Add merge style fast-forward-only (go-gitea#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 go-gitea#24906

custom/conf/app.example.ini

@@ -1044,7 +1044,7 @@ LEVEL = Info
 ;; List of keywords used in Pull Request comments to automatically reopen a related issue
 ;REOPEN_KEYWORDS = reopen,reopens,reopened
 ;;
-;; Set default merge style for repository creating, valid options: merge, rebase, rebase-merge, squash
+;; Set default merge style for repository creating, valid options: merge, rebase, rebase-merge, squash, fast-forward-only
 ;DEFAULT_MERGE_STYLE = merge
 ;;
 ;; In the default merge message for squash commits include at most this many commits

docs/content/administration/config-cheat-sheet.en-us.md

@@ -126,7 +126,7 @@ In addition, there is _`StaticRootPath`_ which can be set as a built-in at build
  keywords used in Pull Request comments to automatically close a related issue
 - `REOPEN_KEYWORDS`: **reopen**, **reopens**, **reopened**: List of keywords used in Pull Request comments to automatically reopen
  a related issue
-- `DEFAULT_MERGE_STYLE`: **merge**: Set default merge style for repository creating, valid options: `merge`, `rebase`, `rebase-merge`, `squash`
+- `DEFAULT_MERGE_STYLE`: **merge**: Set default merge style for repository creating, valid options: `merge`, `rebase`, `rebase-merge`, `squash`, `fast-forward-only`
 - `DEFAULT_MERGE_MESSAGE_COMMITS_LIMIT`: **50**: In the default merge message for squash commits include at most this many commits. Set to `-1` to include all commits
 - `DEFAULT_MERGE_MESSAGE_SIZE`: **5120**: In the default merge message for squash commits limit the size of the commit messages. Set to `-1` to have no limit. Only used if `POPULATE_SQUASH_COMMENT_WITH_COMMIT_MESSAGES` is `true`.
 - `DEFAULT_MERGE_MESSAGE_ALL_AUTHORS`: **false**: In the default merge message for squash commits walk all commits to include all authors in the Co-authored-by otherwise just use those in the limited list

docs/content/administration/config-cheat-sheet.zh-cn.md

@@ -125,7 +125,7 @@ menu:
 - `CLOSE_KEYWORDS`: **close**, **closes**, **closed**, **fix**, **fixes**, **fixed**, **resolve**, **resolves**, **resolved**: 在拉取请求评论中用于自动关闭相关问题的关键词列表。
 - `REOPEN_KEYWORDS`: **reopen**, **reopens**, **reopened**: 在拉取请求评论中用于自动重新打开相关问题的
 关键词列表。
-- `DEFAULT_MERGE_STYLE`: **merge**: 设置创建仓库的默认合并方式，可选: `merge`, `rebase`, `rebase-merge`, `squash`
+- `DEFAULT_MERGE_STYLE`: **merge**: 设置创建仓库的默认合并方式，可选: `merge`, `rebase`, `rebase-merge`, `squash`, `fast-forward-only`
 - `DEFAULT_MERGE_MESSAGE_COMMITS_LIMIT`: **50**: 在默认合并消息中，对于`squash`提交，最多包括此数量的提交。设置为 -1 以包括所有提交。
 - `DEFAULT_MERGE_MESSAGE_SIZE`: **5120**: 在默认的合并消息中，对于`squash`提交，限制提交消息的大小。设置为 `-1`以取消限制。仅在`POPULATE_SQUASH_COMMENT_WITH_COMMIT_MESSAGES`为`true`时使用。
 - `DEFAULT_MERGE_MESSAGE_ALL_AUTHORS`: **false**: 在默认合并消息中，对于`squash`提交，遍历所有提交以包括所有作者的`Co-authored-by`，否则仅使用限定列表中的作者。

models/error.go

@@ -493,6 +493,23 @@ func (err ErrMergeUnrelatedHistories) Error() string {
 	return fmt.Sprintf("Merge UnrelatedHistories Error: %v: %s\n%s", err.Err, err.StdErr, err.StdOut)
 }
 
+// ErrMergeDivergingFastForwardOnly represents an error if a fast-forward-only merge fails because the branches diverge
+type ErrMergeDivergingFastForwardOnly struct {
+	StdOut string
+	StdErr string
+	Err    error
+}
+
+// IsErrMergeDivergingFastForwardOnly checks if an error is a ErrMergeDivergingFastForwardOnly.
+func IsErrMergeDivergingFastForwardOnly(err error) bool {
+	_, ok := err.(ErrMergeDivergingFastForwardOnly)
+	return ok
+}
+
+func (err ErrMergeDivergingFastForwardOnly) Error() string {
+	return fmt.Sprintf("Merge DivergingFastForwardOnly Error: %v: %s\n%s", err.Err, err.StdErr, err.StdOut)
+}
+
 // ErrRebaseConflicts represents an error if rebase fails with a conflict
 type ErrRebaseConflicts struct {
 	Style     repo_model.MergeStyle

models/repo/git.go

@@ -21,6 +21,8 @@ const (
 	MergeStyleRebaseMerge MergeStyle = "rebase-merge"
 	// MergeStyleSquash squash commits into single commit before merging
 	MergeStyleSquash MergeStyle = "squash"
+	// MergeStyleFastForwardOnly fast-forward merge if possible, otherwise fail
+	MergeStyleFastForwardOnly MergeStyle = "fast-forward-only"
 	// MergeStyleManuallyMerged pr has been merged manually, just mark it as merged directly
 	MergeStyleManuallyMerged MergeStyle = "manually-merged"
 	// MergeStyleRebaseUpdate not a merge style, used to update pull head by rebase

models/repo/repo_unit.go

@@ -122,6 +122,7 @@ type PullRequestsConfig struct {
 	AllowRebase                   bool
 	AllowRebaseMerge              bool
 	AllowSquash                   bool
+	AllowFastForwardOnly          bool
 	AllowManualMerge              bool
 	AutodetectManualMerge         bool
 	AllowRebaseUpdate             bool
@@ -148,6 +149,7 @@ func (cfg *PullRequestsConfig) IsMergeStyleAllowed(mergeStyle MergeStyle) bool {
 		mergeStyle == MergeStyleRebase && cfg.AllowRebase ||
 		mergeStyle == MergeStyleRebaseMerge && cfg.AllowRebaseMerge ||
 		mergeStyle == MergeStyleSquash && cfg.AllowSquash ||
+		mergeStyle == MergeStyleFastForwardOnly && cfg.AllowFastForwardOnly ||
 		mergeStyle == MergeStyleManuallyMerged && cfg.AllowManualMerge
 }
 

modules/git/error.go

@@ -96,7 +96,7 @@ func (err ErrBranchNotExist) Unwrap() error {
 	return util.ErrNotExist
 }
 
-// ErrPushOutOfDate represents an error if merging fails due to unrelated histories
+// ErrPushOutOfDate represents an error if merging fails due to the base branch being updated
 type ErrPushOutOfDate struct {
 	StdOut string
 	StdErr string

modules/repository/create.go

@@ -87,7 +87,11 @@ func CreateRepositoryByExample(ctx context.Context, doer, u *user_model.User, re
 			units = append(units, repo_model.RepoUnit{
 				RepoID: repo.ID,
 				Type:   tp,
-				Config: &repo_model.PullRequestsConfig{AllowMerge: true, AllowRebase: true, AllowRebaseMerge: true, AllowSquash: true, DefaultMergeStyle: repo_model.MergeStyle(setting.Repository.PullRequest.DefaultMergeStyle), AllowRebaseUpdate: true},
+				Config: &repo_model.PullRequestsConfig{
+					AllowMerge: true, AllowRebase: true, AllowRebaseMerge: true, AllowSquash: true, AllowFastForwardOnly: true,
+					DefaultMergeStyle: repo_model.MergeStyle(setting.Repository.PullRequest.DefaultMergeStyle),
+					AllowRebaseUpdate: true,
+				},
 			})
 		} else {
 			units = append(units, repo_model.RepoUnit{

modules/structs/repo.go

@@ -98,6 +98,7 @@ type Repository struct {
 	AllowRebase                   bool             `json:"allow_rebase"`
 	AllowRebaseMerge              bool             `json:"allow_rebase_explicit"`
 	AllowSquash                   bool             `json:"allow_squash_merge"`
+	AllowFastForwardOnly          bool             `json:"allow_fast_forward_only_merge"`
 	AllowRebaseUpdate             bool             `json:"allow_rebase_update"`
 	DefaultDeleteBranchAfterMerge bool             `json:"default_delete_branch_after_merge"`
 	DefaultMergeStyle             string           `json:"default_merge_style"`
@@ -195,6 +196,8 @@ type EditRepoOption struct {
 	AllowRebaseMerge *bool `json:"allow_rebase_explicit,omitempty"`
 	// either `true` to allow squash-merging pull requests, or `false` to prevent squash-merging.
 	AllowSquash *bool `json:"allow_squash_merge,omitempty"`
+	// either `true` to allow fast-forward-only merging pull requests, or `false` to prevent fast-forward-only merging.
+	AllowFastForwardOnly *bool `json:"allow_fast_forward_only_merge,omitempty"`
 	// either `true` to allow mark pr as merged manually, or `false` to prevent it.
 	AllowManualMerge *bool `json:"allow_manual_merge,omitempty"`
 	// either `true` to enable AutodetectManualMerge, or `false` to prevent it. Note: In some special cases, misjudgments can occur.
@@ -203,7 +206,7 @@ type EditRepoOption struct {
 	AllowRebaseUpdate *bool `json:"allow_rebase_update,omitempty"`
 	// set to `true` to delete pr branch after merge by default
 	DefaultDeleteBranchAfterMerge *bool `json:"default_delete_branch_after_merge,omitempty"`
-	// set to a merge style to be used by this repository: "merge", "rebase", "rebase-merge", or "squash".
+	// set to a merge style to be used by this repository: "merge", "rebase", "rebase-merge", "squash", or "fast-forward-only".
 	DefaultMergeStyle *string `json:"default_merge_style,omitempty"`
 	// set to `true` to allow edits from maintainers by default
 	DefaultAllowMaintainerEdit *bool `json:"default_allow_maintainer_edit,omitempty"`

options/locale/locale_en-US.ini

@@ -1775,6 +1775,7 @@ pulls.merge_pull_request = Create merge commit
 pulls.rebase_merge_pull_request = Rebase then fast-forward
 pulls.rebase_merge_commit_pull_request = Rebase then create merge commit
 pulls.squash_merge_pull_request = Create squash commit
+pulls.fast_forward_only_merge_pull_request = Fast-forward only
 pulls.merge_manually = Manually merged
 pulls.merge_commit_id = The merge commit ID
 pulls.require_signed_wont_sign = The branch requires signed commits but this merge will not be signed

routers/api/v1/repo/repo.go

@@ -885,6 +885,7 @@ func updateRepoUnits(ctx *context.APIContext, opts api.EditRepoOption) error {
 					AllowRebase:                   true,
 					AllowRebaseMerge:              true,
 					AllowSquash:                   true,
+					AllowFastForwardOnly:          true,
 					AllowManualMerge:              true,
 					AutodetectManualMerge:         false,
 					AllowRebaseUpdate:             true,
@@ -911,6 +912,9 @@ func updateRepoUnits(ctx *context.APIContext, opts api.EditRepoOption) error {
 			if opts.AllowSquash != nil {
 				config.AllowSquash = *opts.AllowSquash
 			}
+			if opts.AllowFastForwardOnly != nil {
+				config.AllowFastForwardOnly = *opts.AllowFastForwardOnly
+			}
 			if opts.AllowManualMerge != nil {
 				config.AllowManualMerge = *opts.AllowManualMerge
 			}

routers/api/v1/repo/repo_test.go

@@ -35,6 +35,7 @@ func TestRepoEdit(t *testing.T) {
 	allowRebase := false
 	allowRebaseMerge := false
 	allowSquashMerge := false
+	allowFastForwardOnlyMerge := false
 	archived := true
 	opts := api.EditRepoOption{
 		Name:                      &ctx.Repo.Repository.Name,
@@ -50,6 +51,7 @@ func TestRepoEdit(t *testing.T) {
 		AllowRebase:               &allowRebase,
 		AllowRebaseMerge:          &allowRebaseMerge,
 		AllowSquash:               &allowSquashMerge,
+		AllowFastForwardOnly:      &allowFastForwardOnlyMerge,
 		Archived:                  &archived,
 	}
 

routers/web/repo/issue.go

@@ -1862,6 +1862,8 @@ func ViewIssue(ctx *context.Context) {
 				mergeStyle = repo_model.MergeStyleRebaseMerge
 			} else if prConfig.AllowSquash {
 				mergeStyle = repo_model.MergeStyleSquash
+			} else if prConfig.AllowFastForwardOnly {
+				mergeStyle = repo_model.MergeStyleFastForwardOnly
 			} else if prConfig.AllowManualMerge {
 				mergeStyle = repo_model.MergeStyleManuallyMerged
 			}

routers/web/repo/setting/setting.go

@@ -576,6 +576,7 @@ func SettingsPost(ctx *context.Context) {
 					AllowRebase:                   form.PullsAllowRebase,
 					AllowRebaseMerge:              form.PullsAllowRebaseMerge,
 					AllowSquash:                   form.PullsAllowSquash,
+					AllowFastForwardOnly:          form.PullsAllowFastForwardOnly,
 					AllowManualMerge:              form.PullsAllowManualMerge,
 					AutodetectManualMerge:         form.EnableAutodetectManualMerge,
 					AllowRebaseUpdate:             form.PullsAllowRebaseUpdate,

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()
@@ -219,6 +221,7 @@ 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

@@ -151,6 +151,7 @@ type RepoSettingForm struct {
 	PullsAllowRebase                      bool
 	PullsAllowRebaseMerge                 bool
 	PullsAllowSquash                      bool
+	PullsAllowFastForwardOnly             bool
 	PullsAllowManualMerge                 bool
 	PullsDefaultMergeStyle                string
 	EnableAutodetectManualMerge           bool
@@ -598,8 +599,8 @@ 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

@@ -267,6 +267,10 @@ 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}
 	}
@@ -377,6 +381,13 @@ 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 {

templates/repo/issue/view_content/pull.tmpl

@@ -197,7 +197,7 @@
 				{{if .AllowMerge}} {{/* user is allowed to merge */}}
 					{{$prUnit := .Repository.MustGetUnit $.Context $.UnitTypePullRequests}}
 					{{$approvers := (.Issue.PullRequest.GetApprovers ctx)}}
-					{{if or $prUnit.PullRequestsConfig.AllowMerge $prUnit.PullRequestsConfig.AllowRebase $prUnit.PullRequestsConfig.AllowRebaseMerge $prUnit.PullRequestsConfig.AllowSquash}}
+					{{if or $prUnit.PullRequestsConfig.AllowMerge $prUnit.PullRequestsConfig.AllowRebase $prUnit.PullRequestsConfig.AllowRebaseMerge $prUnit.PullRequestsConfig.AllowSquash $prUnit.PullRequestsConfig.AllowFastForwardOnly}}
 						{{$hasPendingPullRequestMergeTip := ""}}
 						{{if .HasPendingPullRequestMerge}}
 							{{$createdPRMergeStr := TimeSinceUnix .PendingPullRequestMerge.CreatedUnix ctx.Locale}}
@@ -268,6 +268,13 @@
 									'mergeMessageFieldText': {{.GetCommitMessages}} + defaultSquashMergeMessage,
 									'hideAutoMerge': generalHideAutoMerge,
 								},
+								{
+									'name': 'fast-forward-only',
+									'allowed': {{and $prUnit.PullRequestsConfig.AllowFastForwardOnly (eq .Issue.PullRequest.CommitsBehind 0)}},
+									'textDoMerge': {{ctx.Locale.Tr "repo.pulls.fast_forward_only_merge_pull_request"}},
+									'hideMergeMessageTexts': true,
+									'hideAutoMerge': generalHideAutoMerge,
+								},
 								{
 									'name': 'manually-merged',
 									'allowed': {{$prUnit.PullRequestsConfig.AllowManualMerge}},

templates/repo/issue/view_content/pull_merge_instruction.tmpl

@@ -35,6 +35,10 @@
 			<div>git checkout {{.PullRequest.BaseBranch}}</div>
 			<div>git merge --squash {{$localBranch}}</div>
 		</div>
+		<div class="gt-hidden" data-pull-merge-style="fast-forward-only">
+			<div>git checkout {{.PullRequest.BaseBranch}}</div>
+			<div>git merge --ff-only {{$localBranch}}</div>
+		</div>
 		<div class="gt-hidden" data-pull-merge-style="manually-merged">
 			<div>git checkout {{.PullRequest.BaseBranch}}</div>
 			<div>git merge {{$localBranch}}</div>