feat: Add default PR branch update style setting (#37410)

Adds repository-level settings for pull request branch updates so admins
can choose the default update method and disable merge or rebase
updates.

<img width="1025" height="158"
src="https://github.com/user-attachments/assets/d030973b-0ddd-4035-b04f-145c445084d7"
/>

---------

Co-authored-by: OpenAI Codex (GPT-5) <codex@openai.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/repo/git.go

@@ -29,6 +29,16 @@ const (
 	MergeStyleRebaseUpdate MergeStyle = "rebase-update-only"
 )
 
+// UpdateStyle is a pull request branch update style
+type UpdateStyle string
+
+const (
+	// UpdateStyleMerge merges the base branch into the pull request branch
+	UpdateStyleMerge UpdateStyle = "merge"
+	// UpdateStyleRebase rebases the pull request branch onto the base branch
+	UpdateStyleRebase UpdateStyle = "rebase"
+)
+
 // UpdateDefaultBranch updates the default branch
 func UpdateDefaultBranch(ctx context.Context, repo *Repository) error {
 	_, err := db.GetEngine(ctx).ID(repo.ID).Cols("default_branch").Update(repo)

models/repo/pull_request_default_test.go

@@ -8,6 +8,7 @@ import (
 
 	"code.gitea.io/gitea/models/unit"
 	"code.gitea.io/gitea/models/unittest"
+	"code.gitea.io/gitea/modules/util"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -30,3 +31,82 @@ func TestDefaultTargetBranchSelection(t *testing.T) {
 	repo.Units = nil
 	assert.Equal(t, "branch2", repo.GetPullRequestTargetBranch(ctx))
 }
+
+func TestPullRequestConfigFromDB(t *testing.T) {
+	cases := []struct {
+		// name describes the row shape under test; the comments capture why each row matters.
+		name              string
+		json              string
+		wantMergeUpdate   bool
+		wantRebaseUpdate  bool
+		wantDefaultStyle  UpdateStyle
+		wantValidatesPass bool
+	}{
+		{
+			// Empty object exercises the all-defaults path (e.g. fresh repos created via low-level paths).
+			name: "defaults", json: "{}",
+			wantMergeUpdate: true, wantRebaseUpdate: true,
+			wantDefaultStyle: UpdateStyleMerge, wantValidatesPass: true,
+		},
+		{
+			// Realistic upgrade case: pre-PR JSON lacks the new fields and has AllowRebaseUpdate=false.
+			// Historical setting must be preserved while new fields take safe defaults.
+			name:            "legacy without new fields",
+			json:            `{"AllowMerge":true,"AllowRebase":true,"AllowRebaseMerge":true,"AllowSquash":true,"AllowRebaseUpdate":false}`,
+			wantMergeUpdate: true, wantRebaseUpdate: false,
+			wantDefaultStyle: UpdateStyleMerge, wantValidatesPass: true,
+		},
+		{
+			// Partially-migrated row with explicit empty string must normalize so ValidateUpdateSettings passes.
+			name: "empty default style", json: `{"DefaultUpdateStyle":""}`,
+			wantMergeUpdate: true, wantRebaseUpdate: true,
+			wantDefaultStyle: UpdateStyleMerge, wantValidatesPass: true,
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			cfg := new(PullRequestsConfig)
+			assert.NoError(t, cfg.FromDB([]byte(tc.json)))
+			assert.Equal(t, tc.wantMergeUpdate, cfg.AllowMergeUpdate)
+			assert.Equal(t, tc.wantRebaseUpdate, cfg.AllowRebaseUpdate)
+			assert.Equal(t, tc.wantDefaultStyle, cfg.DefaultUpdateStyle)
+			if tc.wantValidatesPass {
+				assert.NoError(t, cfg.ValidateUpdateSettings())
+			}
+		})
+	}
+}
+
+func TestPullRequestConfigValidateUpdateSettingsInvalidArgument(t *testing.T) {
+	cases := []struct {
+		name string
+		cfg  PullRequestsConfig
+	}{
+		{
+			name: "invalid default style",
+			cfg: PullRequestsConfig{
+				AllowMergeUpdate:   true,
+				AllowRebaseUpdate:  true,
+				DefaultUpdateStyle: "invalid",
+			},
+		},
+		{
+			name: "no update style enabled",
+			cfg: PullRequestsConfig{
+				DefaultUpdateStyle: UpdateStyleMerge,
+			},
+		},
+		{
+			name: "default update style disabled",
+			cfg: PullRequestsConfig{
+				AllowRebaseUpdate:  true,
+				DefaultUpdateStyle: UpdateStyleMerge,
+			},
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			assert.ErrorIs(t, tc.cfg.ValidateUpdateSettings(), util.ErrInvalidArgument)
+		})
+	}
+}

models/repo/repo_unit.go

@@ -125,7 +125,9 @@ type PullRequestsConfig struct {
 	AllowFastForwardOnly          bool
 	AllowManualMerge              bool
 	AutodetectManualMerge         bool
+	AllowMergeUpdate              bool
 	AllowRebaseUpdate             bool
+	DefaultUpdateStyle            UpdateStyle
 	DefaultDeleteBranchAfterMerge bool
 	DefaultMergeStyle             MergeStyle
 	DefaultAllowMaintainerEdit    bool
@@ -139,7 +141,9 @@ func DefaultPullRequestsConfig() *PullRequestsConfig {
 		AllowRebaseMerge:           true,
 		AllowSquash:                true,
 		AllowFastForwardOnly:       true,
+		AllowMergeUpdate:           true,
 		AllowRebaseUpdate:          true,
+		DefaultUpdateStyle:         UpdateStyleMerge,
 		DefaultAllowMaintainerEdit: true,
 	}
 	cfg.DefaultDeleteBranchAfterMerge = setting.Repository.PullRequest.DefaultDeleteBranchAfterMerge
@@ -152,7 +156,9 @@ func DefaultPullRequestsConfig() *PullRequestsConfig {
 func (cfg *PullRequestsConfig) FromDB(bs []byte) error {
 	// set default values for existing PullRequestConfig in DB
 	*cfg = *DefaultPullRequestsConfig()
-	return json.UnmarshalHandleDoubleEncode(bs, &cfg)
+	_ = json.UnmarshalHandleDoubleEncode(bs, &cfg) // don't let corrupted database value cause unnecessary 500 error
+	cfg.DefaultUpdateStyle = util.IfZero(cfg.DefaultUpdateStyle, UpdateStyleMerge)
+	return nil
 }
 
 // ToDB exports a PullRequestsConfig to a serialized format.
@@ -170,6 +176,32 @@ func (cfg *PullRequestsConfig) IsMergeStyleAllowed(mergeStyle MergeStyle) bool {
 		mergeStyle == MergeStyleManuallyMerged && cfg.AllowManualMerge
 }
 
+// IsUpdateStyleAllowed returns if a pull request branch update style is allowed
+func (cfg *PullRequestsConfig) IsUpdateStyleAllowed(updateStyle UpdateStyle) bool {
+	switch updateStyle {
+	case UpdateStyleMerge:
+		return cfg.AllowMergeUpdate
+	case UpdateStyleRebase:
+		return cfg.AllowRebaseUpdate
+	default:
+		return false
+	}
+}
+
+// ValidateUpdateSettings checks that the AllowMerge/RebaseUpdate flags and DefaultUpdateStyle are mutually consistent.
+func (cfg *PullRequestsConfig) ValidateUpdateSettings() error {
+	if cfg.DefaultUpdateStyle != UpdateStyleMerge && cfg.DefaultUpdateStyle != UpdateStyleRebase {
+		return util.NewInvalidArgumentErrorf("default update style must be merge or rebase")
+	}
+	if !cfg.AllowMergeUpdate && !cfg.AllowRebaseUpdate {
+		return util.NewInvalidArgumentErrorf("at least one pull request branch update style must be enabled")
+	}
+	if !cfg.IsUpdateStyleAllowed(cfg.DefaultUpdateStyle) {
+		return util.NewInvalidArgumentErrorf("default update style must be enabled")
+	}
+	return nil
+}
+
 func DefaultPullRequestsUnit(repoID int64) RepoUnit {
 	return RepoUnit{RepoID: repoID, Type: unit.TypePullRequests, Config: DefaultPullRequestsConfig()}
 }