Support updating branch via API (#35951)

Resolve #35368

services/repository/branch.go

@@ -482,6 +482,64 @@ func RenameBranch(ctx context.Context, repo *repo_model.Repository, doer *user_m
 	return "", nil
 }
 
+// UpdateBranch moves a branch reference to the provided commit. permission check should be done before calling this function.
+func UpdateBranch(ctx context.Context, repo *repo_model.Repository, gitRepo *git.Repository, doer *user_model.User, branchName, newCommitID, expectedOldCommitID string, force bool) error {
+	branch, err := git_model.GetBranch(ctx, repo.ID, branchName)
+	if err != nil {
+		return err
+	}
+	if branch.IsDeleted {
+		return git_model.ErrBranchNotExist{
+			BranchName: branchName,
+		}
+	}
+
+	if expectedOldCommitID != "" {
+		expectedID, err := gitRepo.ConvertToGitID(expectedOldCommitID)
+		if err != nil {
+			return fmt.Errorf("ConvertToGitID(old): %w", err)
+		}
+		if expectedID.String() != branch.CommitID {
+			return util.NewInvalidArgumentErrorf("branch commit does not match [expected: %s, given: %s]", expectedID.String(), branch.CommitID)
+		}
+	}
+
+	newID, err := gitRepo.ConvertToGitID(newCommitID)
+	if err != nil {
+		return fmt.Errorf("ConvertToGitID(new): %w", err)
+	}
+	newCommit, err := gitRepo.GetCommit(newID.String())
+	if err != nil {
+		return err
+	}
+
+	if newCommit.ID.String() == branch.CommitID {
+		return nil
+	}
+
+	isForcePush, err := newCommit.IsForcePush(branch.CommitID)
+	if err != nil {
+		return err
+	}
+	if isForcePush && !force {
+		return util.NewInvalidArgumentErrorf("Force push %s need a confirm force parameter", branchName)
+	}
+
+	pushOpts := git.PushOptions{
+		Remote: repo.RepoPath(),
+		Branch: fmt.Sprintf("%s:%s%s", newCommit.ID.String(), git.BranchPrefix, branchName),
+		Env:    repo_module.PushingEnvironment(doer, repo),
+		Force:  isForcePush || force,
+	}
+
+	if expectedOldCommitID != "" {
+		pushOpts.ForceWithLease = fmt.Sprintf("%s:%s", git.BranchPrefix+branchName, branch.CommitID)
+	}
+
+	// branch protection will be checked in the pre received hook, so that we don't need any check here
+	return gitrepo.Push(ctx, repo, repo, pushOpts)
+}
+
 var ErrBranchIsDefault = util.ErrorWrap(util.ErrPermissionDenied, "branch is default")
 
 func CanDeleteBranch(ctx context.Context, repo *repo_model.Repository, branchName string, doer *user_model.User) error {