fs: track most recent parent

Introduce the new struct inodeParents that wraps a map
and one special slot for the most recent parent. Unit tests
included.

Because the map is lazily initialized, we should save some
memory on the common single-parent case (= file with no hard links)
compared to before.

Benchmarking with gocryptfs shows no discernible change
in performance. fsstress testing with gocryptfs shows no issues.

TestStaleHardlinks the previous commit passes now.

Change-Id: I8d69093abc906addde751a9e70dbd78a3a61371a

fs/inode.go

@@ -18,11 +18,6 @@ import (
 	"github.com/hanwen/go-fuse/v2/fuse"
 )
 
-type parentData struct {
-	name   string
-	parent *Inode
-}
-
 // StableAttr holds immutable attributes of a object in the filesystem.
 type StableAttr struct {
 	// Each Inode has a type, which does not change over the
@@ -105,7 +100,7 @@ type Inode struct {
 
 	// Parents of this Inode. Can be more than one due to hard links.
 	// When you change this, you MUST increment changeCounter.
-	parents map[parentData]struct{}
+	parents inodeParents
 }
 
 func (n *Inode) IsDir() bool {
@@ -126,7 +121,6 @@ func initInode(n *Inode, ops InodeEmbedder, attr StableAttr, bridge *rawBridge,
 	n.bridge = bridge
 	n.persistent = persistent
 	n.nodeId = nodeId
-	n.parents = make(map[parentData]struct{})
 	if attr.Mode == fuse.S_IFDIR {
 		n.children = make(map[string]*Inode)
 	}
@@ -265,7 +259,7 @@ func unlockNodes(ns ...*Inode) {
 func (n *Inode) Forgotten() bool {
 	n.mu.Lock()
 	defer n.mu.Unlock()
-	return n.lookupCount == 0 && len(n.parents) == 0 && !n.persistent
+	return n.lookupCount == 0 && n.parents.count() == 0 && !n.persistent
 }
 
 // Operations returns the object implementing the file system
@@ -283,26 +277,16 @@ func (n *Inode) Path(root *Inode) string {
 	var segments []string
 	p := n
 	for p != nil && p != root {
-		var pd parentData
-
 		// We don't try to take all locks at the same time, because
 		// the caller won't use the "path" string under lock anyway.
-		found := false
 		p.mu.Lock()
-		// Select an arbitrary parent
-		for pd = range p.parents {
-			found = true
-			break
-		}
+		// Get last known parent
+		pd := p.parents.get()
 		p.mu.Unlock()
-		if found == false {
+		if pd == nil {
 			p = nil
 			break
 		}
-		if pd.parent == nil {
-			break
-		}
-
 		segments = append(segments, pd.name)
 		p = pd.parent
 	}
@@ -340,11 +324,9 @@ func (iparent *Inode) setEntry(name string, ichild *Inode) {
 		// Directories cannot have more than one parent. Clear the map.
 		// This special-case is neccessary because ichild may still have a
 		// parent that was forgotten (i.e. removed from bridge.inoMap).
-		for i := range ichild.parents {
-			delete(ichild.parents, i)
-		}
+		ichild.parents.clear()
 	}
-	ichild.parents[newParent] = struct{}{}
+	ichild.parents.add(newParent)
 	iparent.children[name] = ichild
 	ichild.changeCounter++
 	iparent.changeCounter++
@@ -421,7 +403,7 @@ retry:
 		parents = parents[:0]
 		nChange := n.changeCounter
 		live = n.lookupCount > 0 || len(n.children) > 0 || n.persistent
-		for p := range n.parents {
+		for _, p := range n.parents.all() {
 			parents = append(parents, p)
 			lockme = append(lockme, p.parent)
 		}
@@ -447,7 +429,7 @@ retry:
 			delete(p.parent.children, p.name)
 			p.parent.changeCounter++
 		}
-		n.parents = map[parentData]struct{}{}
+		n.parents.clear()
 		n.changeCounter++
 
 		if n.lookupCount != 0 {
@@ -488,7 +470,7 @@ retry:
 		parentCounter := n.changeCounter
 		if !ok {
 			n.children[name] = ch
-			ch.parents[parentData{name, n}] = struct{}{}
+			ch.parents.add(parentData{name, n})
 			n.changeCounter++
 			ch.changeCounter++
 			unlockNode2(n, ch)
@@ -506,9 +488,9 @@ retry:
 			continue retry
 		}
 
-		delete(prev.parents, parentData{name, n})
+		prev.parents.delete(parentData{name, n})
 		n.children[name] = ch
-		ch.parents[parentData{name, n}] = struct{}{}
+		ch.parents.add(parentData{name, n})
 		n.changeCounter++
 		ch.changeCounter++
 		prev.changeCounter++
@@ -534,10 +516,11 @@ func (n *Inode) Children() map[string]*Inode {
 func (n *Inode) Parent() (string, *Inode) {
 	n.mu.Lock()
 	defer n.mu.Unlock()
-	for k := range n.parents {
-		return k.name, k.parent
-	}
+	p := n.parents.get()
+	if p == nil {
 		return "", nil
+	}
+	return p.name, p.parent
 }
 
 // RmAllChildren recursively drops a tree, forgetting all persistent
@@ -589,7 +572,7 @@ retry:
 		for _, nm := range names {
 			ch := n.children[nm]
 			delete(n.children, nm)
-			delete(ch.parents, parentData{nm, n})
+			ch.parents.delete(parentData{nm, n})
 
 			ch.changeCounter++
 		}
@@ -640,7 +623,7 @@ retry:
 
 		if oldChild != nil {
 			delete(n.children, old)
-			delete(oldChild.parents, parentData{old, n})
+			oldChild.parents.delete(parentData{old, n})
 			n.changeCounter++
 			oldChild.changeCounter++
 		}
@@ -649,7 +632,7 @@ retry:
 			// This can cause the child to be slated for
 			// removal; see below
 			delete(newParent.children, newName)
-			delete(destChild.parents, parentData{newName, newParent})
+			destChild.parents.delete(parentData{newName, newParent})
 			destChild.changeCounter++
 			newParent.changeCounter++
 		}
@@ -658,7 +641,7 @@ retry:
 			newParent.children[newName] = oldChild
 			newParent.changeCounter++
 
-			oldChild.parents[parentData{newName, newParent}] = struct{}{}
+			oldChild.parents.add(parentData{newName, newParent})
 			oldChild.changeCounter++
 		}
 
@@ -698,14 +681,14 @@ retry:
 		// Detach
 		if oldChild != nil {
 			delete(oldParent.children, oldName)
-			delete(oldChild.parents, parentData{oldName, oldParent})
+			oldChild.parents.delete(parentData{oldName, oldParent})
 			oldParent.changeCounter++
 			oldChild.changeCounter++
 		}
 
 		if destChild != nil {
 			delete(newParent.children, newName)
-			delete(destChild.parents, parentData{newName, newParent})
+			destChild.parents.delete(parentData{newName, newParent})
 			destChild.changeCounter++
 			newParent.changeCounter++
 		}
@@ -715,7 +698,7 @@ retry:
 			newParent.children[newName] = oldChild
 			newParent.changeCounter++
 
-			oldChild.parents[parentData{newName, newParent}] = struct{}{}
+			oldChild.parents.add(parentData{newName, newParent})
 			oldChild.changeCounter++
 		}
 
@@ -723,7 +706,7 @@ retry:
 			oldParent.children[oldName] = oldChild
 			oldParent.changeCounter++
 
-			destChild.parents[parentData{oldName, oldParent}] = struct{}{}
+			destChild.parents.add(parentData{oldName, oldParent})
 			destChild.changeCounter++
 		}
 		unlockNodes(oldParent, newParent, oldChild, destChild)

fs/inode_parents.go

+// Copyright 2021 the Go-FUSE Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fs
+
+// inodeParents stores zero or more parents of an Inode,
+// remembering which one is the most recent.
+//
+// No internal locking: the caller is responsible for preventing
+// concurrent access.
+type inodeParents struct {
+	// newest is the most-recently add()'ed parent.
+	// nil when we don't have any parents.
+	newest *parentData
+	// other are parents in addition to the newest.
+	// nil or empty when we have <= 1 parents.
+	other map[parentData]struct{}
+}
+
+// add adds a parent to the store.
+func (p *inodeParents) add(n parentData) {
+	// one and only parent
+	if p.newest == nil {
+		p.newest = &n
+	}
+	// already known as `newest`
+	if *p.newest == n {
+		return
+	}
+	// old `newest` gets displaced into `other`
+	if p.other == nil {
+		p.other = make(map[parentData]struct{})
+	}
+	p.other[*p.newest] = struct{}{}
+	// new parent becomes `newest` (possibly moving up from `other`)
+	delete(p.other, n)
+	p.newest = &n
+}
+
+// get returns the most recent parent
+// or nil if there is no parent at all.
+func (p *inodeParents) get() *parentData {
+	return p.newest
+}
+
+// all returns all known parents
+// or nil if there is no parent at all.
+func (p *inodeParents) all() []parentData {
+	count := p.count()
+	if count == 0 {
+		return nil
+	}
+	out := make([]parentData, 0, count)
+	out = append(out, *p.newest)
+	for i := range p.other {
+		out = append(out, i)
+	}
+	return out
+}
+
+func (p *inodeParents) delete(n parentData) {
+	// We have zero parents, so we can't delete any.
+	if p.newest == nil {
+		return
+	}
+	// If it's not the `newest` it must be in `other` (or nowhere).
+	if *p.newest != n {
+		delete(p.other, n)
+		return
+	}
+	// We want to delete `newest`, but there is no other to replace it.
+	if len(p.other) == 0 {
+		p.newest = nil
+		return
+	}
+	// Move random entry from `other` over `newest`.
+	var i parentData
+	for i = range p.other {
+		p.newest = &i
+		break
+	}
+	delete(p.other, i)
+}
+
+func (p *inodeParents) clear() {
+	p.newest = nil
+	p.other = nil
+}
+
+func (p *inodeParents) count() int {
+	if p.newest == nil {
+		return 0
+	}
+	return 1 + len(p.other)
+}
+
+type parentData struct {
+	name   string
+	parent *Inode
+}

fs/inode_parents_test.go

+package fs
+
+import (
+	"testing"
+)
+
+func TestInodeParents(t *testing.T) {
+	var p inodeParents
+	var ino1, ino2, ino3 Inode
+
+	// empty store should be empty without panicing
+	if count := p.count(); count != 0 {
+		t.Error(count)
+	}
+	if p.all() != nil {
+		t.Error("empty store should return nil but did not")
+	}
+
+	// non-dupes should be stored
+	all := []parentData{
+		parentData{"foo", &ino1},
+		parentData{"foo2", &ino1},
+		parentData{"foo3", &ino1},
+		parentData{"foo", &ino2},
+		parentData{"foo", &ino3},
+	}
+	for i, v := range all {
+		p.add(v)
+		if count := p.count(); count != i+1 {
+			t.Errorf("want=%d have=%d", i+1, count)
+		}
+		last := p.get()
+		if *last != v {
+			t.Error("get did not give us last-known parent")
+		}
+	}
+
+	// adding dupes should not cause the count to increase, but
+	// must cause get() to return the most recently added dupe.
+	for _, v := range all {
+		p.add(v)
+		if count := p.count(); count != len(all) {
+			t.Errorf("want=%d have=%d", len(all), count)
+		}
+		last := p.get()
+		if *last != v {
+			t.Error("get did not give us last-known parent")
+		}
+	}
+
+	all2 := p.all()
+	if len(all) != len(all2) {
+		t.Errorf("want=%d have=%d", len(all), len(all2))
+	}
+}