fusefronted: enable writing to write-only files

Due to RMW, we always need read permissions on the backing file. This is a
problem if the file permissions do not allow reading (i.e. 0200 permissions).
This patch works around that problem by chmod'ing the file, obtaining a fd,
and chmod'ing it back.

Test included.

Issue reported at: https://github.com/rfjakob/gocryptfs/issues/125

1 files changed, 30 insertions, 0 deletions

diff --git a/tests/defaults/main_test.go b/tests/defaults/main_test.go
index ca11b43..5c6bb84 100644
--- a/tests/defaults/main_test.go
+++ b/tests/defaults/main_test.go
@@ -4,6 +4,7 @@ package defaults
 import (
 	"bytes"
 	"io"
+	"io/ioutil"
 	"os"
 	"os/exec"
 	"runtime"
@@ -161,3 +162,32 @@ func TestXfs124(t *testing.T) {
 
 	wg.Wait()
 }
+
+func TestWrite0200File(t *testing.T) {
+	fn := test_helpers.DefaultPlainDir + "/TestWrite0200File"
+	err := ioutil.WriteFile(fn, nil, 0200)
+	if err != nil {
+		t.Fatalf("creating empty file failed: %v", err)
+	}
+	fd, err := os.OpenFile(fn, os.O_WRONLY, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	fi, err := fd.Stat()
+	if err != nil {
+		t.Fatal(err)
+	}
+	perms := fi.Mode().Perm()
+	if perms != 0200 {
+		t.Fatal("wrong initial permissions")
+	}
+	defer fd.Close()
+	_, err = fd.Write(make([]byte, 10))
+	if err != nil {
+		t.Fatal(err)
+	}
+	perms = fi.Mode().Perm()
+	if perms != 0200 {
+		t.Fatal("wrong restored permissions")
+	}
+}