Fix potential BSD locking race. (#98)

Author: ncruces

vfs/const.go

@@ -51,6 +51,7 @@ const (
 	_IOERR_BEGIN_ATOMIC      _ErrorCode = util.IOERR_BEGIN_ATOMIC
 	_IOERR_COMMIT_ATOMIC     _ErrorCode = util.IOERR_COMMIT_ATOMIC
 	_IOERR_ROLLBACK_ATOMIC   _ErrorCode = util.IOERR_ROLLBACK_ATOMIC
+	_BUSY_SNAPSHOT           _ErrorCode = util.BUSY_SNAPSHOT
 	_CANTOPEN_FULLPATH       _ErrorCode = util.CANTOPEN_FULLPATH
 	_CANTOPEN_ISDIR          _ErrorCode = util.CANTOPEN_ISDIR
 	_READONLY_CANTINIT       _ErrorCode = util.READONLY_CANTINIT

vfs/os_bsd.go

@@ -29,5 +29,12 @@ func osReadLock(file *os.File, _ /*start*/, _ /*len*/ int64, _ /*timeout*/ time.
 }
 
 func osWriteLock(file *os.File, _ /*start*/, _ /*len*/ int64, _ /*timeout*/ time.Duration) _ErrorCode {
-	return osLock(file, unix.LOCK_EX|unix.LOCK_NB, _IOERR_LOCK)
+	rc := osLock(file, unix.LOCK_EX|unix.LOCK_NB, _IOERR_LOCK)
+	if rc == _BUSY {
+		// The documentation states the lock is upgraded by releasing the previous lock,
+		// then acquiring the new lock.
+		// This is a race, so return BUSY_SNAPSHOT to ensure the transaction is aborted.
+		return _BUSY_SNAPSHOT
+	}
+	return rc
 }

vfs/os_unix_lock.go

@@ -48,7 +48,7 @@ func osDowngradeLock(file *os.File, state LockLevel) _ErrorCode {
 			// In theory, the downgrade to a SHARED cannot fail because another
 			// process is holding an incompatible lock. If it does, this
 			// indicates that the other process is not following the locking
-			// protocol. If this happens, return _IOERR_RDLOCK. Returning
+			// protocol. If this happens, return IOERR_RDLOCK. Returning
 			// BUSY would confuse the upper layer.
 			return _IOERR_RDLOCK
 		}

vfs/shm_bsd.go

@@ -128,10 +128,11 @@ func (s *vfsShm) shmOpen() (rc _ErrorCode) {
 	}
 
 	// Lock and truncate the file, if not readonly.
+	// The lock is only released by closing the file.
 	if s.readOnly {
 		rc = _READONLY_CANTINIT
 	} else {
-		if rc := osWriteLock(f, 0, 0, 0); rc != _OK {
+		if rc := osLock(f, unix.LOCK_EX|unix.LOCK_NB, _IOERR_LOCK); rc != _OK {
 			return rc
 		}
 		if err := f.Truncate(0); err != nil {