lib/db: Rework flush hooks (#6838)

lib/db/backend/backend.go

@@ -15,6 +15,8 @@ import (
 	"github.com/syncthing/syncthing/lib/locations"
 )
 
+type CommitHook func(WriteTransaction) error
+
 // The Reader interface specifies the read-only operations available on the
 // main database and on read-only transactions (snapshots). Note that when
 // called directly on the database handle these operations may take implicit
@@ -61,7 +63,7 @@ type ReadTransaction interface {
 type WriteTransaction interface {
 	ReadTransaction
 	Writer
-	Checkpoint(...func() error) error
+	Checkpoint() error
 	Commit() error
 }
 
@@ -108,7 +110,7 @@ type Backend interface {
 	Reader
 	Writer
 	NewReadTransaction() (ReadTransaction, error)
-	NewWriteTransaction() (WriteTransaction, error)
+	NewWriteTransaction(hooks ...CommitHook) (WriteTransaction, error)
 	Close() error
 	Compact() error
 }

lib/db/backend/badger_backend.go

@@ -69,7 +69,7 @@ func (b *badgerBackend) NewReadTransaction() (ReadTransaction, error) {
 	}, nil
 }
 
-func (b *badgerBackend) NewWriteTransaction() (WriteTransaction, error) {
+func (b *badgerBackend) NewWriteTransaction(hooks ...CommitHook) (WriteTransaction, error) {
 	rel1, err := newReleaser(b.closeWG)
 	if err != nil {
 		return nil, err
@@ -90,9 +90,10 @@ func (b *badgerBackend) NewWriteTransaction() (WriteTransaction, error) {
 			txn: rtxn,
 			rel: rel1,
 		},
-		txn: wtxn,
-		bdb: b.bdb,
-		rel: rel2,
+		txn:         wtxn,
+		bdb:         b.bdb,
+		rel:         rel2,
+		commitHooks: hooks,
 	}, nil
 }
 
@@ -249,10 +250,11 @@ func (l badgerSnapshot) Release() {
 
 type badgerTransaction struct {
 	badgerSnapshot
-	txn  *badger.Txn
-	bdb  *badger.DB
-	rel  *releaser
-	size int
+	txn         *badger.Txn
+	bdb         *badger.DB
+	rel         *releaser
+	size        int
+	commitHooks []CommitHook
 }
 
 func (t *badgerTransaction) Delete(key []byte) error {
@@ -295,15 +297,20 @@ func (t *badgerTransaction) transactionRetried(fn func(*badger.Txn) error) error
 func (t *badgerTransaction) Commit() error {
 	defer t.rel.Release()
 	defer t.badgerSnapshot.Release()
+	for _, hook := range t.commitHooks {
+		if err := hook(t); err != nil {
+			return err
+		}
+	}
 	return wrapBadgerErr(t.txn.Commit())
 }
 
-func (t *badgerTransaction) Checkpoint(preFlush ...func() error) error {
+func (t *badgerTransaction) Checkpoint() error {
 	if t.size < checkpointFlushMinSize {
 		return nil
 	}
-	for _, hook := range preFlush {
-		if err := hook(); err != nil {
+	for _, hook := range t.commitHooks {
+		if err := hook(t); err != nil {
 			return err
 		}
 	}

lib/db/backend/leveldb_backend.go

@@ -59,7 +59,7 @@ func (b *leveldbBackend) newSnapshot() (leveldbSnapshot, error) {
 	}, nil
 }
 
-func (b *leveldbBackend) NewWriteTransaction() (WriteTransaction, error) {
+func (b *leveldbBackend) NewWriteTransaction(hooks ...CommitHook) (WriteTransaction, error) {
 	rel, err := newReleaser(b.closeWG)
 	if err != nil {
 		return nil, err
@@ -74,6 +74,7 @@ func (b *leveldbBackend) NewWriteTransaction() (WriteTransaction, error) {
 		ldb:             b.ldb,
 		batch:           new(leveldb.Batch),
 		rel:             rel,
+		commitHooks:     hooks,
 	}, nil
 }
 
@@ -142,9 +143,10 @@ func (l leveldbSnapshot) Release() {
 // an actual leveldb transaction)
 type leveldbTransaction struct {
 	leveldbSnapshot
-	ldb   *leveldb.DB
-	batch *leveldb.Batch
-	rel   *releaser
+	ldb         *leveldb.DB
+	batch       *leveldb.Batch
+	rel         *releaser
+	commitHooks []CommitHook
 }
 
 func (t *leveldbTransaction) Delete(key []byte) error {
@@ -157,8 +159,8 @@ func (t *leveldbTransaction) Put(key, val []byte) error {
 	return t.checkFlush(dbFlushBatchMax)
 }
 
-func (t *leveldbTransaction) Checkpoint(preFlush ...func() error) error {
-	return t.checkFlush(dbFlushBatchMin, preFlush...)
+func (t *leveldbTransaction) Checkpoint() error {
+	return t.checkFlush(dbFlushBatchMin)
 }
 
 func (t *leveldbTransaction) Commit() error {
@@ -174,19 +176,19 @@ func (t *leveldbTransaction) Release() {
 }
 
 // checkFlush flushes and resets the batch if its size exceeds the given size.
-func (t *leveldbTransaction) checkFlush(size int, preFlush ...func() error) error {
+func (t *leveldbTransaction) checkFlush(size int) error {
 	if len(t.batch.Dump()) < size {
 		return nil
 	}
-	for _, hook := range preFlush {
-		if err := hook(); err != nil {
-			return err
-		}
-	}
 	return t.flush()
 }
 
 func (t *leveldbTransaction) flush() error {
+	for _, hook := range t.commitHooks {
+		if err := hook(t); err != nil {
+			return err
+		}
+	}
 	if t.batch.Len() == 0 {
 		return nil
 	}