mount: add Options{RootStableAttr} field

- provides an optional way to set e.g. Ino and/or Gen for
  the root directory when calling fs.Mount()
- provides unit test cases
- partial fix for #399

Change-Id: If45a0ec3c0ea06c0419913e34b3415808f9349da

fs/api.go

@@ -610,4 +610,8 @@ type Options struct {
 	// return error, but want to signal something seems off
 	// anyway. If unset, no messages are printed.
 	Logger *log.Logger
+
+	// RootStableAttr is an optional way to set e.g. Ino and/or Gen for
+	// the root directory when calling fs.Mount(), Mode is ignored.
+	RootStableAttr *StableAttr
 }

fs/bridge.go

@@ -283,11 +283,17 @@ func NewNodeFS(root InodeEmbedder, opts *Options) fuse.RawFileSystem {
 		bridge.options.AttrTimeout = &oneSec
 	}
 
+	stableAttr := StableAttr{
+		Ino:  root.embed().StableAttr().Ino,
+		Mode: fuse.S_IFDIR,
+	}
+	if opts.RootStableAttr != nil {
+		stableAttr.Ino = opts.RootStableAttr.Ino
+		stableAttr.Gen = opts.RootStableAttr.Gen
+	}
+
 	initInode(root.embed(), root,
-		StableAttr{
-			Ino:  root.embed().StableAttr().Ino,
-			Mode: fuse.S_IFDIR,
-		},
+		stableAttr,
 		bridge,
 		false,
 		1,

fs/mem_test.go

@@ -70,6 +70,26 @@ func TestDefaultOwner(t *testing.T) {
 	}
 }
 
+func TestRootInode(t *testing.T) {
+	var rootIno uint64 = 42
+	root := &Inode{}
+
+	mntDir, _, clean := testMount(t, root, &Options{
+		RootStableAttr: &StableAttr{
+			Ino: rootIno,
+			Gen: 1,
+		},
+	})
+	defer clean()
+
+	var st syscall.Stat_t
+	if err := syscall.Lstat(mntDir, &st); err != nil {
+		t.Fatalf("Lstat: %v", err)
+	} else if st.Ino != rootIno {
+		t.Fatalf("Got Lstat inode %d, want %d", st.Ino, rootIno)
+	}
+}
+
 func TestDataFile(t *testing.T) {
 	want := "hello"
 	root := &Inode{}