cephfs: make the Release function more idempotent to callers

Previously, calling Release more than once for the same MountInfo would
abort due to a double free in the ceph libs. As this is somewhat user
hostile we add some simple state tracking in our wrapper type such that
one can make redundant calls to Release without crashing the
application.

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

cephfs/cephfs.go

@@ -155,8 +155,15 @@ func (mount *MountInfo) Unmount() error {
 // Implements:
 //  int ceph_release(struct ceph_mount_info *cmount);
 func (mount *MountInfo) Release() error {
+	if mount.mount == nil {
+		return nil
+	}
 	ret := C.ceph_release(mount.mount)
-	return getError(ret)
+	if err := getError(ret); err != nil {
+		return err
+	}
+	mount.mount = nil
+	return nil
 }
 
 // SyncFs synchronizes all filesystem data to persistent media.

cephfs/cephfs_test.go

@@ -187,8 +187,9 @@ func TestReleaseMount(t *testing.T) {
 	assert.NoError(t, err)
 	require.NotNil(t, mount)
 
-	err = mount.Release()
-	assert.NoError(t, err)
+	assert.NoError(t, mount.Release())
+	// call release again to ensure idempotency of the func
+	assert.NoError(t, mount.Release())
 }
 
 func TestChmodDir(t *testing.T) {