fix: avoid FetchRemoteTempCommit touching unnecessary resources (#39583)

Also rename FetchRemoteCommit to FetchRemoteTempCommit to clarify its
purpose

modules/git/fetch.go

@@ -10,20 +10,16 @@ import (
 	"gitea.dev/modules/git/gitrepo"
 )
 
-// FetchRemoteCommit fetches a specific commit and its related objects from a remote
-// repository into the managed repository.
+// FetchRemoteTempCommit fetches a specific commit and its related objects from a remote repository
+// into the managed repository for temporary use (e.g.: compare, check merge base, etc.).
 //
-// If no reference (branch, tag, or other ref) points to the fetched commit, it will
-// be treated as unreachable and cleaned up by `git gc` after the default prune
-// expiration period (2 weeks). Ref: https://www.kernel.org/pub/software/scm/git/docs/git-gc.html
-//
-// This behavior is sufficient for temporary operations, such as determining the
-// merge base between commits.
-func FetchRemoteCommit(ctx context.Context, repo, remoteRepo RepositoryFacade, commitID string) error {
-	return LockWriteAndDo(ctx, repo, func(ctx context.Context) error {
-		return gitcmd.NewCommand("fetch", "--no-tags").
-			AddDynamicArguments(gitrepo.RepoLocalPath(remoteRepo)).
-			AddDynamicArguments(commitID).
-			WithRepo(repo).Run(ctx)
-	})
+// If no reference (branch, tag, or other ref) points to the fetched commit, it will be treated as unreachable
+// and cleaned up by auto-gc (a future push) or Gitea's "git gc" cronjob after the default prune expiration period.
+// Ref: https://www.kernel.org/pub/software/scm/git/docs/git-gc.html
+func FetchRemoteTempCommit(ctx context.Context, repo, remoteRepo RepositoryFacade, commitID string) error {
+	// since it is for temporary use, we don't need to update any ref or lock anything or maintain anything
+	return gitcmd.NewCommand("fetch", "--no-tags", "--no-write-commit-graph", "--no-write-fetch-head", "--no-auto-maintenance").
+		AddDynamicArguments(gitrepo.RepoLocalPath(remoteRepo)).
+		AddDynamicArguments(commitID).
+		WithRepo(repo).Run(ctx)
 }

modules/git/git.go

@@ -206,7 +206,3 @@ func runGitTests(m interface{ Run() int }) int {
 func LockConfigAndDo(ctx context.Context, repo RepositoryFacade, fn func(ctx context.Context) error) error {
 	return globallock.LockAndDo(ctx, "repo-config:"+repo.GitRepoManagedID(), fn)
 }
-
-func LockWriteAndDo(ctx context.Context, repo RepositoryFacade, fn func(ctx context.Context) error) error {
-	return globallock.LockAndDo(ctx, "repo-write:"+repo.GitRepoManagedID(), fn)
-}

services/git/compare.go

@@ -73,8 +73,8 @@ func GetCompareInfo(ctx context.Context, baseRepo, headRepo *repo_model.Reposito
 	if baseRepo.ID != headRepo.ID {
 		exist := headGitRepo.IsReferenceExist(ctx, compareInfo.BaseCommitID)
 		if !exist {
-			if err := git.FetchRemoteCommit(ctx, headRepo, baseRepo, compareInfo.BaseCommitID); err != nil {
-				return compareInfo, fmt.Errorf("FetchRemoteCommit: %w", err)
+			if err := git.FetchRemoteTempCommit(ctx, headRepo, baseRepo, compareInfo.BaseCommitID); err != nil {
+				return compareInfo, fmt.Errorf("FetchRemoteTempCommit: %w", err)
 			}
 		}
 	}

services/pull/merge_tree.go

@@ -90,11 +90,10 @@ func checkPullRequestMergeableByMergeTree(ctx context.Context, pr *issues_model.
 	}
 
 	// 4. fetch head commit id into the current repository
-	// it will be checked in 2 weeks by default from git if the pull request created failure.
 	if !pr.IsSameRepo() {
 		if !baseGitRepo.IsReferenceExist(ctx, pr.HeadCommitID) {
-			if err := git.FetchRemoteCommit(ctx, pr.BaseRepo, pr.HeadRepo, pr.HeadCommitID); err != nil {
-				return fmt.Errorf("FetchRemoteCommit: %w", err)
+			if err := git.FetchRemoteTempCommit(ctx, pr.BaseRepo, pr.HeadRepo, pr.HeadCommitID); err != nil {
+				return fmt.Errorf("FetchRemoteTempCommit: %w", err)
 			}
 		}
 	}