Add ff_only parameter to POST /repos/{owner}/{repo}/merge-upstream

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.

Author: dcermak

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

@@ -147,5 +147,59 @@ 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)
+
+			// Test ff_only=true when fast-forward is not possible (using fork-branch which has diverged)
+			// The fork-branch already has merge commits from previous tests, so this 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)
+
+			// same scenario but with ff_only=false (should succeed with merge)
+			req = NewRequestWithJSON(t, "POST", fmt.Sprintf("/api/v1/repos/%s/test-repo-fork/merge-upstream", forkUser.Name), &api.MergeUpstreamRequest{
+				Branch: "fork-branch",
+				FfOnly: false,
+			}).AddTokenAuth(token)
+			resp = MakeRequest(t, req, http.StatusOK)
+
+			DecodeJSON(t, resp, &mergeResp)
+			assert.Equal(t, "merge", mergeResp.MergeStyle)
+
+			// ff_only not specified (should use default behavior - merge)
+			require.NoError(t, createOrReplaceFileInBranch(baseUser, baseRepo, "final-file.txt", "master", "final-content"))
+
+			req = NewRequestWithJSON(t, "POST", fmt.Sprintf("/api/v1/repos/%s/test-repo-fork/merge-upstream", forkUser.Name), &api.MergeUpstreamRequest{
+				Branch: "fork-branch",
+			}).AddTokenAuth(token)
+			resp = MakeRequest(t, req, http.StatusOK)
+
+			DecodeJSON(t, resp, &mergeResp)
+			assert.Equal(t, "merge", mergeResp.MergeStyle)
+		})
 	})
 }