Some refactors about GetMergeBase (#36186)

Maybe fix #32018

- Use `gitrepo.GetMergeBase` method instead of other two
implementations.
- Add `FetchRemoteCommit` so that we don't need to add many `remote` to
the git repository to avoid possible git lock conflicts. A lock will
start when invoke the function, it will be invoked when cross-repository
comparing. The head repository will fetch the base repository's base
commit id. In most situations, it should lock the fork repositories so
that it should not become a bottleneck.
- Improve `GetCompareInfo` to remove unnecessarily adding remote.
- Remove unnecessary parameters of `SignMerge`.

modules/git/repo_compare.go

@@ -18,32 +18,6 @@ import (
 	"code.gitea.io/gitea/modules/git/gitcmd"
 )
 
-// GetMergeBase checks and returns merge base of two branches and the reference used as base.
-func (repo *Repository) GetMergeBase(tmpRemote, base, head string) (string, string, error) {
-	if tmpRemote == "" {
-		tmpRemote = "origin"
-	}
-
-	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 := gitcmd.NewCommand("fetch", "--no-tags").
-			AddDynamicArguments(tmpRemote).
-			AddDashesAndList(base + ":" + tmpBaseName).
-			WithDir(repo.Path).
-			RunStdString(repo.Ctx)
-		if err == nil {
-			base = tmpBaseName
-		}
-	}
-
-	stdout, _, err := gitcmd.NewCommand("merge-base").
-		AddDashesAndList(base, head).
-		WithDir(repo.Path).
-		RunStdString(repo.Ctx)
-	return strings.TrimSpace(stdout), base, err
-}
-
 type lineCountWriter struct {
 	numLines int
 }

modules/gitrepo/fetch.go

@@ -0,0 +1,28 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gitrepo
+
+import (
+	"context"
+
+	"code.gitea.io/gitea/modules/git/gitcmd"
+	"code.gitea.io/gitea/modules/globallock"
+)
+
+// FetchRemoteCommit fetches a specific commit and its related objects from a remote
+// repository into the managed repository.
+//
+// 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 Repository, commitID string) error {
+	return globallock.LockAndDo(ctx, getRepoWriteLockKey(repo.RelativePath()), func(ctx context.Context) error {
+		return RunCmd(ctx, repo, gitcmd.NewCommand("fetch", "--no-tags").
+			AddDynamicArguments(repoPath(remoteRepo)).
+			AddDynamicArguments(commitID))
+	})
+}

modules/gitrepo/merge.go

@@ -0,0 +1,22 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gitrepo
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"code.gitea.io/gitea/modules/git/gitcmd"
+)
+
+// MergeBase checks and returns merge base of two commits.
+func MergeBase(ctx context.Context, repo Repository, baseCommitID, headCommitID string) (string, error) {
+	mergeBase, err := RunCmdString(ctx, repo, gitcmd.NewCommand("merge-base").
+		AddDashesAndList(baseCommitID, headCommitID))
+	if err != nil {
+		return "", fmt.Errorf("get merge-base of %s and %s failed: %w", baseCommitID, headCommitID, err)
+	}
+	return strings.TrimSpace(mergeBase), nil
+}

modules/gitrepo/repo_lock.go

@@ -0,0 +1,10 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gitrepo
+
+// getRepoWriteLockKey returns the global lock key for write operations on the repository.
+// Parallel write operations on the same git repository should be avoided to prevent data corruption.
+func getRepoWriteLockKey(repoStoragePath string) string {
+	return "repo-write:" + repoStoragePath
+}

routers/web/repo/issue_view.go

@@ -495,7 +495,7 @@ func preparePullViewSigning(ctx *context.Context, issue *issues_model.Issue) {
 	pull := issue.PullRequest
 	ctx.Data["WillSign"] = false
 	if ctx.Doer != nil {
-		sign, key, _, err := asymkey_service.SignMerge(ctx, pull, ctx.Doer, ctx.Repo.GitRepo, pull.BaseBranch, pull.GetGitHeadRefName())
+		sign, key, _, err := asymkey_service.SignMerge(ctx, pull, ctx.Doer, ctx.Repo.GitRepo)
 		ctx.Data["WillSign"] = sign
 		ctx.Data["SigningKeyMergeDisplay"] = asymkey_model.GetDisplaySigningKey(key)
 		if err != nil {

services/asymkey/sign.go

@@ -271,13 +271,22 @@ Loop:
 }
 
 // SignMerge determines if we should sign a PR merge commit to the base repository
-func SignMerge(ctx context.Context, pr *issues_model.PullRequest, u *user_model.User, gitRepo *git.Repository, baseCommit, headCommit string) (bool, *git.SigningKey, *git.Signature, error) {
+func SignMerge(ctx context.Context, pr *issues_model.PullRequest, u *user_model.User, gitRepo *git.Repository) (bool, *git.SigningKey, *git.Signature, error) {
 	if err := pr.LoadBaseRepo(ctx); err != nil {
 		log.Error("Unable to get Base Repo for pull request")
 		return false, nil, nil, err
 	}
 	repo := pr.BaseRepo
 
+	baseCommit, err := gitRepo.GetCommit(pr.BaseBranch)
+	if err != nil {
+		return false, nil, nil, err
+	}
+	headCommit, err := gitRepo.GetCommit(pr.GetGitHeadRefName())
+	if err != nil {
+		return false, nil, nil, err
+	}
+
 	signingKey, signer := gitrepo.GetSigningKey(ctx)
 	if signingKey == nil {
 		return false, nil, nil, &ErrWontSign{noKey}
@@ -319,38 +328,26 @@ Loop:
 				return false, nil, nil, &ErrWontSign{approved}
 			}
 		case baseSigned:
-			commit, err := gitRepo.GetCommit(baseCommit)
+			verification := ParseCommitWithSignature(ctx, baseCommit)
-			if err != nil {
-				return false, nil, nil, err
-			}
-			verification := ParseCommitWithSignature(ctx, commit)
 			if !verification.Verified {
 				return false, nil, nil, &ErrWontSign{baseSigned}
 			}
 		case headSigned:
-			commit, err := gitRepo.GetCommit(headCommit)
+			verification := ParseCommitWithSignature(ctx, headCommit)
-			if err != nil {
-				return false, nil, nil, err
-			}
-			verification := ParseCommitWithSignature(ctx, commit)
 			if !verification.Verified {
 				return false, nil, nil, &ErrWontSign{headSigned}
 			}
 		case commitsSigned:
-			commit, err := gitRepo.GetCommit(headCommit)
+			verification := ParseCommitWithSignature(ctx, headCommit)
-			if err != nil {
-				return false, nil, nil, err
-			}
-			verification := ParseCommitWithSignature(ctx, commit)
 			if !verification.Verified {
 				return false, nil, nil, &ErrWontSign{commitsSigned}
 			}
 			// need to work out merge-base
-			mergeBaseCommit, _, err := gitRepo.GetMergeBase("", baseCommit, headCommit)
+			mergeBaseCommit, err := gitrepo.MergeBase(ctx, pr.BaseRepo, baseCommit.ID.String(), headCommit.ID.String())
 			if err != nil {
 				return false, nil, nil, err
 			}
-			commitList, err := commit.CommitsBeforeUntil(mergeBaseCommit)
+			commitList, err := headCommit.CommitsBeforeUntil(mergeBaseCommit)
 			if err != nil {
 				return false, nil, nil, err
 			}

services/git/compare.go

@@ -6,14 +6,10 @@ package git
 import (
 	"context"
 	"fmt"
-	"strconv"
-	"time"
 
 	repo_model "code.gitea.io/gitea/models/repo"
 	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/gitrepo"
-	"code.gitea.io/gitea/modules/graceful"
-	logger "code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/util"
 )
 
@@ -48,25 +44,6 @@ func (ci *CompareInfo) DirectComparison() bool {
 
 // GetCompareInfo generates and returns compare information between base and head branches of repositories.
 func GetCompareInfo(ctx context.Context, baseRepo, headRepo *repo_model.Repository, headGitRepo *git.Repository, baseRef, headRef git.RefName, directComparison, fileOnly bool) (_ *CompareInfo, err error) {
-	var (
-		remoteBranch string
-		tmpRemote    string
-	)
-
-	// We don't need a temporary remote for same repository.
-	if baseRepo.ID != headRepo.ID {
-		// Add a temporary remote
-		tmpRemote = strconv.FormatInt(time.Now().UnixNano(), 10)
-		if err = gitrepo.GitRemoteAdd(ctx, headRepo, tmpRemote, baseRepo.RepoPath()); err != nil {
-			return nil, fmt.Errorf("GitRemoteAdd: %w", err)
-		}
-		defer func() {
-			if err := gitrepo.GitRemoteRemove(graceful.GetManager().ShutdownContext(), headRepo, tmpRemote); err != nil {
-				logger.Error("GetPullRequestInfo: GitRemoteRemove: %v", err)
-			}
-		}()
-	}
-
 	compareInfo := &CompareInfo{
 		BaseRepo:         baseRepo,
 		BaseRef:          baseRef,
@@ -76,47 +53,49 @@ func GetCompareInfo(ctx context.Context, baseRepo, headRepo *repo_model.Reposito
 		CompareSeparator: util.Iif(directComparison, "..", "..."),
 	}
 
+	compareInfo.BaseCommitID, err = gitrepo.GetFullCommitID(ctx, baseRepo, baseRef.String())
+	if err != nil {
+		return nil, err
+	}
 	compareInfo.HeadCommitID, err = gitrepo.GetFullCommitID(ctx, headRepo, headRef.String())
 	if err != nil {
-		compareInfo.HeadCommitID = headRef.String()
+		return nil, err
 	}
 
-	// FIXME: It seems we don't need mergebase if it's a direct comparison?
+	// if they are not the same repository, then we need to fetch the base commit into the head repository
-	compareInfo.MergeBase, remoteBranch, err = headGitRepo.GetMergeBase(tmpRemote, baseRef.String(), headRef.String())
+	// because we will use headGitRepo in the following code
-	if err == nil {
+	if baseRepo.ID != headRepo.ID {
-		compareInfo.BaseCommitID, err = gitrepo.GetFullCommitID(ctx, headRepo, remoteBranch)
+		exist := headGitRepo.IsReferenceExist(compareInfo.BaseCommitID)
-		if err != nil {
+		if !exist {
-			compareInfo.BaseCommitID = remoteBranch
+			if err := gitrepo.FetchRemoteCommit(ctx, headRepo, baseRepo, compareInfo.BaseCommitID); err != nil {
-		}
+				return nil, fmt.Errorf("FetchRemoteCommit: %w", err)
-		separator := "..."
-		baseCommitID := compareInfo.MergeBase
-		if directComparison {
-			separator = ".."
-			baseCommitID = compareInfo.BaseCommitID
-		}
-
-		// We have a common base - therefore we know that ... should work
-		if !fileOnly {
-			compareInfo.Commits, err = headGitRepo.ShowPrettyFormatLogToList(ctx, baseCommitID+separator+headRef.String())
-			if err != nil {
-				return nil, fmt.Errorf("ShowPrettyFormatLogToList: %w", err)
 			}
-		} else {
+		}
-			compareInfo.Commits = []*git.Commit{}
+	}
+
+	if !directComparison {
+		compareInfo.MergeBase, err = gitrepo.MergeBase(ctx, headRepo, compareInfo.BaseCommitID, compareInfo.HeadCommitID)
+		if err != nil {
+			return nil, fmt.Errorf("MergeBase: %w", err)
+		}
+	} else {
+		compareInfo.MergeBase = compareInfo.BaseCommitID
+	}
+
+	// We have a common base - therefore we know that ... should work
+	if !fileOnly {
+		compareInfo.Commits, err = headGitRepo.ShowPrettyFormatLogToList(ctx, compareInfo.BaseCommitID+compareInfo.CompareSeparator+compareInfo.HeadCommitID)
+		if err != nil {
+			return nil, fmt.Errorf("ShowPrettyFormatLogToList: %w", err)
 		}
 	} else {
 		compareInfo.Commits = []*git.Commit{}
-		compareInfo.MergeBase, err = gitrepo.GetFullCommitID(ctx, headRepo, remoteBranch)
-		if err != nil {
-			compareInfo.MergeBase = remoteBranch
-		}
-		compareInfo.BaseCommitID = compareInfo.MergeBase
 	}
 
 	// Count number of changed files.
 	// This probably should be removed as we need to use shortstat elsewhere
 	// Now there is git diff --shortstat but this appears to be slower than simply iterating with --nameonly
-	compareInfo.NumFiles, err = headGitRepo.GetDiffNumChangedFiles(remoteBranch, headRef.String(), directComparison)
+	compareInfo.NumFiles, err = headGitRepo.GetDiffNumChangedFiles(compareInfo.BaseCommitID, compareInfo.HeadCommitID, directComparison)
 	if err != nil {
 		return nil, err
 	}

services/issue/pull.go

@@ -7,35 +7,16 @@ import (
 	"context"
 	"fmt"
 	"slices"
-	"time"
 
 	issues_model "code.gitea.io/gitea/models/issues"
 	org_model "code.gitea.io/gitea/models/organization"
-	repo_model "code.gitea.io/gitea/models/repo"
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/gitrepo"
-	"code.gitea.io/gitea/modules/graceful"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
 )
 
-func getMergeBase(ctx context.Context, repo *repo_model.Repository, gitRepo *git.Repository, pr *issues_model.PullRequest, baseBranch, headBranch string) (string, error) {
-	// Add a temporary remote
-	tmpRemote := fmt.Sprintf("mergebase-%d-%d", pr.ID, time.Now().UnixNano())
-	if err := gitrepo.GitRemoteAdd(ctx, repo, tmpRemote, gitRepo.Path); err != nil {
-		return "", fmt.Errorf("GitRemoteAdd: %w", err)
-	}
-	defer func() {
-		if err := gitrepo.GitRemoteRemove(graceful.GetManager().ShutdownContext(), repo, tmpRemote); err != nil {
-			log.Error("getMergeBase: GitRemoteRemove: %v", err)
-		}
-	}()
-
-	mergeBase, _, err := gitRepo.GetMergeBase(tmpRemote, baseBranch, headBranch)
-	return mergeBase, err
-}
-
 type ReviewRequestNotifier struct {
 	Comment    *issues_model.Comment
 	IsAdd      bool
@@ -99,11 +80,10 @@ func PullRequestCodeOwnersReview(ctx context.Context, pr *issues_model.PullReque
 	}
 
 	// get the mergebase
-	mergeBase, err := getMergeBase(ctx, pr.BaseRepo, repo, pr, git.BranchPrefix+pr.BaseBranch, pr.GetGitHeadRefName())
+	mergeBase, err := gitrepo.MergeBase(ctx, pr.BaseRepo, git.BranchPrefix+pr.BaseBranch, pr.GetGitHeadRefName())
 	if err != nil {
 		return nil, err
 	}
-
 	// https://github.com/go-gitea/gitea/issues/29763, we need to get the files changed
 	// between the merge base and the head commit but not the base branch and the head commit
 	changedFiles, err := repo.GetFilesChangedBetween(mergeBase, pr.GetGitHeadRefName())

services/migrations/gitea_uploader.go

@@ -732,7 +732,7 @@ func (g *GiteaLocalUploader) newPullRequest(ctx context.Context, pr *base.PullRe
 		if pr.Base.Ref != "" && pr.Head.SHA != "" {
 			// A PR against a tag base does not make sense - therefore pr.Base.Ref must be a branch
 			// TODO: should we be checking for the refs/heads/ prefix on the pr.Base.Ref? (i.e. are these actually branches or refs)
-			pr.Base.SHA, _, err = g.gitRepo.GetMergeBase("", git.BranchPrefix+pr.Base.Ref, pr.Head.SHA)
+			pr.Base.SHA, err = gitrepo.MergeBase(ctx, g.repo, git.BranchPrefix+pr.Base.Ref, pr.Head.SHA)
 			if err != nil {
 				log.Error("Cannot determine the merge base for PR #%d in %s/%s. Error: %v", pr.Number, g.repoOwner, g.repoName, err)
 			}

services/pull/check.go

@@ -238,7 +238,7 @@ func isSignedIfRequired(ctx context.Context, pr *issues_model.PullRequest, doer
 	}
 	defer closer.Close()
 
-	sign, _, _, err := asymkey_service.SignMerge(ctx, pr, doer, gitRepo, pr.BaseBranch, pr.GetGitHeadRefName())
+	sign, _, _, err := asymkey_service.SignMerge(ctx, pr, doer, gitRepo)
 
 	return sign, err
 }

services/pull/merge_prepare.go

@@ -19,6 +19,7 @@ import (
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/git/gitcmd"
+	"code.gitea.io/gitea/modules/gitrepo"
 	"code.gitea.io/gitea/modules/log"
 	asymkey_service "code.gitea.io/gitea/services/asymkey"
 )
@@ -105,7 +106,7 @@ func createTemporaryRepoForMerge(ctx context.Context, pr *issues_model.PullReque
 	mergeCtx.sig = doer.NewGitSig()
 	mergeCtx.committer = mergeCtx.sig
 
-	gitRepo, err := git.OpenRepository(ctx, mergeCtx.tmpBasePath)
+	gitRepo, err := gitrepo.OpenRepository(ctx, pr.BaseRepo)
 	if err != nil {
 		defer cancel()
 		return nil, nil, fmt.Errorf("failed to open temp git repo for pr[%d]: %w", mergeCtx.pr.ID, err)
@@ -113,7 +114,7 @@ func createTemporaryRepoForMerge(ctx context.Context, pr *issues_model.PullReque
 	defer gitRepo.Close()
 
 	// Determine if we should sign
-	sign, key, signer, _ := asymkey_service.SignMerge(ctx, mergeCtx.pr, mergeCtx.doer, gitRepo, "HEAD", trackingBranch)
+	sign, key, signer, _ := asymkey_service.SignMerge(ctx, pr, doer, gitRepo)
 	if sign {
 		mergeCtx.signKey = key
 		if pr.BaseRepo.GetTrustModel() == repo_model.CommitterTrustModel || pr.BaseRepo.GetTrustModel() == repo_model.CollaboratorCommitterTrustModel {

services/pull/pull.go

@@ -521,14 +521,7 @@ func checkIfPRContentChanged(ctx context.Context, pr *issues_model.PullRequest,
 	}
 	defer cancel()
 
-	tmpRepo, err := git.OpenRepository(ctx, prCtx.tmpBasePath)
+	mergeBase, err = gitrepo.MergeBase(ctx, pr.BaseRepo, pr.BaseBranch, pr.GetGitHeadRefName())
-	if err != nil {
-		return false, "", fmt.Errorf("OpenRepository: %w", err)
-	}
-	defer tmpRepo.Close()
-
-	// Find the merge-base
-	mergeBase, _, err = tmpRepo.GetMergeBase("", "base", "tracking")
 	if err != nil {
 		return false, "", fmt.Errorf("GetMergeBase: %w", err)
 	}