Improve sync fork behavior (#33319)

Fix #33271

Suppose there is a `branch-a` in fork repo:

1. if `branch-a` exists in base repo: try to sync `base:branch-a` to `fork:branch-a`
2. if `branch-a` doesn't exist in base repo: try to sync `base:main` to `fork:branch-a`

services/repository/branch.go

@@ -668,9 +668,12 @@ func SetRepoDefaultBranch(ctx context.Context, repo *repo_model.Repository, gitR
 }
 
 // BranchDivergingInfo contains the information about the divergence of a head branch to the base branch.
-// This struct is also used in templates, so it needs to search for all references before changing it.
 type BranchDivergingInfo struct {
+	// whether the base branch contains new commits which are not in the head branch
 	BaseHasNewCommits bool
+
+	// behind/after are number of commits that the head branch is behind/after the base branch, it's 0 if it's unable to calculate.
+	// there could be a case that BaseHasNewCommits=true while the behind/after are both 0 (unable to calculate).
 	HeadCommitsBehind int
 	HeadCommitsAhead  int
 }
@@ -681,11 +684,20 @@ func GetBranchDivergingInfo(ctx reqctx.RequestContext, baseRepo *repo_model.Repo
 	if err != nil {
 		return nil, err
 	}
-
+	if headGitBranch.IsDeleted {
+		return nil, git_model.ErrBranchNotExist{
+			BranchName: headBranch,
+		}
+	}
 	baseGitBranch, err := git_model.GetBranch(ctx, baseRepo.ID, baseBranch)
 	if err != nil {
 		return nil, err
 	}
+	if baseGitBranch.IsDeleted {
+		return nil, git_model.ErrBranchNotExist{
+			BranchName: baseBranch,
+		}
+	}
 
 	info := &BranchDivergingInfo{}
 	if headGitBranch.CommitID == baseGitBranch.CommitID {
@@ -720,5 +732,6 @@ func GetBranchDivergingInfo(ctx reqctx.RequestContext, baseRepo *repo_model.Repo
 	}
 
 	info.HeadCommitsBehind, info.HeadCommitsAhead = diff.Behind, diff.Ahead
+	info.BaseHasNewCommits = info.HeadCommitsBehind > 0
 	return info, nil
 }

services/repository/merge_upstream.go

@@ -4,7 +4,7 @@
 package repository
 
 import (
-	"context"
+	"errors"
 	"fmt"
 
 	issue_model "code.gitea.io/gitea/models/issues"
@@ -18,16 +18,24 @@ import (
 )
 
 // MergeUpstream merges the base repository's default branch into the fork repository's current branch.
-func MergeUpstream(ctx context.Context, doer *user_model.User, repo *repo_model.Repository, branch string) (mergeStyle string, err error) {
+func MergeUpstream(ctx reqctx.RequestContext, doer *user_model.User, repo *repo_model.Repository, branch string) (mergeStyle string, err error) {
 	if err = repo.MustNotBeArchived(); err != nil {
 		return "", err
 	}
 	if err = repo.GetBaseRepo(ctx); err != nil {
 		return "", err
 	}
+	divergingInfo, err := GetUpstreamDivergingInfo(ctx, repo, branch)
+	if err != nil {
+		return "", err
+	}
+	if !divergingInfo.BaseBranchHasNewCommits {
+		return "up-to-date", nil
+	}
+
 	err = git.Push(ctx, repo.BaseRepo.RepoPath(), git.PushOptions{
 		Remote: repo.RepoPath(),
-		Branch: fmt.Sprintf("%s:%s", repo.BaseRepo.DefaultBranch, branch),
+		Branch: fmt.Sprintf("%s:%s", divergingInfo.BaseBranchName, branch),
 		Env:    repo_module.PushingEnvironment(doer, repo),
 	})
 	if err == nil {
@@ -59,7 +67,7 @@ func MergeUpstream(ctx context.Context, doer *user_model.User, repo *repo_model.
 		BaseRepoID: repo.BaseRepo.ID,
 		BaseRepo:   repo.BaseRepo,
 		HeadBranch: branch, // maybe HeadCommitID is not needed
-		BaseBranch: repo.BaseRepo.DefaultBranch,
+		BaseBranch: divergingInfo.BaseBranchName,
 	}
 	fakeIssue.PullRequest = fakePR
 	err = pull.Update(ctx, fakePR, doer, "merge upstream", false)
@@ -69,8 +77,15 @@ func MergeUpstream(ctx context.Context, doer *user_model.User, repo *repo_model.
 	return "merge", nil
 }
 
+// UpstreamDivergingInfo is also used in templates, so it needs to search for all references before changing it.
+type UpstreamDivergingInfo struct {
+	BaseBranchName          string
+	BaseBranchHasNewCommits bool
+	HeadBranchCommitsBehind int
+}
+
 // GetUpstreamDivergingInfo returns the information about the divergence between the fork repository's branch and the base repository's default branch.
-func GetUpstreamDivergingInfo(ctx reqctx.RequestContext, forkRepo *repo_model.Repository, forkBranch string) (*BranchDivergingInfo, error) {
+func GetUpstreamDivergingInfo(ctx reqctx.RequestContext, forkRepo *repo_model.Repository, forkBranch string) (*UpstreamDivergingInfo, error) {
 	if !forkRepo.IsFork {
 		return nil, util.NewInvalidArgumentErrorf("repo is not a fork")
 	}
@@ -83,5 +98,26 @@ func GetUpstreamDivergingInfo(ctx reqctx.RequestContext, forkRepo *repo_model.Re
 		return nil, err
 	}
 
-	return GetBranchDivergingInfo(ctx, forkRepo.BaseRepo, forkRepo.BaseRepo.DefaultBranch, forkRepo, forkBranch)
+	// Do the best to follow the GitHub's behavior, suppose there is a `branch-a` in fork repo:
+	// * if `branch-a` exists in base repo: try to sync `base:branch-a` to `fork:branch-a`
+	// * if `branch-a` doesn't exist in base repo: try to sync `base:main` to `fork:branch-a`
+	info, err := GetBranchDivergingInfo(ctx, forkRepo.BaseRepo, forkBranch, forkRepo, forkBranch)
+	if err == nil {
+		return &UpstreamDivergingInfo{
+			BaseBranchName:          forkBranch,
+			BaseBranchHasNewCommits: info.BaseHasNewCommits,
+			HeadBranchCommitsBehind: info.HeadCommitsBehind,
+		}, nil
+	}
+	if errors.Is(err, util.ErrNotExist) {
+		info, err = GetBranchDivergingInfo(ctx, forkRepo.BaseRepo, forkRepo.BaseRepo.DefaultBranch, forkRepo, forkBranch)
+		if err == nil {
+			return &UpstreamDivergingInfo{
+				BaseBranchName:          forkRepo.BaseRepo.DefaultBranch,
+				BaseBranchHasNewCommits: info.BaseHasNewCommits,
+				HeadBranchCommitsBehind: info.HeadCommitsBehind,
+			}, nil
+		}
+	}
+	return nil, err
 }