fs: return OK for unimplemented Flush

This simplifies writing R/O filesystems, which do not need flushing.

Change-Id: I428627799904e30f486ff82bcafe2f9c4db88d01

fs/api.go

@@ -296,10 +296,11 @@ type NodeFsyncer interface {
 	Fsync(ctx context.Context, f FileHandle, flags uint32) syscall.Errno
 }
 
-// Flush is called for close() call on a file descriptor. In
-// case of duplicated descriptor, it may be called more than
-// once for a file.   The default implementation forwards to the
-// FileHandle.
+// Flush is called for the close(2) call on a file descriptor. In case
+// of a descriptor that was duplicated using dup(2), it may be called
+// more than once for the same FileHandle.  The default implementation
+// forwards to the FileHandle, or if the handle does not support
+// FileFlusher, returns OK.
 type NodeFlusher interface {
 	Flush(ctx context.Context, f FileHandle) syscall.Errno
 }

fs/bridge.go

@@ -743,8 +743,7 @@ func (b *rawBridge) Flush(cancel <-chan struct{}, input *fuse.FlushIn) fuse.Stat
 	if fl, ok := f.file.(FileFlusher); ok {
 		return errnoToStatus(fl.Flush(&fuse.Context{Caller: input.Caller, Cancel: cancel}))
 	}
-	// XXX should return OK to reflect r/o filesystem?
-	return fuse.ENOTSUP
+	return 0
 }
 
 func (b *rawBridge) Fsync(cancel <-chan struct{}, input *fuse.FsyncIn) fuse.Status {