vfs: fix hard link names in String()

This change fixes a bug in MemFS dump printer, as well as an incorrect
"logical" ownwership structure.

Files that are hard linked to the same node incorrectly print the same
file name (matching the name of the file which created the node).

This is due to the fact that we store the file name in memNode. Instead,
we should allow the same memNode to be used under different names. To
fix it, this commit moves the name field from memNode to memFile.

vfs/mem_fs.go

@@ -110,7 +110,7 @@ func (y *MemFS) String() string {
 	defer y.mu.Unlock()
 
 	s := new(bytes.Buffer)
-	y.root.dump(s, 0)
+	y.root.dump(s, 0, sep)
 	return s.String()
 }
 
@@ -209,9 +209,10 @@ func (y *MemFS) Create(fullname string) (File, error) {
 			if frag == "" {
 				return errors.New("pebble/vfs: empty file name")
 			}
-			n := &memNode{name: frag}
+			n := &memNode{}
 			dir.children[frag] = n
 			ret = &memFile{
+				name:  frag,
 				n:     n,
 				fs:    y,
 				read:  true,
@@ -275,13 +276,15 @@ func (y *MemFS) open(fullname string, openForWrite bool) (File, error) {
 		if final {
 			if frag == "" {
 				ret = &memFile{
-					n:  dir,
-					fs: y,
+					name: sep, // this is the root directory
+					n:    dir,
+					fs:   y,
 				}
 				return nil
 			}
 			if n := dir.children[frag]; n != nil {
 				ret = &memFile{
+					name:  frag,
 					n:     n,
 					fs:    y,
 					read:  true,
@@ -406,7 +409,6 @@ func (y *MemFS) Rename(oldname, newname string) error {
 				return errors.New("pebble/vfs: empty file name")
 			}
 			dir.children[frag] = n
-			n.name = frag
 		}
 		return nil
 	})
@@ -442,7 +444,6 @@ func (y *MemFS) MkdirAll(dirname string, perm os.FileMode) error {
 		child := dir.children[frag]
 		if child == nil {
 			dir.children[frag] = &memNode{
-				name:     frag,
 				children: make(map[string]*memNode),
 				isDir:    true,
 			}
@@ -551,9 +552,6 @@ func (*MemFS) GetDiskUsage(string) (DiskUsage, error) {
 
 // memNode holds a file's data or a directory's children.
 type memNode struct {
-	// TODO(pav-kv): file name is a property of a file, not of a node. Multiple
-	// files with different names can hard link to the same node.
-	name  string // protected by MemFS.mu
 	isDir bool
 	refs  atomic.Int32
 
@@ -576,13 +574,12 @@ type memNode struct {
 
 func newRootMemNode() *memNode {
 	return &memNode{
-		name:     "/", // set the name to match what file systems do
 		children: make(map[string]*memNode),
 		isDir:    true,
 	}
 }
 
-func (f *memNode) dump(w *bytes.Buffer, level int) {
+func (f *memNode) dump(w *bytes.Buffer, level int, name string) {
 	if f.isDir {
 		w.WriteString("          ")
 	} else {
@@ -593,7 +590,7 @@ func (f *memNode) dump(w *bytes.Buffer, level int) {
 	for i := 0; i < level; i++ {
 		w.WriteString("  ")
 	}
-	w.WriteString(f.name)
+	w.WriteString(name)
 	if !f.isDir {
 		w.WriteByte('\n')
 		return
@@ -608,7 +605,7 @@ func (f *memNode) dump(w *bytes.Buffer, level int) {
 	}
 	sort.Strings(names)
 	for _, name := range names {
-		f.children[name].dump(w, level+1)
+		f.children[name].dump(w, level+1, name)
 	}
 }
 
@@ -630,6 +627,7 @@ func (f *memNode) resetToSyncedState() {
 
 // memFile is a reader or writer of a node's data. Implements File.
 type memFile struct {
+	name        string
 	n           *memNode
 	fs          *MemFS // nil for a standalone memFile
 	rpos        int
@@ -643,9 +641,9 @@ func (f *memFile) Close() error {
 	if n := f.n.refs.Add(-1); n < 0 {
 		panic(fmt.Sprintf("pebble: close of unopened file: %d", n))
 	}
-	// TODO(pav-kv): set f.n to nil here, to detect double-close and
-	// use-after-close errors automatically. Currently this is not possible
-	// because the lifetime of Stat() can outlive the Close() of this file.
+	// Set node pointer to nil, to cause panic on any subsequent method call. This
+	// is a defence-in-depth to catch use-after-close or double-close bugs.
+	f.n = nil
 	return nil
 }
 
@@ -741,24 +739,11 @@ func (f *memFile) WriteAt(p []byte, ofs int64) (int, error) {
 func (f *memFile) Prefetch(offset int64, length int64) error { return nil }
 func (f *memFile) Preallocate(offset, length int64) error    { return nil }
 
-// name returns the current file name.
-func (f *memFile) name() string {
-	// NB: the file name must be accessed under MemFS.mu lock.
-	if fs := f.fs; fs != nil {
-		fs.mu.Lock()
-		defer fs.mu.Unlock()
-	}
-	return f.n.name
-}
-
 func (f *memFile) Stat() (os.FileInfo, error) {
-	// NB: can not inline f.name() below because it would violate the locking
-	// order: MemFS.mu can not be locked after the f.n.mu lock.
-	name := f.name()
 	f.n.mu.Lock()
 	defer f.n.mu.Unlock()
 	return &memFileInfo{
-		name:    name,
+		name:    f.name,
 		size:    int64(len(f.n.mu.data)),
 		modTime: f.n.mu.modTime,
 		isDir:   f.n.isDir,

vfs/mem_fs_test.go

@@ -232,14 +232,12 @@ func TestList(t *testing.T) {
 
 	{
 		got := fs.String()
-		// TODO(pav-kv): fix the bar/link-to-3 and bar/another-link-to-3 link names.
-		// Currently String() erroneously prints the name of the linked file.
 		const want = `          /
        0    a
             bar/
-       0      3
+       0      another-link-to-3
        0      baz
-       0      3
+       0      link-to-3
             foo/
        0      0
        0      1