fuse: cache fs.Node instances

A particular node should always be represented by a single instance.
This is necessary to allow the fuse library to assign a stable nodeId to
a node. macOS Sonoma trips over the previous, unstable behavior when
using fuse-t.

internal/fuse/tree_cache.go

@@ -0,0 +1,38 @@
+//go:build darwin || freebsd || linux
+// +build darwin freebsd linux
+
+package fuse
+
+import (
+	"sync"
+
+	"github.com/anacrolix/fuse/fs"
+)
+
+type treeCache struct {
+	nodes map[string]fs.Node
+	m     sync.Mutex
+}
+
+func newTreeCache() *treeCache {
+	return &treeCache{
+		nodes: map[string]fs.Node{},
+	}
+}
+
+func (t *treeCache) lookupOrCreate(name string, create func() (fs.Node, error)) (fs.Node, error) {
+	t.m.Lock()
+	defer t.m.Unlock()
+
+	if node, ok := t.nodes[name]; ok {
+		return node, nil
+	}
+
+	node, err := create()
+	if err != nil {
+		return nil, err
+	}
+
+	t.nodes[name] = node
+	return node, nil
+}