https://github.com/go-gitea/gitea/issues/22578#issuecomment-1444180053

dontnod@07b38b7

modules/git/repo_compare.go

@@ -39,10 +39,17 @@ func (repo *Repository) GetMergeBase(tmpRemote, base, head string) (string, stri
 	if tmpRemote != "origin" {
 		tmpBaseName := RemotePrefix + tmpRemote + "/tmp_" + base
 		// Fetch commit into a temporary branch in order to be able to handle commits and tags
-		_, _, err := NewCommand(repo.Ctx, "fetch", "--no-tags").AddDynamicArguments(tmpRemote).AddDashesAndList(base + ":" + tmpBaseName).RunStdString(&RunOpts{Dir: repo.Path})
-		if err == nil {
-			base = tmpBaseName
+		// --no-write-commit-graph: fetch will try to generate commit graph by default
+		// this process require a lock-file (objects/info/commit-graphs/commit-graph-chain.lock)
+		// and will make fetch return an error code if the file already exists
+		_, _, err := NewCommand(repo.Ctx, "fetch", "--no-write-commit-graph", "--no-tags").
+			AddDynamicArguments(tmpRemote).AddDashesAndList(base + ":" + tmpBaseName).
+			RunStdString(&RunOpts{Dir: repo.Path})
+		if err != nil {
+			logger.Error("Failed to fetch new remote for GetMergeBase %v", err)
+			return "", "", err
 		}
+		base = tmpBaseName
 	}
 
 	stdout, _, err := NewCommand(repo.Ctx, "merge-base").AddDashesAndList(base, head).RunStdString(&RunOpts{Dir: repo.Path})