cephfs: add GetFsCid to implement ceph_get_fs_cid

This function can be used to uniquely id the file system that the
mount object is connecting to.

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

cephfs/conn_nautilus.go

@@ -0,0 +1,30 @@
+// +build !luminous,!mimic
+
+package cephfs
+
+/*
+#cgo LDFLAGS: -lcephfs
+#cgo CPPFLAGS: -D_FILE_OFFSET_BITS=64
+#include <cephfs/libcephfs.h>
+*/
+import "C"
+
+// Some general connectivity and mounting functions are new in
+// Ceph Nautilus.
+
+// GetFsCid returns the cluster ID for a mounted ceph file system.
+// If the object does not refer to a mounted file system, an error
+// will be returned.
+//
+// Note:
+//  Only supported in Ceph Nautilus and newer.
+//
+// Implements:
+//  int64_t ceph_get_fs_cid(struct ceph_mount_info *cmount);
+func (mount *MountInfo) GetFsCid() (int64, error) {
+	ret := C.ceph_get_fs_cid(mount.mount)
+	if ret < 0 {
+		return 0, getError(C.int(ret))
+	}
+	return int64(ret), nil
+}

cephfs/conn_nautilus_test.go

@@ -0,0 +1,32 @@
+// +build !luminous,!mimic
+
+package cephfs
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGetFsCid(t *testing.T) {
+	t.Run("unmounted", func(t *testing.T) {
+		mount, err := CreateMount()
+		require.NoError(t, err)
+		require.NotNil(t, mount)
+
+		err = mount.ReadDefaultConfigFile()
+		require.NoError(t, err)
+
+		cid, err := mount.GetFsCid()
+		assert.Error(t, err)
+		assert.Equal(t, cid, int64(0))
+	})
+	t.Run("mounted", func(t *testing.T) {
+		mount := fsConnect(t)
+
+		cid, err := mount.GetFsCid()
+		assert.NoError(t, err)
+		assert.GreaterOrEqual(t, cid, int64(0))
+	})
+}