Add merge style fast-forward-only (#28954)

With this option, it is possible to require a linear commit history with
the following benefits over the next best option `Rebase+fast-forward`:
The original commits continue existing, with the original signatures
continuing to stay valid instead of being rewritten, there is no merge
commit, and reverting commits becomes easier.

Closes #24906

services/convert/repository.go

@@ -93,6 +93,7 @@ func innerToRepo(ctx context.Context, repo *repo_model.Repository, permissionInR
 	allowRebase := false
 	allowRebaseMerge := false
 	allowSquash := false
+	allowFastForwardOnly := false
 	allowRebaseUpdate := false
 	defaultDeleteBranchAfterMerge := false
 	defaultMergeStyle := repo_model.MergeStyleMerge
@@ -105,6 +106,7 @@ func innerToRepo(ctx context.Context, repo *repo_model.Repository, permissionInR
 		allowRebase = config.AllowRebase
 		allowRebaseMerge = config.AllowRebaseMerge
 		allowSquash = config.AllowSquash
+		allowFastForwardOnly = config.AllowFastForwardOnly
 		allowRebaseUpdate = config.AllowRebaseUpdate
 		defaultDeleteBranchAfterMerge = config.DefaultDeleteBranchAfterMerge
 		defaultMergeStyle = config.GetDefaultMergeStyle()
@@ -220,6 +222,7 @@ func innerToRepo(ctx context.Context, repo *repo_model.Repository, permissionInR
 		AllowRebase:                   allowRebase,
 		AllowRebaseMerge:              allowRebaseMerge,
 		AllowSquash:                   allowSquash,
+		AllowFastForwardOnly:          allowFastForwardOnly,
 		AllowRebaseUpdate:             allowRebaseUpdate,
 		DefaultDeleteBranchAfterMerge: defaultDeleteBranchAfterMerge,
 		DefaultMergeStyle:             string(defaultMergeStyle),

services/forms/repo_form.go

@@ -170,6 +170,7 @@ type RepoUnitSettingForm struct {
 	PullsAllowRebase                      bool
 	PullsAllowRebaseMerge                 bool
 	PullsAllowSquash                      bool
+	PullsAllowFastForwardOnly             bool
 	PullsAllowManualMerge                 bool
 	PullsDefaultMergeStyle                string
 	EnableAutodetectManualMerge           bool
@@ -609,8 +610,8 @@ func (f *InitializeLabelsForm) Validate(req *http.Request, errs binding.Errors)
 // swagger:model MergePullRequestOption
 type MergePullRequestForm struct {
 	// required: true
-	// enum: merge,rebase,rebase-merge,squash,manually-merged
-	Do                     string `binding:"Required;In(merge,rebase,rebase-merge,squash,manually-merged)"`
+	// enum: merge,rebase,rebase-merge,squash,fast-forward-only,manually-merged
+	Do                     string `binding:"Required;In(merge,rebase,rebase-merge,squash,fast-forward-only,manually-merged)"`
 	MergeTitleField        string
 	MergeMessageField      string
 	MergeCommitID          string // only used for manually-merged

services/pull/merge.go

@@ -273,6 +273,10 @@ func doMergeAndPush(ctx context.Context, pr *issues_model.PullRequest, doer *use
 		if err := doMergeStyleSquash(mergeCtx, message); err != nil {
 			return "", err
 		}
+	case repo_model.MergeStyleFastForwardOnly:
+		if err := doMergeStyleFastForwardOnly(mergeCtx); err != nil {
+			return "", err
+		}
 	default:
 		return "", models.ErrInvalidMergeStyle{ID: pr.BaseRepo.ID, Style: mergeStyle}
 	}
@@ -383,6 +387,13 @@ func runMergeCommand(ctx *mergeContext, mergeStyle repo_model.MergeStyle, cmd *g
 				StdErr: ctx.errbuf.String(),
 				Err:    err,
 			}
+		} else if mergeStyle == repo_model.MergeStyleFastForwardOnly && strings.Contains(ctx.errbuf.String(), "Not possible to fast-forward, aborting") {
+			log.Debug("MergeDivergingFastForwardOnly %-v: %v\n%s\n%s", ctx.pr, err, ctx.outbuf.String(), ctx.errbuf.String())
+			return models.ErrMergeDivergingFastForwardOnly{
+				StdOut: ctx.outbuf.String(),
+				StdErr: ctx.errbuf.String(),
+				Err:    err,
+			}
 		}
 		log.Error("git merge %-v: %v\n%s\n%s", ctx.pr, err, ctx.outbuf.String(), ctx.errbuf.String())
 		return fmt.Errorf("git merge %v: %w\n%s\n%s", ctx.pr, err, ctx.outbuf.String(), ctx.errbuf.String())

services/pull/merge_ff_only.go

@@ -0,0 +1,21 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package pull
+
+import (
+	repo_model "code.gitea.io/gitea/models/repo"
+	"code.gitea.io/gitea/modules/git"
+	"code.gitea.io/gitea/modules/log"
+)
+
+// doMergeStyleFastForwardOnly merges the tracking into the current HEAD - which is assumed to be staging branch (equal to the pr.BaseBranch)
+func doMergeStyleFastForwardOnly(ctx *mergeContext) error {
+	cmd := git.NewCommand(ctx, "merge", "--ff-only").AddDynamicArguments(trackingBranch)
+	if err := runMergeCommand(ctx, repo_model.MergeStyleFastForwardOnly, cmd); err != nil {
+		log.Error("%-v Unable to merge tracking into base: %v", ctx.pr, err)
+		return err
+	}
+
+	return nil
+}

services/pull/merge_merge.go

@@ -9,7 +9,7 @@ import (
 	"code.gitea.io/gitea/modules/log"
 )
 
-// doMergeStyleMerge merges the tracking into the current HEAD - which is assumed to tbe staging branch (equal to the pr.BaseBranch)
+// doMergeStyleMerge merges the tracking branch into the current HEAD - which is assumed to be the staging branch (equal to the pr.BaseBranch)
 func doMergeStyleMerge(ctx *mergeContext, message string) error {
 	cmd := git.NewCommand(ctx, "merge", "--no-ff", "--no-commit").AddDynamicArguments(trackingBranch)
 	if err := runMergeCommand(ctx, repo_model.MergeStyleMerge, cmd); err != nil {