fix: add test

Signed-off-by: Michael Crenshaw <michael@crenshaw.dev>

reposerver/repository/repository.go

@@ -1678,15 +1678,19 @@ func directoryPermissionInitializer(rootPath string) goio.Closer {
 // nolint:unparam
 func (s *Service) checkoutRevision(gitClient git.Client, revision string, submoduleEnabled bool) (goio.Closer, error) {
 	closer := s.gitRepoInitializer(gitClient.Root())
+	return closer, checkoutRevision(gitClient, revision, submoduleEnabled)
+}
+
+func checkoutRevision(gitClient git.Client, revision string, submoduleEnabled bool) error {
 	err := gitClient.Init()
 	if err != nil {
-		return closer, status.Errorf(codes.Internal, "Failed to initialize git repo: %v", err)
+		return status.Errorf(codes.Internal, "Failed to initialize git repo: %v", err)
 	}
 
 	// Fetching with no revision first. Fetching with an explicit version can cause repo bloat. https://github.com/argoproj/argo-cd/issues/8845
 	err = gitClient.Fetch("")
 	if err != nil {
-		return closer, status.Errorf(codes.Internal, "Failed to fetch default: %v", err)
+		return status.Errorf(codes.Internal, "Failed to fetch default: %v", err)
 	}
 
 	err = gitClient.Checkout(revision, submoduleEnabled)
@@ -1698,16 +1702,16 @@ func (s *Service) checkoutRevision(gitClient git.Client, revision string, submod
 
 		err = gitClient.Fetch(revision)
 		if err != nil {
-			return closer, status.Errorf(codes.Internal, "Failed to checkout revision %s: %v", revision, err)
+			return status.Errorf(codes.Internal, "Failed to checkout revision %s: %v", revision, err)
 		}
 
 		err = gitClient.Checkout("FETCH_HEAD", submoduleEnabled)
 		if err != nil {
-			return closer, status.Errorf(codes.Internal, "Failed to checkout FETCH_HEAD: %v", err)
+			return status.Errorf(codes.Internal, "Failed to checkout FETCH_HEAD: %v", err)
 		}
 	}
 
-	return closer, err
+	return err
 }
 
 func (s *Service) GetHelmCharts(ctx context.Context, q *apiclient.HelmChartsRequest) (*apiclient.HelmChartsResponse, error) {

reposerver/repository/repository_test.go

@@ -1779,3 +1779,48 @@ func TestInit(t *testing.T) {
 	require.Error(t, err)
 	require.NoError(t, initGitRepo(path.Join(dir, "repo2"), "https://github.com/argo-cd/test-repo2"))
 }
+
+// TestCheckoutRevisionCanGetNonstandardRefs shows that we can fetch a revision that points to a non-standard ref. In
+// other words, we haven't regressed and caused this issue again: https://github.com/argoproj/argo-cd/issues/4935
+func TestCheckoutRevision(t *testing.T) {
+	rootPath, err := ioutil.TempDir("", "")
+	require.NoError(t, err)
+
+	sourceRepoPath, err := ioutil.TempDir(rootPath, "")
+	require.NoError(t, err)
+
+	// Create a repo such that one commit is on a non-standard ref _and nowhere else_. This is meant to simulate, for
+	// example, a GitHub ref for a pull into one repo from a fork of that repo.
+	run(t, sourceRepoPath, "git", "init")
+	run(t, sourceRepoPath, "git", "checkout", "-b", "main")  // make sure there's a main branch to switch back to
+	run(t, sourceRepoPath, "git", "commit", "-m", "empty", "--allow-empty")
+	run(t, sourceRepoPath, "git", "checkout", "-b", "branch")
+	run(t, sourceRepoPath, "git", "commit", "-m", "empty", "--allow-empty")
+	sha := run(t, sourceRepoPath, "git", "rev-parse", "HEAD")
+	run(t, sourceRepoPath, "git", "update-ref", "refs/pull/123/head", strings.TrimSuffix(sha, "\n"))
+	run(t, sourceRepoPath, "git", "checkout", "main")
+	run(t, sourceRepoPath, "git", "branch", "-D", "branch")
+
+	destRepoPath, err := ioutil.TempDir(rootPath, "")
+	require.NoError(t, err)
+
+	gitClient, err := git.NewClientExt("file://"+sourceRepoPath, destRepoPath, &git.NopCreds{}, true, false, "")
+	require.NoError(t, err)
+
+	pullSha, err := gitClient.LsRemote("refs/pull/123/head")
+	require.NoError(t, err)
+
+	err = checkoutRevision(gitClient, pullSha, false)
+	require.NoError(t, err)
+}
+
+// run runs a command in the given working directory. If the command succeeds, it returns the combined standard and
+// error output. If it fails, it stops the test with a failure message.
+func run(t *testing.T, workDir string, command string, args ...string) string {
+	cmd := exec.Command(command, args...)
+	cmd.Dir = workDir
+	out, err := cmd.CombinedOutput()
+	stringOut := string(out)
+	require.NoError(t, err, stringOut)
+	return stringOut
+}