fs: document FileHandle argument for Getattr

Change-Id: I48236bf879939537944e617baa620884e7601055

fs/api.go

@@ -225,7 +225,10 @@ type NodeAccesser interface {
 // returning zeroed permissions, the default behavior is to change the
 // mode of 0755 (directory) or 0644 (files). This can be switched off
 // with the Options.NullPermissions setting. If blksize is unset, 4096
-// is assumed, and the 'blocks' field is set accordingly.
+// is assumed, and the 'blocks' field is set accordingly. The 'f'
+// argument is provided for consistency, however, in practice the
+// kernel never sends a file handle, even if the Getattr call
+// originated from a fstat system call.
 type NodeGetattrer interface {
 	Getattr(ctx context.Context, f FileHandle, out *fuse.AttrOut) syscall.Errno
 }

fs/bridge.go

@@ -548,15 +548,10 @@ func (b *rawBridge) GetAttr(cancel <-chan struct{}, input *fuse.GetAttrIn, out *
 func (b *rawBridge) getattr(ctx context.Context, n *Inode, f FileHandle, out *fuse.AttrOut) syscall.Errno {
 	var errno syscall.Errno
 
-	var fg FileGetattrer
-	if f != nil {
-		fg, _ = f.(FileGetattrer)
-	}
-
-	if fops, ok := n.ops.(NodeGetattrer); ok {
-		errno = fops.Getattr(ctx, f, out)
-	} else if fg != nil {
-		errno = fg.Getattr(ctx, out)
+	if nodeOps, ok := n.ops.(NodeGetattrer); ok {
+		errno = nodeOps.Getattr(ctx, f, out)
+	} else if fileOps, ok := f.(FileGetattrer); ok {
+		errno = fileOps.Getattr(ctx, out)
 	} else {
 		// We set Mode below, which is the minimum for success
 	}