vfs: fix crash reading the metadata of a file which is being written

With --vfs-metadata-extension set, looking up the metadata file of a
file which was open for write and not yet uploaded caused a nil pointer
panic. Such a file has no object to read the modification time from.

Use the modification time of the VFS node instead, which is valid
whether or not the file has been uploaded.

vfs/dir.go

@@ -845,7 +845,7 @@ func (d *Dir) statMetadata(leaf, baseLeaf string) (metaNode Node, err error) {
 	}
 	// Make a memory based file with metadataDump in
 	remote := path.Join(d.path, leaf)
-	o := object.NewMemoryObject(remote, entry.ModTime(d.vfs.ctx), metadataDump)
+	o := object.NewMemoryObject(remote, node.ModTime(), metadataDump)
 	f := newFile(d, d.path, o, leaf)
 	// Base the metadata inode number off the real file inode number
 	// to keep it constant

vfs/dir_test.go

@@ -762,4 +762,16 @@ func TestDirMetadataExtension(t *testing.T) {
 	if features.ReadDirMetadata {
 		assert.Equal(t, modTime.Format(time.RFC3339Nano), metadata["mtime"])
 	}
+
+	// Check metadata for a file being written, which has no object
+	// yet, exists and is empty
+	fd, err := vfs.OpenFile("dir/newfile", os.O_WRONLY|os.O_CREATE, 0666)
+	require.NoError(t, err)
+	node, err = vfs.Stat("dir/newfile.metadata")
+	require.NoError(t, err)
+	require.True(t, node.IsFile())
+	blob, err = vfs.ReadFile("dir/newfile.metadata")
+	require.NoError(t, err)
+	assert.Equal(t, "{}", string(blob))
+	require.NoError(t, fd.Close())
 }