mfs: clean directory cache on flush

Flushing represents a sort of "I'm done" with the folder.

It is less likely that old references to the folder are used after it has been
flushed. As such, we take the chance to clean up the cache then.

CHANGELOG.md

@@ -33,7 +33,7 @@ The following emojis are used to highlight certain changes:
 
 ### Fixed
 
-* `mfs`: directory cache is now cleared every time the directory node is read, somewhat limiting unbounded growth and time to sync it to the underlying unixfs.
+* `mfs`: directory cache is now cleared on Flush(), liberating the memory used by the otherwise ever-growing cache. References to directories and sub-directories should be renewed after flushing.
 
 ### Security
 

mfs/dir.go

@@ -323,7 +323,7 @@ func (d *Directory) Unlink(name string) error {
 }
 
 func (d *Directory) Flush() error {
-	nd, err := d.GetNode()
+	nd, err := d.getNode(true)
 	if err != nil {
 		return err
 	}
@@ -349,7 +349,7 @@ func (d *Directory) AddChild(name string, nd ipld.Node) error {
 	return d.unixfsDir.AddChild(d.ctx, name, nd)
 }
 
-func (d *Directory) syncCache(clean bool) error {
+func (d *Directory) cacheSync(clean bool) error {
 	for name, entry := range d.entriesCache {
 		nd, err := entry.GetNode()
 		if err != nil {
@@ -385,10 +385,14 @@ func (d *Directory) Path() string {
 }
 
 func (d *Directory) GetNode() (ipld.Node, error) {
+	return d.getNode(false)
+}
+
+func (d *Directory) getNode(cacheClean bool) (ipld.Node, error) {
 	d.lock.Lock()
 	defer d.lock.Unlock()
 
-	err := d.syncCache(true)
+	err := d.cacheSync(cacheClean)
 	if err != nil {
 		return nil, err
 	}

mfs/file.go

@@ -270,10 +270,9 @@ func (fi *File) setNodeData(data []byte) error {
 	}
 
 	fi.nodeLock.Lock()
-	defer fi.nodeLock.Unlock()
 	fi.node = nd
 	parent := fi.inode.parent
 	name := fi.inode.name
-
+	fi.nodeLock.Unlock()
 	return parent.updateChildEntry(child{name, fi.node})
 }

mfs/root.go

@@ -170,16 +170,7 @@ func (kr *Root) Flush() error {
 func (kr *Root) FlushMemFree(ctx context.Context) error {
 	dir := kr.GetDirectory()
 
-	if err := dir.Flush(); err != nil {
-		return err
-	}
-
-	dir.lock.Lock()
-	defer dir.lock.Unlock()
-
-	clear(dir.entriesCache)
-
-	return nil
+	return dir.Flush()
 }
 
 // updateChildEntry implements the `parent` interface, and signals