Add default_branch support for Project API

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/26261

Set `default_branch` value as a default branch name for non-empty
repositories.

Changelog: fixed

app/services/projects/create_service.rb

@@ -11,6 +11,7 @@ module Projects
       @initialize_with_readme = Gitlab::Utils.to_boolean(@params.delete(:initialize_with_readme))
       @import_data = @params.delete(:import_data)
       @relations_block = @params.delete(:relations_block)
+      @default_branch = @params.delete(:default_branch)
 
       build_topics
     end
@@ -151,7 +152,7 @@ module Projects
 
     def create_readme
       commit_attrs = {
-        branch_name: @project.default_branch_or_main,
+        branch_name: @default_branch.presence || @project.default_branch_or_main,
         commit_message: 'Initial commit',
         file_path: 'README.md',
         file_content: "# #{@project.name}\n\n#{@project.description}"

doc/api/projects.md

@@ -1171,7 +1171,7 @@ POST /projects
 | `ci_config_path`                                            | string  | **{dotted-circle}** No | The path to CI configuration file. |
 | `container_expiration_policy_attributes`                    | hash    | **{dotted-circle}** No | Update the image cleanup policy for this project. Accepts: `cadence` (string), `keep_n` (integer), `older_than` (string), `name_regex` (string), `name_regex_delete` (string), `name_regex_keep` (string), `enabled` (boolean). Valid values for `cadence` are: `1d` (every day), `7d` (every week), `14d` (every two weeks), `1month` (every month), or `3month` (every quarter). |
 | `container_registry_enabled`                                | boolean | **{dotted-circle}** No | Enable container registry for this project. |
-| `default_branch`                                            | string  | **{dotted-circle}** No | The [default branch](../user/project/repository/branches/default.md) name. |
+| `default_branch`                                            | string  | **{dotted-circle}** No | The [default branch](../user/project/repository/branches/default.md) name. Requires `initialize_with_readme` to be `true`. |
 | `description`                                               | string  | **{dotted-circle}** No | Short project description. |
 | `emails_disabled`                                           | boolean | **{dotted-circle}** No | Disable email notifications. |
 | `external_authorization_classification_label` **(PREMIUM)** | string  | **{dotted-circle}** No | The classification label for the project. |
@@ -1246,6 +1246,7 @@ POST /projects/user/:user_id
 | `ci_config_path`                                            | string  | **{dotted-circle}** No | The path to CI configuration file. |
 | `container_registry_enabled`                                | boolean | **{dotted-circle}** No | Enable container registry for this project. |
 | `description`                                               | string  | **{dotted-circle}** No | Short project description. |
+| `default_branch`                                            | string  | **{dotted-circle}** No | The [default branch](../user/project/repository/branches/default.md) name. Requires `initialize_with_readme` to be `true`. |
 | `emails_disabled`                                           | boolean | **{dotted-circle}** No | Disable email notifications. |
 | `external_authorization_classification_label` **(PREMIUM)** | string  | **{dotted-circle}** No | The classification label for the project. |
 | `forking_access_level`                                      | string  | **{dotted-circle}** No | One of `disabled`, `private`, or `enabled`. |

lib/api/projects.rb

@@ -234,6 +234,7 @@ module API
       params do
         optional :name, type: String, desc: 'The name of the project'
         optional :path, type: String, desc: 'The path of the repository'
+        optional :default_branch, type: String, desc: 'The default branch of the project'
         at_least_one_of :name, :path
         use :optional_create_project_params
         use :create_params

spec/requests/api/projects_spec.rb

@@ -109,6 +109,43 @@ RSpec.describe API::Projects do
     end
   end
 
+  shared_examples_for 'create project with default branch parameter' do
+    let(:params) { { name: 'Foo Project', initialize_with_readme: true, default_branch: default_branch } }
+    let(:default_branch) { 'main' }
+
+    it 'creates project with provided default branch name' do
+      expect { request }.to change { Project.count }.by(1)
+      expect(response).to have_gitlab_http_status(:created)
+
+      project = Project.find(json_response['id'])
+      expect(project.default_branch).to eq(default_branch)
+    end
+
+    context 'when branch name is empty' do
+      let(:default_branch) { '' }
+
+      it 'creates project with a default project branch name' do
+        expect { request }.to change { Project.count }.by(1)
+        expect(response).to have_gitlab_http_status(:created)
+
+        project = Project.find(json_response['id'])
+        expect(project.default_branch).to eq('master')
+      end
+    end
+
+    context 'when initialize with readme is not set' do
+      let(:params) { super().merge(initialize_with_readme: nil) }
+
+      it 'creates project with a default project branch name' do
+        expect { request }.to change { Project.count }.by(1)
+        expect(response).to have_gitlab_http_status(:created)
+
+        project = Project.find(json_response['id'])
+        expect(project.default_branch).to be_nil
+      end
+    end
+  end
+
   describe 'GET /projects' do
     shared_examples_for 'projects response' do
       it 'returns an array of projects' do
@@ -921,6 +958,10 @@ RSpec.describe API::Projects do
       expect(project.path).to eq('path-project-Foo')
     end
 
+    it_behaves_like 'create project with default branch parameter' do
+      let(:request) { post api('/projects', user), params: params }
+    end
+
     it 'creates last project before reaching project limit' do
       allow_any_instance_of(User).to receive(:projects_limit_left).and_return(1)
       post api('/projects', user2), params: { name: 'foo' }
@@ -1401,6 +1442,10 @@ RSpec.describe API::Projects do
       expect(project.path).to eq('path-project-Foo')
     end
 
+    it_behaves_like 'create project with default branch parameter' do
+      let(:request) { post api("/projects/user/#{user.id}", admin), params: params }
+    end
+
     it 'responds with 400 on failure and not project' do
       expect { post api("/projects/user/#{user.id}", admin) }
         .not_to change { Project.count }

spec/services/projects/create_service_spec.rb

@@ -298,7 +298,7 @@ RSpec.describe Projects::CreateService, '#execute' do
 
   context 'error handling' do
     it 'handles invalid options' do
-      opts[:default_branch] = 'master'
+      opts[:invalid] = 'option'
       expect(create_project(user, opts)).to eq(nil)
     end
   end