gitutil: Fix git pull on non-git repository directory

* Add conditional check: if the git repo directory is newly created, we
clone.
* Add tests

Fixes #641

pkg/gitutil/gitutil.go

@@ -73,7 +73,7 @@ func CloneOrPull(remote, repoPath, tempDirPrefix string) (path, head string, err
 		path = repoPath
 	}
 
-	if _, pathExists := os.Stat(path); os.IsNotExist(pathExists) {
+	if _, pathExists := os.Stat(path); repoPath == "" || os.IsNotExist(pathExists) {
 		head, err = clone(remote, path)
 		return
 	}

pkg/gitutil/gitutil_test.go

@@ -0,0 +1,77 @@
+// Copyright 2018 clair authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package gitutil
+
+import (
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func createTemporaryGitRepo(t *testing.T) string {
+	// create temporary folder
+	path, err := ioutil.TempDir(os.TempDir(), "1b750a87bbfc")
+	require.Nil(t, err)
+
+	// initialize git
+	cmd := exec.Command("git", "init", path)
+	out, err := cmd.CombinedOutput()
+	require.Nil(t, err, "Failed to initialize temporary git repo, output=%s", string(out))
+	return path
+}
+
+func createEmptyCommit(t *testing.T, repoPath string) {
+	cmd := exec.Command("git", "commit", "-m", "\"init\"", "--allow-empty")
+	cmd.Dir = repoPath
+	out, err := cmd.CombinedOutput()
+	require.Nil(t, err, "Failed to submit first empty git commit, output=%s", string(out))
+}
+
+func getHeadCommitRev(t *testing.T, repoPath string) string {
+	cmd := exec.Command("git", "rev-parse", "HEAD")
+	cmd.Dir = repoPath
+	out, err := cmd.CombinedOutput()
+	require.Nil(t, err, "Failed to get head revision, output=%s", string(out))
+	return strings.TrimSuffix(string(out), "\n")
+}
+
+func TestCloneOrPull(t *testing.T) {
+	remote := createTemporaryGitRepo(t)
+	createEmptyCommit(t, remote)
+	expectedHead := getHeadCommitRev(t, remote)
+	t.Log(expectedHead)
+	repoPath := ""
+	tempDirPrefix := "9c2d4181"
+	path, head, err := CloneOrPull(remote, repoPath, tempDirPrefix)
+	require.Nil(t, err)
+	_, err = os.Stat(path)
+	require.Nil(t, err, "Expect generated repo to exist")
+	require.Equal(t, expectedHead, head)
+
+	// create second empty commit to try pull
+	createEmptyCommit(t, remote)
+	expectedHead = getHeadCommitRev(t, remote)
+	t.Log(expectedHead)
+	newPath, newHead, err := CloneOrPull(remote, path, tempDirPrefix)
+	require.Nil(t, err)
+	_, err = os.Stat(path)
+	require.Nil(t, err, "Expect generated repo to exist")
+	require.Equal(t, path, newPath, "No new path should be created when pulling")
+	require.Equal(t, expectedHead, newHead)
+}