Merge pull request #104 from benbjohnson/remove-sync-lock

Remove SQLite write lock during WAL sync

db.go

@@ -733,27 +733,6 @@ func (db *DB) Sync() (err error) {
 		return fmt.Errorf("ensure wal exists: %w", err)
 	}
 
-	// Start a transaction. This will be promoted immediately after.
-	tx, err := db.db.Begin()
-	if err != nil {
-		return fmt.Errorf("begin: %w", err)
-	}
-
-	// Ensure write transaction rolls back before returning.
-	defer func() {
-		if e := rollback(tx); e != nil && err == nil {
-			err = e
-		}
-	}()
-
-	// Insert into the lock table to promote to a write tx. The lock table
-	// insert will never actually occur because our tx will be rolled back,
-	// however, it will ensure our tx grabs the write lock. Unfortunately,
-	// we can't call "BEGIN IMMEDIATE" as we are already in a transaction.
-	if _, err := tx.ExecContext(db.ctx, `INSERT INTO _litestream_lock (id) VALUES (1);`); err != nil {
-		return fmt.Errorf("_litestream_lock: %w", err)
-	}
-
 	// Verify our last sync matches the current state of the WAL.
 	// This ensures that we have an existing generation & that the last sync
 	// position of the real WAL hasn't been overwritten by another process.
@@ -800,11 +779,6 @@ func (db *DB) Sync() (err error) {
 		checkpoint = true
 	}
 
-	// Release write lock before checkpointing & exiting.
-	if err := tx.Rollback(); err != nil {
-		return fmt.Errorf("rollback write tx: %w", err)
-	}
-
 	// Issue the checkpoint.
 	if checkpoint {
 		changed = true
@@ -875,7 +849,7 @@ func (db *DB) verify() (info syncInfo, err error) {
 	if err != nil {
 		return info, err
 	}
-	info.walSize = fi.Size()
+	info.walSize = frameAlign(fi.Size(), db.pageSize)
 	info.walModTime = fi.ModTime()
 	db.walSizeGauge.Set(float64(fi.Size()))
 
@@ -899,7 +873,6 @@ func (db *DB) verify() (info syncInfo, err error) {
 	}
 	info.shadowWALSize = frameAlign(fi.Size(), db.pageSize)
 
-	// Truncate shadow WAL if there is a partial page.
 	// Exit if shadow WAL does not contain a full header.
 	if info.shadowWALSize < WALHeaderSize {
 		info.reason = "short shadow wal"
@@ -1353,6 +1326,21 @@ func (db *DB) checkpointAndInit(generation, mode string) error {
 		return nil
 	}
 
+	// Start a transaction. This will be promoted immediately after.
+	tx, err := db.db.Begin()
+	if err != nil {
+		return fmt.Errorf("begin: %w", err)
+	}
+	defer func() { _ = rollback(tx) }()
+
+	// Insert into the lock table to promote to a write tx. The lock table
+	// insert will never actually occur because our tx will be rolled back,
+	// however, it will ensure our tx grabs the write lock. Unfortunately,
+	// we can't call "BEGIN IMMEDIATE" as we are already in a transaction.
+	if _, err := tx.ExecContext(db.ctx, `INSERT INTO _litestream_lock (id) VALUES (1);`); err != nil {
+		return fmt.Errorf("_litestream_lock: %w", err)
+	}
+
 	// Copy the end of the previous WAL before starting a new shadow WAL.
 	if _, err := db.copyToShadowWAL(shadowWALPath); err != nil {
 		return fmt.Errorf("cannot copy to end of shadow wal: %w", err)
@@ -1370,6 +1358,10 @@ func (db *DB) checkpointAndInit(generation, mode string) error {
 		return fmt.Errorf("cannot init shadow wal file: name=%s err=%w", newShadowWALPath, err)
 	}
 
+	// Release write lock before checkpointing & exiting.
+	if err := tx.Rollback(); err != nil {
+		return fmt.Errorf("rollback post-checkpoint tx: %w", err)
+	}
 	return nil
 }