fix

modules/structs/repo_branch.go

@@ -133,3 +133,11 @@ type EditBranchProtectionOption struct {
 type UpdateBranchProtectionPriories struct {
 	IDs []int64 `json:"ids"`
 }
+
+type MergeUpstreamRequest struct {
+	Branch string `json:"branch"`
+}
+
+type MergeUpstreamResponse struct {
+	MergeStyle string `json:"merge_type"`
+}

routers/api/v1/api.go

@@ -1190,6 +1190,7 @@ func Routes() *web.Router {
 				m.Get("/archive/*", reqRepoReader(unit.TypeCode), repo.GetArchive)
 				m.Combo("/forks").Get(repo.ListForks).
 					Post(reqToken(), reqRepoReader(unit.TypeCode), bind(api.CreateForkOption{}), repo.CreateFork)
+				m.Post("/merge-upstream", reqToken(), mustNotBeArchived, reqRepoWriter(unit.TypeCode), bind(api.MergeUpstreamRequest{}), repo.MergeUpstream)
 				m.Group("/branches", func() {
 					m.Get("", repo.ListBranches)
 					m.Get("/*", repo.GetBranch)

routers/api/v1/repo/branch.go

@@ -18,6 +18,7 @@ import (
 	"code.gitea.io/gitea/modules/optional"
 	repo_module "code.gitea.io/gitea/modules/repository"
 	api "code.gitea.io/gitea/modules/structs"
+	"code.gitea.io/gitea/modules/util"
 	"code.gitea.io/gitea/modules/web"
 	"code.gitea.io/gitea/routers/api/v1/utils"
 	"code.gitea.io/gitea/services/context"
@@ -1186,3 +1187,47 @@ func UpdateBranchProtectionPriories(ctx *context.APIContext) {
 
 	ctx.Status(http.StatusNoContent)
 }
+
+func MergeUpstream(ctx *context.APIContext) {
+	// swagger:operation POST /repos/{owner}/{repo}/merge-upstream repository repoMergeUpstream
+	// ---
+	// summary: Merge a branch from upstream
+	// produces:
+	// - application/json
+	// parameters:
+	// - name: owner
+	//   in: path
+	//   description: owner of the repo
+	//   type: string
+	//   required: true
+	// - name: repo
+	//   in: path
+	//   description: name of the repo
+	//   type: string
+	//   required: true
+	// - name: body
+	//   in: body
+	//   schema:
+	//     "$ref": "#/definitions/MergeUpstreamRequest"
+	// responses:
+	//   "200":
+	//     "$ref": "#/responses/MergeUpstreamResponse"
+	//   "400":
+	//     "$ref": "#/responses/error"
+	//   "404":
+	//     "$ref": "#/responses/notFound"
+	form := web.GetForm(ctx).(*api.MergeUpstreamRequest)
+	mergeStyle, err := repo_service.MergeUpstream(ctx, ctx.Doer, ctx.Repo.Repository, form.Branch)
+	if err != nil {
+		if errors.Is(err, util.ErrInvalidArgument) {
+			ctx.Error(http.StatusBadRequest, "MergeUpstream", err)
+			return
+		} else if errors.Is(err, util.ErrNotExist) {
+			ctx.Error(http.StatusNotFound, "MergeUpstream", err)
+			return
+		}
+		ctx.Error(http.StatusInternalServerError, "MergeUpstream", err)
+		return
+	}
+	ctx.JSON(http.StatusOK, &api.MergeUpstreamResponse{MergeStyle: mergeStyle})
+}

routers/api/v1/swagger/repo.go

@@ -448,3 +448,15 @@ type swaggerCompare struct {
 	// in:body
 	Body api.Compare `json:"body"`
 }
+
+// swagger:response MergeUpstreamRequest
+type swaggerMergeUpstreamRequest struct {
+	// in:body
+	Body api.MergeUpstreamRequest `json:"body"`
+}
+
+// swagger:response MergeUpstreamResponse
+type swaggerMergeUpstreamResponse struct {
+	// in:body
+	Body api.MergeUpstreamResponse `json:"body"`
+}

tests/integration/repo_merge_upstream_test.go

@@ -0,0 +1,91 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package integration
+
+import (
+	"fmt"
+	"net/http"
+	"net/url"
+	"strings"
+	"testing"
+	"time"
+
+	auth_model "code.gitea.io/gitea/models/auth"
+	repo_model "code.gitea.io/gitea/models/repo"
+	"code.gitea.io/gitea/models/unittest"
+	user_model "code.gitea.io/gitea/models/user"
+	api "code.gitea.io/gitea/modules/structs"
+	"code.gitea.io/gitea/modules/util"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRepoMergeUpstream(t *testing.T) {
+	onGiteaRun(t, func(*testing.T, *url.URL) {
+		forkUser := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 4})
+
+		baseRepo := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 1})
+		baseUser := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: baseRepo.OwnerID})
+
+		checkFileContent := func(exp string) {
+			req := NewRequest(t, "GET", fmt.Sprintf("/%s/test-repo-fork/raw/branch/master/new-file.txt", forkUser.Name))
+			resp := MakeRequest(t, req, http.StatusOK)
+			require.Equal(t, exp, resp.Body.String())
+		}
+
+		session := loginUser(t, forkUser.Name)
+		token := getTokenForLoggedInUser(t, session, auth_model.AccessTokenScopeWriteRepository)
+
+		// create a fork
+		req := NewRequestWithJSON(t, "POST", fmt.Sprintf("/api/v1/repos/%s/%s/forks", baseUser.Name, baseRepo.Name), &api.CreateForkOption{
+			Name: util.ToPointer("test-repo-fork"),
+		}).AddTokenAuth(token)
+		MakeRequest(t, req, http.StatusAccepted)
+		forkRepo := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{OwnerID: forkUser.ID, Name: "test-repo-fork"})
+
+		// add a file in base repo
+		require.NoError(t, createOrReplaceFileInBranch(baseUser, baseRepo, "new-file.txt", "master", "test-content-1"))
+
+		// the repo shows a prompt to "sync fork"
+		var mergeUpstreamLink string
+		require.Eventually(t, func() bool {
+			resp := session.MakeRequest(t, NewRequestf(t, "GET", "/%s/test-repo-fork", forkUser.Name), http.StatusOK)
+			htmlDoc := NewHTMLParser(t, resp.Body)
+			respMsg, _ := htmlDoc.Find(".ui.message").Html()
+			if !strings.Contains(respMsg, `This branch is 1 commit behind <a href="/user2/repo1/src/branch/master">user2/repo1:master</a>`) {
+				return false
+			}
+			mergeUpstreamLink = htmlDoc.Find("button[data-url*='merge-upstream']").AttrOr("data-url", "")
+			require.NotEmpty(t, mergeUpstreamLink)
+			return true
+		}, 5*time.Second, 100*time.Millisecond)
+
+		// click the "sync fork" button
+		req = NewRequestWithValues(t, "POST", mergeUpstreamLink, map[string]string{"_csrf": GetUserCSRFToken(t, session)})
+		session.MakeRequest(t, req, http.StatusOK)
+		checkFileContent("test-content-1")
+
+		// update the files
+		require.NoError(t, createOrReplaceFileInBranch(forkUser, forkRepo, "new-file-other.txt", "master", "test-content-other"))
+		require.NoError(t, createOrReplaceFileInBranch(baseUser, baseRepo, "new-file.txt", "master", "test-content-2"))
+		require.Eventually(t, func() bool {
+			resp := session.MakeRequest(t, NewRequestf(t, "GET", "/%s/test-repo-fork", forkUser.Name), http.StatusOK)
+			htmlDoc := NewHTMLParser(t, resp.Body)
+			respMsg, _ := htmlDoc.Find(".ui.message:not(.positive)").Html()
+			return strings.Contains(respMsg, `The base branch <a href="/user2/repo1/src/branch/master">user2/repo1:master</a> has new changes`)
+		}, 5*time.Second, 100*time.Millisecond)
+
+		// and do the merge-upstream by API
+		req = NewRequestWithJSON(t, "POST", fmt.Sprintf("/api/v1/repos/%s/test-repo-fork/merge-upstream", forkUser.Name), &api.MergeUpstreamRequest{
+			Branch: "master",
+		}).AddTokenAuth(token)
+		resp := MakeRequest(t, req, http.StatusOK)
+		checkFileContent("test-content-2")
+
+		var mergeResp api.MergeUpstreamResponse
+		DecodeJSON(t, resp, &mergeResp)
+		assert.Equal(t, "merge", mergeResp.MergeStyle)
+	})
+}