fuse: do not print warning for ENOENT on RELEASE

This avoids spurious error messages when unmounting a file system
directly after closing a file.

Fixes #505

Change-Id: I3a6a6ada0b5e5abe95a1d7965a48ddb46664310a

fuse/server.go

@@ -529,15 +529,22 @@ func (ms *Server) handleRequest(req *request) Status {
 
 	errNo := ms.write(req)
 	if errNo != 0 {
-		// Unless debugging is enabled, ignore ENOENT for INTERRUPT responses
-		// which indicates that the referred request is no longer known by the
-		// kernel. This is a normal if the referred request already has
-		// completed.
-		if ms.opts.Debug || !(req.inHeader.Opcode == _OP_INTERRUPT && errNo == ENOENT) {
+		// Ignore ENOENT for INTERRUPT responses which
+		// indicates that the referred request is no longer
+		// known by the kernel. This is a normal if the
+		// referred request already has completed.
+		//
+		// Ignore ENOENT for RELEASE responses.  When the FS
+		// is unmounted directly after a file close, the
+		// device can go away while we are still processing
+		// RELEASE. This is because RELEASE is analogous to
+		// FORGET, and is not synchronized with the calling
+		// process, but does require a response.
+		if ms.opts.Debug || !(errNo == ENOENT && (req.inHeader.Opcode == _OP_INTERRUPT ||
+			req.inHeader.Opcode == _OP_RELEASE)) {
 			ms.opts.Logger.Printf("writer: Write/Writev failed, err: %v. opcode: %v",
 				errNo, operationName(req.inHeader.Opcode))
 		}
-
 	}
 	ms.returnRequest(req)
 	return Status(errNo)