fuse: forget fs.Node instances on request by the kernel

Forget fs.Node instances once the kernel frees the corresponding nodeId.
This ensures that restic does not run out of memory on large snapshots.

internal/fuse/tree_cache.go

@@ -14,13 +14,15 @@ type treeCache struct {
 	m     sync.Mutex
 }
 
+type forgetFn func()
+
 func newTreeCache() *treeCache {
 	return &treeCache{
 		nodes: map[string]fs.Node{},
 	}
 }
 
-func (t *treeCache) lookupOrCreate(name string, create func() (fs.Node, error)) (fs.Node, error) {
+func (t *treeCache) lookupOrCreate(name string, create func(forget forgetFn) (fs.Node, error)) (fs.Node, error) {
 	t.m.Lock()
 	defer t.m.Unlock()
 
@@ -28,7 +30,12 @@ func (t *treeCache) lookupOrCreate(name string, create func() (fs.Node, error))
 		return node, nil
 	}
 
-	node, err := create()
+	node, err := create(func() {
+		t.m.Lock()
+		defer t.m.Unlock()
+
+		delete(t.nodes, name)
+	})
 	if err != nil {
 		return nil, err
 	}