cephfs: add Fsync and Sync calls for open files

Add wrappers for ceph_fsync. The Fsync call directly wraps ceph_fsync
which provides options to behave more like fsync or more like fdatasync.
Add Sync, a simpler wrapper over Fsync, to match any interfaces that
make use of `File.Sync` from Go's os package.

Signed-off-by: John Mulligan <jmulligan@redhat.com>

cephfs/file.go

@@ -25,6 +25,17 @@ const (
 	SeekEnd = int(C.SEEK_END)
 )
 
+// SyncChoice is used to control how metadata and/or data is sync'ed to
+// the file system.
+type SyncChoice int
+
+const (
+	// SyncAll will synchronize both data and metadata.
+	SyncAll = SyncChoice(0)
+	// SyncDataOnly will synchronize only data.
+	SyncDataOnly = SyncChoice(1)
+)
+
 // File represents an open file descriptor in cephfs.
 type File struct {
 	mount *MountInfo
@@ -293,3 +304,31 @@ func (f *File) Flock(operation LockOp, owner uint64) error {
 	ret := C.ceph_flock(f.mount.mount, f.fd, C.int(operation), C.uint64_t(owner))
 	return getError(ret)
 }
+
+// Fsync ensures the file content that may be cached is committed to stable
+// storage.
+// Pass SyncAll to have this call behave like standard fsync and synchronize
+// all data and metadata.
+// Pass SyncDataOnly to have this call behave more like fdatasync (on linux).
+//
+// Implements:
+//  int ceph_fsync(struct ceph_mount_info *cmount, int fd, int syncdataonly);
+func (f *File) Fsync(sync SyncChoice) error {
+	if err := f.validate(); err != nil {
+		return err
+	}
+
+	ret := C.ceph_fsync(
+		f.mount.mount,
+		f.fd,
+		C.int(sync),
+	)
+	return getError(ret)
+}
+
+// Sync ensures the file content that may be cached is committed to stable
+// storage.
+// Sync behaves like Go's os package File.Sync function.
+func (f *File) Sync() error {
+	return f.Fsync(SyncAll)
+}

cephfs/file_test.go

@@ -711,3 +711,61 @@ func TestFlock(t *testing.T) {
 		})
 	})
 }
+
+func TestFsync(t *testing.T) {
+	mount := fsConnect(t)
+	defer fsDisconnect(t, mount)
+
+	fname := "test_fsync.txt"
+	defer mount.Unlink(fname)
+
+	// unfortunately there's not much to assert around the the behavior of
+	// fsync in these simple tests so we sort-of have to trust ceph on this :-)
+	t.Run("simpleFsync", func(t *testing.T) {
+		f, err := mount.Open(fname, os.O_RDWR|os.O_CREATE, 0666)
+		defer func() { assert.NoError(t, f.Close()) }()
+		assert.NoError(t, err)
+		_, err = f.Write([]byte("batman"))
+		assert.NoError(t, err)
+		err = f.Fsync(SyncAll)
+		assert.NoError(t, err)
+	})
+	t.Run("DataOnly", func(t *testing.T) {
+		f, err := mount.Open(fname, os.O_RDWR|os.O_CREATE, 0666)
+		defer func() { assert.NoError(t, f.Close()) }()
+		assert.NoError(t, err)
+		_, err = f.Write([]byte("superman"))
+		assert.NoError(t, err)
+		err = f.Fsync(SyncDataOnly)
+		assert.NoError(t, err)
+	})
+	t.Run("invalid", func(t *testing.T) {
+		f := &File{}
+		err := f.Fsync(SyncAll)
+		assert.Error(t, err)
+	})
+}
+
+func TestSync(t *testing.T) {
+	mount := fsConnect(t)
+	defer fsDisconnect(t, mount)
+
+	fname := "test_sync.txt"
+	defer mount.Unlink(fname)
+
+	// see fsync
+	t.Run("simple", func(t *testing.T) {
+		f, err := mount.Open(fname, os.O_RDWR|os.O_CREATE, 0666)
+		defer func() { assert.NoError(t, f.Close()) }()
+		assert.NoError(t, err)
+		_, err = f.Write([]byte("question"))
+		assert.NoError(t, err)
+		err = f.Sync()
+		assert.NoError(t, err)
+	})
+	t.Run("invalid", func(t *testing.T) {
+		f := &File{}
+		err := f.Sync()
+		assert.Error(t, err)
+	})
+}