Open snapshot database only once

Prevents having multiple database connections open to a connection,
preventing bugs with locking by relying on the boltdb database
object to handle synchronization rather than file locking.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

snapshot/storage/bolt.go

@@ -30,21 +30,6 @@ var (
 	ErrNoTransaction = errors.New("no transaction in context")
 )
 
-type boltFileTransactor struct {
-	db *bolt.DB
-	tx *bolt.Tx
-}
-
-func (bft *boltFileTransactor) Rollback() error {
-	defer bft.db.Close()
-	return bft.tx.Rollback()
-}
-
-func (bft *boltFileTransactor) Commit() error {
-	defer bft.db.Close()
-	return bft.tx.Commit()
-}
-
 // parentKey returns a composite key of the parent and child identifiers. The
 // parts of the key are separated by a zero byte.
 func parentKey(parent, child uint64) []byte {
@@ -405,11 +390,11 @@ func CommitActive(ctx context.Context, key, name string, usage snapshot.Usage, o
 }
 
 func withSnapshotBucket(ctx context.Context, key string, fn func(context.Context, *bolt.Bucket, *bolt.Bucket) error) error {
-	t, ok := ctx.Value(transactionKey{}).(*boltFileTransactor)
+	tx, ok := ctx.Value(transactionKey{}).(*bolt.Tx)
 	if !ok {
 		return ErrNoTransaction
 	}
-	bkt := t.tx.Bucket(bucketKeyStorageVersion)
+	bkt := tx.Bucket(bucketKeyStorageVersion)
 	if bkt == nil {
 		return errors.Wrap(errdefs.ErrNotFound, "bucket does not exist")
 	}
@@ -426,11 +411,11 @@ func withSnapshotBucket(ctx context.Context, key string, fn func(context.Context
 }
 
 func withBucket(ctx context.Context, fn func(context.Context, *bolt.Bucket, *bolt.Bucket) error) error {
-	t, ok := ctx.Value(transactionKey{}).(*boltFileTransactor)
+	tx, ok := ctx.Value(transactionKey{}).(*bolt.Tx)
 	if !ok {
 		return ErrNoTransaction
 	}
-	bkt := t.tx.Bucket(bucketKeyStorageVersion)
+	bkt := tx.Bucket(bucketKeyStorageVersion)
 	if bkt == nil {
 		return errors.Wrap(errdefs.ErrNotFound, "bucket does not exist")
 	}
@@ -438,12 +423,12 @@ func withBucket(ctx context.Context, fn func(context.Context, *bolt.Bucket, *bol
 }
 
 func createBucketIfNotExists(ctx context.Context, fn func(context.Context, *bolt.Bucket, *bolt.Bucket) error) error {
-	t, ok := ctx.Value(transactionKey{}).(*boltFileTransactor)
+	tx, ok := ctx.Value(transactionKey{}).(*bolt.Tx)
 	if !ok {
 		return ErrNoTransaction
 	}
 
-	bkt, err := t.tx.CreateBucketIfNotExists(bucketKeyStorageVersion)
+	bkt, err := tx.CreateBucketIfNotExists(bucketKeyStorageVersion)
 	if err != nil {
 		return errors.Wrap(err, "failed to create version bucket")
 	}

snapshot/storage/metastore.go

@@ -7,6 +7,7 @@ package storage
 
 import (
 	"context"
+	"sync"
 
 	"github.com/boltdb/bolt"
 	"github.com/containerd/containerd/snapshot"
@@ -46,6 +47,9 @@ type Snapshot struct {
 // complexities of a driver implementation.
 type MetaStore struct {
 	dbfile string
+
+	dbL sync.Mutex
+	db  *bolt.DB
 }
 
 // NewMetaStore returns a snapshot MetaStore for storage of metadata related to
@@ -63,22 +67,33 @@ type transactionKey struct{}
 // TransactionContext creates a new transaction context. The writable value
 // should be set to true for transactions which are expected to mutate data.
 func (ms *MetaStore) TransactionContext(ctx context.Context, writable bool) (context.Context, Transactor, error) {
-	db, err := bolt.Open(ms.dbfile, 0600, nil)
-	if err != nil {
-		return ctx, nil, errors.Wrap(err, "failed to open database file")
+	ms.dbL.Lock()
+	if ms.db == nil {
+		db, err := bolt.Open(ms.dbfile, 0600, nil)
+		if err != nil {
+			ms.dbL.Unlock()
+			return ctx, nil, errors.Wrap(err, "failed to open database file")
+		}
+		ms.db = db
 	}
+	ms.dbL.Unlock()
 
-	tx, err := db.Begin(writable)
+	tx, err := ms.db.Begin(writable)
 	if err != nil {
 		return ctx, nil, errors.Wrap(err, "failed to start transaction")
 	}
 
-	t := &boltFileTransactor{
-		db: db,
-		tx: tx,
-	}
+	ctx = context.WithValue(ctx, transactionKey{}, tx)
 
-	ctx = context.WithValue(ctx, transactionKey{}, t)
-
-	return ctx, t, nil
+	return ctx, tx, nil
+}
+
+// Close closes the metastore and any underlying database connections
+func (ms *MetaStore) Close() error {
+	ms.dbL.Lock()
+	defer ms.dbL.Unlock()
+	if ms.db == nil {
+		return nil
+	}
+	return ms.db.Close()
 }