Optimize FUSE - make command restic mount faster and consume less memory

- Add Open() functionality to dir
- only access index for blobs when file is read
- Implement NodeOpener and put one-time file stuff there
- Add comment about locking as suggested by bazil.org/fuse

=> Thanks at Michael Eischer for suggesting the last two improvements

internal/fuse/fuse_test.go

@@ -62,7 +62,7 @@ func TestCache(t *testing.T) {
 	rtest.Equals(t, cacheSize, c.free)
 }
 
-func testRead(t testing.TB, f *file, offset, length int, data []byte) {
+func testRead(t testing.TB, f fs.Handle, offset, length int, data []byte) {
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 
@@ -73,7 +73,8 @@ func testRead(t testing.TB, f *file, offset, length int, data []byte) {
 	resp := &fuse.ReadResponse{
 		Data: data,
 	}
-	rtest.OK(t, f.Read(ctx, req, resp))
+	fr := f.(fs.HandleReader)
+	rtest.OK(t, fr.Read(ctx, req, resp))
 }
 
 func firstSnapshotID(t testing.TB, repo restic.Repository) (first restic.ID) {
@@ -156,11 +157,13 @@ func TestFuseFile(t *testing.T) {
 		Size:    filesize,
 		Content: content,
 	}
-	root := NewRoot(repo, Config{})
+	root := &Root{repo: repo, blobCache: newBlobCache(blobCacheSize)}
 
 	inode := fs.GenerateDynamicInode(1, "foo")
 	f, err := newFile(context.TODO(), root, inode, node)
 	rtest.OK(t, err)
+	of, err := f.Open(context.TODO(), nil, nil)
+	rtest.OK(t, err)
 
 	attr := fuse.Attr{}
 	rtest.OK(t, f.Attr(ctx, &attr))
@@ -178,7 +181,7 @@ func TestFuseFile(t *testing.T) {
 
 		buf := make([]byte, length)
 
-		testRead(t, f, offset, length, buf)
+		testRead(t, of, offset, length, buf)
 		if !bytes.Equal(b, buf) {
 			t.Errorf("test %d failed, wrong data returned (offset %v, length %v)", i, offset, length)
 		}