mmm: use flock instead of file presence for locking the database.

eventstore/mmm/mmmm.go

@@ -35,6 +35,7 @@ type MultiMmapManager struct {
 	mmapfEnd  uint64
 
 	writeMutex sync.Mutex
+	lockfile   *os.File
 
 	lmdbEnv     *lmdb.Env
 	stuff       lmdb.DBI
@@ -55,28 +56,38 @@ const (
 	maxuint32          = 4294967295
 )
 
-func (b *MultiMmapManager) Init() error {
+func (b *MultiMmapManager) Init() (err error) {
 	if b.Logger == nil {
 		nopLogger := zerolog.Nop()
 		b.Logger = &nopLogger
 	}
 
+	defer func() {
+		if err != nil {
+			b.releaseLock()
+		}
+	}()
+
 	// create directory if it doesn't exist
 	dbpath := filepath.Join(b.Dir, "mmmm")
 	if err := os.MkdirAll(dbpath, 0755); err != nil {
 		return fmt.Errorf("failed to create directory %s: %w", dbpath, err)
 	}
 
-	if !b.ReadOnly {
-		// create lockfile to prevent multiple instances
-		lockfilePath := filepath.Join(b.Dir, "mmmm.lock")
-		if _, err := os.OpenFile(lockfilePath, os.O_CREATE|os.O_EXCL|os.O_RDWR, 0644); err != nil {
-			if os.IsExist(err) {
-				return fmt.Errorf("database at %s is already in use by another instance", b.Dir)
-			}
-			return fmt.Errorf("failed to create lockfile %s: %w", lockfilePath, err)
-		}
+	// lock database directory to prevent multiple instances
+	lockfilePath := filepath.Join(b.Dir, "mmmm.lock")
+	lockfile, err := os.OpenFile(lockfilePath, os.O_CREATE|os.O_RDWR, 0644)
+	if err != nil {
+		return fmt.Errorf("failed to open lockfile %s: %w", lockfilePath, err)
 	}
+	if err := syscall.Flock(int(lockfile.Fd()), syscall.LOCK_EX|syscall.LOCK_NB); err != nil {
+		lockfile.Close()
+		if errors.Is(err, syscall.EWOULDBLOCK) || errors.Is(err, syscall.EAGAIN) {
+			return fmt.Errorf("database at %s is already in use by another instance", b.Dir)
+		}
+		return fmt.Errorf("failed to lock database at %s: %w", b.Dir, err)
+	}
+	b.lockfile = lockfile
 
 	// open a huge mmapped file
 	b.mmapfPath = filepath.Join(b.Dir, "events")
@@ -365,6 +376,19 @@ func (b *MultiMmapManager) getNextAvailableLayerId(txn *lmdb.Txn) (uint16, error
 	return id, nil
 }
 
+func (b *MultiMmapManager) releaseLock() {
+	if b.lockfile == nil {
+		return
+	}
+
+	_ = syscall.Flock(int(b.lockfile.Fd()), syscall.LOCK_UN)
+	_ = b.lockfile.Close()
+	b.lockfile = nil
+
+	lockfilePath := filepath.Join(b.Dir, "mmmm.lock")
+	_ = os.Remove(lockfilePath)
+}
+
 func (b *MultiMmapManager) Close() {
 	b.lmdbEnv.Close()
 	for _, il := range b.layers {
@@ -373,7 +397,5 @@ func (b *MultiMmapManager) Close() {
 
 	syscall.Munmap(b.mmapf)
 
-	// remove lockfile
-	lockfilePath := filepath.Join(b.Dir, "mmmm.lock")
-	os.Remove(lockfilePath)
+	b.releaseLock()
 }