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

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Remove SQLite write lock during WAL sync
This commit is contained in:
Ben Johnson
2021-03-06 08:08:23 -07:00
committed by GitHub
parent b2ca113fb5 998e831c5c
commit 9341863bdb
1 changed files with 20 additions and 28 deletions
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48
db.go
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@@ -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
}