Compare dropdown fails when selecting branch with no common merge-base (#37470) (#37472)

## Summary

- handle compare requests where base and head refs have no common merge
base without returning 500
- keep the compare branch selectors usable and show a clear warning
message
- add regression coverage for unrelated-history compare selection and
merge-base error detection

Fixes #37469 



Manuel Backport of: https://github.com/go-gitea/gitea/pull/37470

---------

Co-authored-by: Codex <codex@openai.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/gitrepo/compare_test.go

@@ -4,9 +4,18 @@
 package gitrepo
 
 import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"strings"
 	"testing"
+	"time"
+
+	"code.gitea.io/gitea/modules/git/gitcmd"
+	"code.gitea.io/gitea/modules/util"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 type mockRepository struct {
@@ -17,6 +26,61 @@ func (r *mockRepository) RelativePath() string {
 	return r.path
 }
 
+func commitRootTree(t *testing.T, repoDir, fileName, content, message string) string {
+	t.Helper()
+
+	require.NoError(t, gitcmd.NewCommand("read-tree", "--empty").WithDir(repoDir).Run(t.Context()))
+
+	stdout, _, err := gitcmd.NewCommand("hash-object", "-w", "--stdin").
+		WithDir(repoDir).
+		WithStdinBytes([]byte(content)).
+		RunStdString(t.Context())
+	require.NoError(t, err)
+	blobSHA := strings.TrimSpace(stdout)
+
+	_, _, err = gitcmd.NewCommand("update-index", "--add", "--replace", "--cacheinfo").
+		AddDynamicArguments("100644", blobSHA, fileName).
+		WithDir(repoDir).
+		RunStdString(t.Context())
+	require.NoError(t, err)
+
+	stdout, _, err = gitcmd.NewCommand("write-tree").WithDir(repoDir).RunStdString(t.Context())
+	require.NoError(t, err)
+	treeSHA := strings.TrimSpace(stdout)
+
+	commitTimeStr := time.Now().Format(time.RFC3339)
+	env := append(os.Environ(),
+		"GIT_AUTHOR_NAME=Test",
+		"GIT_AUTHOR_EMAIL=test@example.com",
+		"GIT_AUTHOR_DATE="+commitTimeStr,
+		"GIT_COMMITTER_NAME=Test",
+		"GIT_COMMITTER_EMAIL=test@example.com",
+		"GIT_COMMITTER_DATE="+commitTimeStr,
+	)
+
+	messageBytes := bytes.NewBufferString(message + "\n")
+	stdout, _, err = gitcmd.NewCommand("commit-tree").AddDynamicArguments(treeSHA).
+		WithEnv(env).
+		WithDir(repoDir).
+		WithStdinBytes(messageBytes.Bytes()).
+		RunStdString(t.Context())
+	require.NoError(t, err)
+
+	return strings.TrimSpace(stdout)
+}
+
+func TestMergeBaseNoCommonHistory(t *testing.T) {
+	repoDir := filepath.Join(t.TempDir(), "repo.git")
+	require.NoError(t, gitcmd.NewCommand("init").AddDynamicArguments(repoDir).Run(t.Context()))
+
+	baseCommit := commitRootTree(t, repoDir, "base.txt", "base", "base")
+	headCommit := commitRootTree(t, repoDir, "head.txt", "head", "head")
+
+	mergeBase, err := MergeBase(t.Context(), &mockRepository{path: repoDir}, baseCommit, headCommit)
+	assert.Empty(t, mergeBase)
+	assert.ErrorIs(t, err, util.ErrNotExist)
+}
+
 func TestRepoGetDivergingCommits(t *testing.T) {
 	repo := &mockRepository{path: "repo1_bare"}
 	do, err := GetDivergingCommits(t.Context(), repo, "master", "branch2")

modules/gitrepo/merge.go

@@ -9,6 +9,7 @@ import (
 	"strings"
 
 	"code.gitea.io/gitea/modules/git/gitcmd"
+	"code.gitea.io/gitea/modules/util"
 )
 
 // MergeBase checks and returns merge base of two commits.
@@ -16,6 +17,9 @@ func MergeBase(ctx context.Context, repo Repository, baseCommitID, headCommitID
 	mergeBase, _, err := RunCmdString(ctx, repo, gitcmd.NewCommand("merge-base").
 		AddDashesAndList(baseCommitID, headCommitID))
 	if err != nil {
+		if gitcmd.IsErrorExitCode(err, 1) {
+			return "", util.NewNotExistErrorf("get merge-base of %s and %s failed", baseCommitID, headCommitID)
+		}
 		return "", fmt.Errorf("get merge-base of %s and %s failed: %w", baseCommitID, headCommitID, err)
 	}
 	return strings.TrimSpace(mergeBase), nil