	}
 
 	// Determine latest generation if one is not specified.
@@ -218,11 +226,7 @@ func (c *RestoreCommand) loadReplicaFromConfig(ctx context.Context, config Confi
 	}
 
 	// Determine latest replica to restore from.
-	r, err := litestream.LatestReplica(ctx, db.Replicas)
+	return litestream.LatestReplica(ctx, db.Replicas)
-	if err != nil {
-		return nil, fmt.Errorf("cannot determine latest replica: %w", err)
-	}
-	return r, nil
 }
 
 // Usage prints the help screen to STDOUT.

cmd/litestream/restore_test.go

@@ -138,6 +138,19 @@ func TestRestoreCommand(t *testing.T) {
 		}
 	})
 
+	t.Run("IfReplicaExists/Multiple", func(t *testing.T) {
+		testDir := filepath.Join("testdata", "restore", "if-replica-exists-flag-multiple")
+		defer testingutil.Setenv(t, "LITESTREAM_TESTDIR", testDir)()
+
+		m, _, stdout, _ := newMain()
+		err := m.Run(context.Background(), []string{"restore", "-config", filepath.Join(testDir, "litestream.yml"), "-if-replica-exists", filepath.Join(testDir, "db")})
+		if err != nil {
+			t.Fatal(err)
+		} else if got, want := stdout.String(), string(testingutil.ReadFile(t, filepath.Join(testDir, "stdout"))); got != want {
+			t.Fatalf("stdout=%q, want %q", got, want)
+		}
+	})
+
 	t.Run("ErrNoBackups", func(t *testing.T) {
 		testDir := filepath.Join("testdata", "restore", "no-backups")
 		defer testingutil.Setenv(t, "LITESTREAM_TESTDIR", testDir)()

cmd/litestream/testdata/restore/if-replica-exists-flag-multiple/litestream.yml

@@ -0,0 +1,5 @@
+dbs:
+  - path: $LITESTREAM_TESTDIR/db
+    replicas:
+      - path: $LITESTREAM_TESTDIR/replica0
+      - path: $LITESTREAM_TESTDIR/replica1

cmd/litestream/testdata/restore/if-replica-exists-flag-multiple/stdout

@@ -0,0 +1 @@
+no replicas have generations to restore from, skipping