Add support to sync fork branch with upstream repo (#2337)

Fixes: #2335

github/repos_merging.go

@@ -18,6 +18,20 @@ type RepositoryMergeRequest struct {
 	CommitMessage *string `json:"commit_message,omitempty"`
 }
 
+// RepoMergeUpstreamRequest represents a request to sync a branch of
+// a forked repository to keep it up-to-date with the upstream repository.
+type RepoMergeUpstreamRequest struct {
+	Branch *string `json:"branch,omitempty"`
+}
+
+// RepoMergeUpstreamResult represents the result of syncing a branch of
+// a forked repository with the upstream repository.
+type RepoMergeUpstreamResult struct {
+	Message    *string `json:"message,omitempty"`
+	MergeType  *string `json:"merge_type,omitempty"`
+	BaseBranch *string `json:"base_branch,omitempty"`
+}
+
 // Merge a branch in the specified repository.
 //
 // GitHub API docs: https://docs.github.com/en/free-pro-team@latest/rest/reference/repos/#merge-a-branch
@@ -36,3 +50,23 @@ func (s *RepositoriesService) Merge(ctx context.Context, owner, repo string, req
 
 	return commit, resp, nil
 }
+
+// MergeUpstream syncs a branch of a forked repository to keep it up-to-date
+// with the upstream repository.
+//
+// GitHub API docs: https://docs.github.com/en/rest/reference/branches#sync-a-fork-branch-with-the-upstream-repository
+func (s *RepositoriesService) MergeUpstream(ctx context.Context, owner, repo string, request *RepoMergeUpstreamRequest) (*RepoMergeUpstreamResult, *Response, error) {
+	u := fmt.Sprintf("repos/%v/%v/merge-upstream", owner, repo)
+	req, err := s.client.NewRequest("POST", u, request)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	result := new(RepoMergeUpstreamResult)
+	resp, err := s.client.Do(ctx, req, result)
+	if err != nil {
+		return nil, resp, err
+	}
+
+	return result, resp, nil
+}

github/repos_merging_test.go

@@ -80,3 +80,49 @@ func TestRepositoryMergeRequest_Marshal(t *testing.T) {
 
 	testJSONMarshal(t, u, want)
 }
+
+func TestRepositoriesService_MergeUpstream(t *testing.T) {
+	client, mux, _, teardown := setup()
+	defer teardown()
+
+	input := &RepoMergeUpstreamRequest{
+		Branch: String("b"),
+	}
+
+	mux.HandleFunc("/repos/o/r/merge-upstream", func(w http.ResponseWriter, r *http.Request) {
+		v := new(RepoMergeUpstreamRequest)
+		json.NewDecoder(r.Body).Decode(v)
+
+		testMethod(t, r, "POST")
+		if !cmp.Equal(v, input) {
+			t.Errorf("Request body = %+v, want %+v", v, input)
+		}
+
+		fmt.Fprint(w, `{"merge_type":"m"}`)
+	})
+
+	ctx := context.Background()
+	result, _, err := client.Repositories.MergeUpstream(ctx, "o", "r", input)
+	if err != nil {
+		t.Errorf("Repositories.MergeUpstream returned error: %v", err)
+	}
+
+	want := &RepoMergeUpstreamResult{MergeType: String("m")}
+	if !cmp.Equal(result, want) {
+		t.Errorf("Repositories.MergeUpstream returned %+v, want %+v", result, want)
+	}
+
+	const methodName = "MergeUpstream"
+	testBadOptions(t, methodName, func() (err error) {
+		_, _, err = client.Repositories.MergeUpstream(ctx, "\n", "\n", input)
+		return err
+	})
+
+	testNewRequestAndDoFailure(t, methodName, client, func() (*Response, error) {
+		got, resp, err := client.Repositories.MergeUpstream(ctx, "o", "r", input)
+		if got != nil {
+			t.Errorf("testNewRequestAndDoFailure %v = %#v, want nil", methodName, got)
+		}
+		return resp, err
+	})
+}