Merge message template support for rebase without merge commit (#22669)

Use `default_merge_message/REBASE_TEMPLATE.md` for amending the message
of the last commit in the list of commits that was merged. Previously
this template was mentioned in the documentation but not actually used.

In this template additional variables `CommitTitle` and `CommitBody` are
available, for the title and body of the commit.

Ideally the message of every commit would be updated using the template,
but doing an interactive rebase or merging commits one by one is
complicated, so that is left as a future improvement.

2 files changed, 99 insertions, 21 deletions

diff --git a/services/pull/merge.go b/services/pull/merge.go
index 12e01e8ce7..85bb90b853 100644
--- a/services/pull/merge.go
+++ b/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 {
diff --git a/services/pull/merge_rebase.go b/services/pull/merge_rebase.go
index d3bb86d4aa..a88f805ef0 100644
--- a/services/pull/merge_rebase.go
+++ b/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)
 }