Add ff_only parameter to POST /repos/{owner}/{repo}/merge-upstream (#34770)

The merge-upstream route was so far performing any kind of merge, even
those that would create merge commits and thus make your branch diverge
from upstream, requiring manual intervention via the git cli to undo the
damage.

With the new optional parameter ff_only, we can instruct gitea to error
out, if a non-fast-forward merge would be performed.

modules/structs/repo_branch.go

@@ -136,6 +136,7 @@ type UpdateBranchProtectionPriories struct {
 
 type MergeUpstreamRequest struct {
 	Branch string `json:"branch"`
+	FfOnly bool   `json:"ff_only"`
 }
 
 type MergeUpstreamResponse struct {

routers/api/v1/repo/branch.go

@@ -1181,7 +1181,7 @@ func MergeUpstream(ctx *context.APIContext) {
 	//   "404":
 	//     "$ref": "#/responses/notFound"
 	form := web.GetForm(ctx).(*api.MergeUpstreamRequest)
-	mergeStyle, err := repo_service.MergeUpstream(ctx, ctx.Doer, ctx.Repo.Repository, form.Branch)
+	mergeStyle, err := repo_service.MergeUpstream(ctx, ctx.Doer, ctx.Repo.Repository, form.Branch, form.FfOnly)
 	if err != nil {
 		if errors.Is(err, util.ErrInvalidArgument) {
 			ctx.APIError(http.StatusBadRequest, err)

routers/web/repo/branch.go

@@ -258,7 +258,7 @@ func CreateBranch(ctx *context.Context) {
 
 func MergeUpstream(ctx *context.Context) {
 	branchName := ctx.FormString("branch")
-	_, err := repo_service.MergeUpstream(ctx, ctx.Doer, ctx.Repo.Repository, branchName)
+	_, err := repo_service.MergeUpstream(ctx, ctx.Doer, ctx.Repo.Repository, branchName, false)
 	if err != nil {
 		if errors.Is(err, util.ErrNotExist) {
 			ctx.JSONErrorNotFound()

services/repository/merge_upstream.go

@@ -18,7 +18,7 @@ import (
 )
 
 // MergeUpstream merges the base repository's default branch into the fork repository's current branch.
-func MergeUpstream(ctx reqctx.RequestContext, doer *user_model.User, repo *repo_model.Repository, branch string) (mergeStyle string, err error) {
+func MergeUpstream(ctx reqctx.RequestContext, doer *user_model.User, repo *repo_model.Repository, branch string, ffOnly bool) (mergeStyle string, err error) {
 	if err = repo.MustNotBeArchived(); err != nil {
 		return "", err
 	}
@@ -45,6 +45,11 @@ func MergeUpstream(ctx reqctx.RequestContext, doer *user_model.User, repo *repo_
 		return "", err
 	}
 
+	// If ff_only is requested and fast-forward failed, return error
+	if ffOnly {
+		return "", util.NewInvalidArgumentErrorf("fast-forward merge not possible: branch has diverged")
+	}
+
 	// TODO: FakePR: it is somewhat hacky, but it is the only way to "merge" at the moment
 	// ideally in the future the "merge" functions should be refactored to decouple from the PullRequest
 	fakeIssue := &issue_model.Issue{

templates/swagger/v1_json.tmpl

@@ -24839,6 +24839,10 @@
         "branch": {
           "type": "string",
           "x-go-name": "Branch"
+        },
+        "ff_only": {
+          "type": "boolean",
+          "x-go-name": "FfOnly"
         }
       },
       "x-go-package": "code.gitea.io/gitea/modules/structs"

tests/integration/repo_merge_upstream_test.go

@@ -147,5 +147,37 @@ func TestRepoMergeUpstream(t *testing.T) {
 				return queryMergeUpstreamButtonLink(htmlDoc) == ""
 			}, 5*time.Second, 100*time.Millisecond)
 		})
+
+		t.Run("FastForwardOnly", func(t *testing.T) {
+			// Create a clean branch for fast-forward testing
+			req = NewRequestWithValues(t, "POST", fmt.Sprintf("/%s/test-repo-fork/branches/_new/branch/master", forkUser.Name), map[string]string{
+				"_csrf":           GetUserCSRFToken(t, session),
+				"new_branch_name": "ff-test-branch",
+			})
+			session.MakeRequest(t, req, http.StatusSeeOther)
+
+			// Add content to base repository that can be fast-forwarded
+			require.NoError(t, createOrReplaceFileInBranch(baseUser, baseRepo, "ff-test.txt", "master", "ff-content-1"))
+
+			// ff_only=true with fast-forward possible (should succeed)
+			req = NewRequestWithJSON(t, "POST", fmt.Sprintf("/api/v1/repos/%s/test-repo-fork/merge-upstream", forkUser.Name), &api.MergeUpstreamRequest{
+				Branch: "ff-test-branch",
+				FfOnly: true,
+			}).AddTokenAuth(token)
+			resp := MakeRequest(t, req, http.StatusOK)
+
+			var mergeResp api.MergeUpstreamResponse
+			DecodeJSON(t, resp, &mergeResp)
+			assert.Equal(t, "fast-forward", mergeResp.MergeStyle)
+
+			// ff_only=true when fast-forward is not possible (should fail)
+			require.NoError(t, createOrReplaceFileInBranch(baseUser, baseRepo, "another-file.txt", "master", "more-content"))
+
+			req = NewRequestWithJSON(t, "POST", fmt.Sprintf("/api/v1/repos/%s/test-repo-fork/merge-upstream", forkUser.Name), &api.MergeUpstreamRequest{
+				Branch: "fork-branch",
+				FfOnly: true,
+			}).AddTokenAuth(token)
+			MakeRequest(t, req, http.StatusBadRequest)
+		})
 	})
 }