Have pathInode.Truncate and friends use the file handle, if available

If the file handle is specified we should use it instead of
picking one at random.

Note that the only real change is for ftruncate. For fstat, fchown
and fchmod the kernel currently never passes a file descriptor.

Fixes #95.

fuse/pathfs/pathfs.go

@@ -605,7 +605,9 @@ func (n *pathInode) findChild(fi *fuse.Attr, name string, fullPath string) (out
 func (n *pathInode) GetAttr(out *fuse.Attr, file nodefs.File, context *fuse.Context) (code fuse.Status) {
 	var fi *fuse.Attr
 	if file == nil {
-		// called on a deleted files.
+		// Linux currently (tested on v4.4) does not pass a file descriptor for
+		// fstat. To be able to stat a deleted file we have to find ourselves
+		// an open fd.
 		file = n.Inode().AnyFile()
 	}
 
@@ -632,6 +634,15 @@ func (n *pathInode) GetAttr(out *fuse.Attr, file nodefs.File, context *fuse.Cont
 }
 
 func (n *pathInode) Chmod(file nodefs.File, perms uint32, context *fuse.Context) (code fuse.Status) {
+	// Note that Linux currently (Linux 4.4) DOES NOT pass a file descriptor
+	// to FUSE for fchmod. We still check because that may change in the future.
+	if file != nil {
+		code = file.Chmod(perms)
+		if code != fuse.ENOSYS {
+			return code
+		}
+	}
+
 	files := n.Inode().Files(fuse.O_ANYWRITE)
 	for _, f := range files {
 		// TODO - pass context
@@ -648,6 +659,15 @@ func (n *pathInode) Chmod(file nodefs.File, perms uint32, context *fuse.Context)
 }
 
 func (n *pathInode) Chown(file nodefs.File, uid uint32, gid uint32, context *fuse.Context) (code fuse.Status) {
+	// Note that Linux currently (Linux 4.4) DOES NOT pass a file descriptor
+	// to FUSE for fchown. We still check because it may change in the future.
+	if file != nil {
+		code = file.Chown(uid, gid)
+		if code != fuse.ENOSYS {
+			return code
+		}
+	}
+
 	files := n.Inode().Files(fuse.O_ANYWRITE)
 	for _, f := range files {
 		// TODO - pass context
@@ -664,6 +684,15 @@ func (n *pathInode) Chown(file nodefs.File, uid uint32, gid uint32, context *fus
 }
 
 func (n *pathInode) Truncate(file nodefs.File, size uint64, context *fuse.Context) (code fuse.Status) {
+	// A file descriptor was passed in AND the filesystem implements the
+	// operation on the file handle. This the common case for ftruncate.
+	if file != nil {
+		code = file.Truncate(size)
+		if code != fuse.ENOSYS {
+			return code
+		}
+	}
+
 	files := n.Inode().Files(fuse.O_ANYWRITE)
 	for _, f := range files {
 		// TODO - pass context
@@ -679,6 +708,15 @@ func (n *pathInode) Truncate(file nodefs.File, size uint64, context *fuse.Contex
 }
 
 func (n *pathInode) Utimens(file nodefs.File, atime *time.Time, mtime *time.Time, context *fuse.Context) (code fuse.Status) {
+	// Note that Linux currently (Linux 4.4) DOES NOT pass a file descriptor
+	// to FUSE for futimens. We still check because it may change in the future.
+	if file != nil {
+		code = file.Utimens(atime, mtime)
+		if code != fuse.ENOSYS {
+			return code
+		}
+	}
+
 	files := n.Inode().Files(fuse.O_ANYWRITE)
 	for _, f := range files {
 		// TODO - pass context