Merge message template support for rebase without merge commit

docs/content/doc/usage/merge-message-templates.en-us.md

@@ -48,3 +48,10 @@ You can use the following variables enclosed in `${}` inside these templates whi
 - PullRequestIndex: Pull request's index number
 - PullRequestReference: Pull request's reference char with index number. i.e. #1, !2
 - ClosingIssues: return a string contains all issues which will be closed by this pull request i.e. `close #1, close #2`
+
+## Rebase
+
+When rebasing without a merge commit, `REBASE_TEMPLATE.md` modifies the message of the last commit. The following additional variables are available in this template:
+
+- CommitTitle: Commit's title
+- CommitBody: Commits's body text

services/pull/merge.go

@@ -34,8 +34,8 @@ import (
 	issue_service "code.gitea.io/gitea/services/issue"
 )
 
-// GetDefaultMergeMessage returns default message used when merging pull request
-func GetDefaultMergeMessage(ctx context.Context, baseGitRepo *git.Repository, pr *issues_model.PullRequest, mergeStyle repo_model.MergeStyle) (message, body string, err error) {
+// getMergeMessage composes the message used when merging a pull request.
+func getMergeMessage(ctx context.Context, baseGitRepo *git.Repository, pr *issues_model.PullRequest, mergeStyle repo_model.MergeStyle, extraVars map[string]string) (message, body string, err error) {
 	if err := pr.LoadBaseRepo(ctx); err != nil {
 		return "", "", err
 	}
@@ -81,6 +81,9 @@ func GetDefaultMergeMessage(ctx context.Context, baseGitRepo *git.Repository, pr
 				vars["HeadRepoOwnerName"] = pr.HeadRepo.OwnerName
 				vars["HeadRepoName"] = pr.HeadRepo.Name
 			}
+			for extraKey, extraValue := range extraVars {
+				vars[extraKey] = extraValue
+			}
 			refs, err := pr.ResolveCrossReferences(ctx)
 			if err == nil {
 				closeIssueIndexes := make([]string, 0, len(refs))
@@ -133,6 +136,11 @@ func expandDefaultMergeMessage(template string, vars map[string]string) (message
 	return os.Expand(message, mapping), os.Expand(body, mapping)
 }
 
+// GetDefaultMergeMessage returns default message used when merging pull request
+func GetDefaultMergeMessage(ctx context.Context, baseGitRepo *git.Repository, pr *issues_model.PullRequest, mergeStyle repo_model.MergeStyle) (message, body string, err error) {
+	return getMergeMessage(ctx, baseGitRepo, pr, mergeStyle, nil)
+}
+
 // Merge merges pull request to base repository.
 // Caller should check PR is ready to be merged (review and status checks)
 func Merge(ctx context.Context, pr *issues_model.PullRequest, doer *user_model.User, baseGitRepo *git.Repository, mergeStyle repo_model.MergeStyle, expectedHeadCommitID, message string, wasAutoMerged bool) error {

services/pull/merge_rebase.go

@@ -5,13 +5,99 @@ package pull
 
 import (
 	"fmt"
+	"strings"
 
 	repo_model "code.gitea.io/gitea/models/repo"
 	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/log"
 )
 
-// doMergeStyleRebase rebaases the tracking branch on the base branch as the current HEAD with or with a merge commit to the original pr branch
+// getRebaseAmendMessage composes the message to amend commits in rebase merge of a pull request.
+func getRebaseAmendMessage(ctx *mergeContext, baseGitRepo *git.Repository) (message string, err error) {
+	// Get existing commit message.
+	commitMessage, _, err := git.NewCommand(ctx, "show", "--format=%B", "-s").RunStdString(&git.RunOpts{Dir: ctx.tmpBasePath})
+	if err != nil {
+		return "", err
+	}
+
+	commitTitle, commitBody, _ := strings.Cut(commitMessage, "\n")
+	extraVars := map[string]string{"CommitTitle": strings.TrimSpace(commitTitle), "CommitBody": strings.TrimSpace(commitBody)}
+
+	message, body, err := getMergeMessage(ctx, baseGitRepo, ctx.pr, repo_model.MergeStyleRebase, extraVars)
+	if err != nil || message == "" {
+		return "", err
+	}
+
+	if len(body) > 0 {
+		message = message + "\n\n" + body
+	}
+	return message, err
+}
+
+// Perform rebase merge without merge commit.
+func doMergeRebaseFastForward(ctx *mergeContext) error {
+	baseHeadSHA, err := git.GetFullCommitID(ctx, ctx.tmpBasePath, "HEAD")
+	if err != nil {
+		return fmt.Errorf("Failed to get full commit id for HEAD: %w", err)
+	}
+
+	cmd := git.NewCommand(ctx, "merge", "--ff-only").AddDynamicArguments(stagingBranch)
+	if err := runMergeCommand(ctx, repo_model.MergeStyleRebase, cmd); err != nil {
+		log.Error("Unable to merge staging into base: %v", err)
+		return err
+	}
+
+	// Check if anything actually changed before we amend the message, fast forward can skip commits.
+	newMergeHeadSHA, err := git.GetFullCommitID(ctx, ctx.tmpBasePath, "HEAD")
+	if err != nil {
+		return fmt.Errorf("Failed to get full commit id for HEAD: %w", err)
+	}
+	if baseHeadSHA == newMergeHeadSHA {
+		return nil
+	}
+
+	// Original repo to read template from.
+	baseGitRepo, err := git.OpenRepository(ctx, ctx.pr.BaseRepo.RepoPath())
+	if err != nil {
+		log.Error("Unable to get Git repo for rebase: %v", err)
+		return err
+	}
+	defer baseGitRepo.Close()
+
+	// Amend last commit message based on template, if one exists
+	newMessage, err := getRebaseAmendMessage(ctx, baseGitRepo)
+	if err != nil {
+		log.Error("Unable to get commit message for amend: %v", err)
+		return err
+	}
+
+	if newMessage != "" {
+		if err := git.NewCommand(ctx, "commit", "--amend").AddOptionFormat("--message=%s", newMessage).Run(&git.RunOpts{Dir: ctx.tmpBasePath}); err != nil {
+			log.Error("Unable to amend commit message: %v", err)
+			return err
+		}
+	}
+
+	return nil
+}
+
+// Perform rebase merge with merge commit.
+func doMergeRebaseMergeCommit(ctx *mergeContext, message string) error {
+	cmd := git.NewCommand(ctx, "merge").AddArguments("--no-ff", "--no-commit").AddDynamicArguments(stagingBranch)
+
+	if err := runMergeCommand(ctx, repo_model.MergeStyleRebaseMerge, cmd); err != nil {
+		log.Error("Unable to merge staging into base: %v", err)
+		return err
+	}
+	if err := commitAndSignNoAuthor(ctx, message); err != nil {
+		log.Error("Unable to make final commit: %v", err)
+		return err
+	}
+
+	return nil
+}
+
+// doMergeStyleRebase rebases the tracking branch on the base branch as the current HEAD with or with a merge commit to the original pr branch
 func doMergeStyleRebase(ctx *mergeContext, mergeStyle repo_model.MergeStyle, message string) error {
 	if err := rebaseTrackingOnToBase(ctx, mergeStyle); err != nil {
 		return err
@@ -26,25 +112,9 @@ func doMergeStyleRebase(ctx *mergeContext, mergeStyle repo_model.MergeStyle, mes
 	ctx.outbuf.Reset()
 	ctx.errbuf.Reset()
 
-	cmd := git.NewCommand(ctx, "merge")
 	if mergeStyle == repo_model.MergeStyleRebase {
-		cmd.AddArguments("--ff-only")
-	} else {
-		cmd.AddArguments("--no-ff", "--no-commit")
-	}
-	cmd.AddDynamicArguments(stagingBranch)
-
-	// Prepare merge with commit
-	if err := runMergeCommand(ctx, mergeStyle, cmd); err != nil {
-		log.Error("Unable to merge staging into base: %v", err)
-		return err
-	}
-	if mergeStyle == repo_model.MergeStyleRebaseMerge {
-		if err := commitAndSignNoAuthor(ctx, message); err != nil {
-			log.Error("Unable to make final commit: %v", err)
-			return err
-		}
+		return doMergeRebaseFastForward(ctx)
 	}
 
-	return nil
+	return doMergeRebaseMergeCommit(ctx, message)
 }