fix: parse HEAD ref (#38088)

fix #38086

modules/git/ref.go

@@ -168,7 +168,7 @@ 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

modules/git/repo_ref.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 
 	"gitea.dev/modules/git/gitcmd"
+	"gitea.dev/modules/setting"
 	"gitea.dev/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 "shortName" is either partial commit ID, or it is HEAD
+		if strings.HasPrefix(commitIDString, shortName) || shortName == RefNameHead {
 			return RefName(commitIDString)
+		} else {
+			setting.PanicInDevOrTesting("abuse of UnstableGuessRefByShortName, queried %s, got %s", shortName, commitIDString)
 		}
 	}
 	return ""

tests/integration/compare_test.go

@@ -33,9 +33,17 @@ 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)