From bb4cccc6e90f69e8804c1df98f4b6c249eae17b1 Mon Sep 17 00:00:00 2001
From: Lunny Xiao <xiaolunwen@gmail.com>
Date: Sun, 14 Jun 2026 12:52:44 -0700
Subject: [PATCH] fix: parse HEAD ref (#38119)

backport #38088
---
 modules/git/ref.go                |  2 +-
 modules/git/repo_ref.go           |  6 +++++-
 modules/git/repo_ref_test.go      | 15 +++++++++++++++
 tests/integration/compare_test.go | 10 +++++++++-
 4 files changed, 30 insertions(+), 3 deletions(-)

diff --git a/modules/git/ref.go b/modules/git/ref.go
index 7b63d06b388..42ad9bd63d8 100644
--- a/modules/git/ref.go
+++ b/modules/git/ref.go
@@ -161,7 +161,7 @@ func (ref RefName) ShortName() string {
 	if ref.IsFor() {
 		return ref.ForBranchName()
 	}
-	return string(ref) // usually it is a commit ID
+	return string(ref) // usually it is a commit ID, or "HEAD"
 }
 
 // RefGroup returns the group type of the reference
diff --git a/modules/git/repo_ref.go b/modules/git/repo_ref.go
index 8859a93a578..cef25e85ee9 100644
--- a/modules/git/repo_ref.go
+++ b/modules/git/repo_ref.go
@@ -8,6 +8,7 @@ import (
 	"strings"
 
 	"code.gitea.io/gitea/modules/git/gitcmd"
+	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/util"
 )
 
@@ -86,8 +87,11 @@ func (repo *Repository) UnstableGuessRefByShortName(shortName string) RefName {
 	commit, err := repo.GetCommit(shortName)
 	if err == nil {
 		commitIDString := commit.ID.String()
-		if strings.HasPrefix(commitIDString, shortName) {
+		// Make sure the short name is either a partial commit ID, or the symbolic HEAD ref.
+		if strings.HasPrefix(commitIDString, shortName) || shortName == "HEAD" {
 			return RefName(commitIDString)
+		} else {
+			setting.PanicInDevOrTesting("abuse of UnstableGuessRefByShortName, queried %s, got %s", shortName, commitIDString)
 		}
 	}
 	return ""
diff --git a/modules/git/repo_ref_test.go b/modules/git/repo_ref_test.go
index 29c255098f1..0ff86d3cd3d 100644
--- a/modules/git/repo_ref_test.go
+++ b/modules/git/repo_ref_test.go
@@ -53,3 +53,18 @@ func TestRepository_GetRefsFiltered(t *testing.T) {
 		assert.Equal(t, "3ad28a9149a2864384548f3d17ed7f38014c9e8a", refs[1].Object.String())
 	}
 }
+
+func TestRepository_UnstableGuessRefByShortName(t *testing.T) {
+	bareRepo1Path := filepath.Join(testReposDir, "repo1_bare")
+	bareRepo1, err := OpenRepository(t.Context(), bareRepo1Path)
+	assert.NoError(t, err)
+	defer bareRepo1.Close()
+
+	headCommit, err := bareRepo1.GetCommit("HEAD")
+	assert.NoError(t, err)
+
+	assert.Equal(t, RefName(headCommit.ID.String()), bareRepo1.UnstableGuessRefByShortName("HEAD"))
+	assert.Equal(t, RefName(headCommit.ID.String()), bareRepo1.UnstableGuessRefByShortName(headCommit.ID.String()[:8]))
+	assert.Equal(t, RefNameFromBranch("master"), bareRepo1.UnstableGuessRefByShortName("master"))
+	assert.Empty(t, bareRepo1.UnstableGuessRefByShortName("NotExisting"))
+}
diff --git a/tests/integration/compare_test.go b/tests/integration/compare_test.go
index f926f106967..61e54131f52 100644
--- a/tests/integration/compare_test.go
+++ b/tests/integration/compare_test.go
@@ -36,9 +36,17 @@ func TestCompareTag(t *testing.T) {
 	// A dropdown for both base and head.
 	assert.Lenf(t, selection.Nodes, 2, "The template has changed")
 
+	req = NewRequest(t, "GET", "/user2/repo1/compare/v1.1...HEAD")
+	resp = session.MakeRequest(t, req, http.StatusOK)
+	assert.True(t, test.IsNormalPageCompleted(resp.Body.String()))
+
+	req = NewRequest(t, "GET", "/user2/repo1/compare/v1.1...NotExisting").SetHeader("Accept", "text/html")
+	resp = session.MakeRequest(t, req, http.StatusNotFound)
+	assert.True(t, test.IsNormalPageCompleted(resp.Body.String()))
+
 	req = NewRequest(t, "GET", "/user2/repo1/compare/invalid").SetHeader("Accept", "text/html")
 	resp = session.MakeRequest(t, req, http.StatusNotFound)
-	assert.True(t, test.IsNormalPageCompleted(resp.Body.String()), "expect 404 page not 500")
+	assert.True(t, test.IsNormalPageCompleted(resp.Body.String()))
 }
 
 // Compare with inferred default branch (master)