fusefrontend: Handle PlaintextNames mode in Link

In PlaintextNames mode the "gocryptfs.longname." prefix does not have any
special meaning.

https://github.com/rfjakob/gocryptfs/issues/174

internal/fusefrontend/fs.go

@@ -550,7 +550,7 @@ func (fs *FS) Link(oldPath string, newPath string, context *fuse.Context) (code
 	}
 	// Handle long file name
 	cNewName := filepath.Base(cNewPath)
-	if nametransform.IsLongContent(cNewName) {
+	if !fs.args.PlaintextNames && nametransform.IsLongContent(cNewName) {
 		dirfd, err := os.Open(filepath.Dir(cNewPath))
 		if err != nil {
 			return fuse.ToStatus(err)

tests/matrix/matrix_test.go

@@ -812,3 +812,26 @@ func TestSymlink(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+// Make sure the Link call works with paths starting with "gocryptfs.longname."
+func TestLink(t *testing.T) {
+	target := test_helpers.DefaultPlainDir + "/linktarget"
+	f, err := os.Create(target)
+	if err != nil {
+		t.Fatal(err)
+	}
+	f.Close()
+	path := test_helpers.DefaultPlainDir + "/gocryptfs.longname.XXX"
+	err = syscall.Link(target, path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = os.Remove(target)
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = os.Remove(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+}