Introduce a new `GET /projects/:id/repository/changelog` API endpoint

Changelog: added
Signed-off-by: Rémy Coutable <remy@rymai.me>

app/services/repositories/changelog_service.rb

@@ -60,7 +60,7 @@ module Repositories
     end
     # rubocop: enable Metrics/ParameterLists
 
-    def execute
+    def execute(commit_to_changelog: true)
       config = Gitlab::Changelog::Config.from_git(@project, @user)
       from = start_of_commit_range(config)
 
@@ -93,9 +93,13 @@ module Repositories
         end
       end
 
-      Gitlab::Changelog::Committer
-        .new(@project, @user)
-        .commit(release: release, file: @file, branch: @branch, message: @message)
+      if commit_to_changelog
+        Gitlab::Changelog::Committer
+          .new(@project, @user)
+          .commit(release: release, file: @file, branch: @branch, message: @message)
+      else
+        Gitlab::Changelog::Generator.new.add(release)
+      end
     end
 
     def start_of_commit_range(config)

doc/api/repositories.md

@@ -291,7 +291,7 @@ Example response:
 }
 ```
 
-## Generate changelog data
+## Add changelog data to a changelog file
 
 > [Introduced](https://gitlab.com/groups/gitlab-com/gl-infra/-/epics/351) in GitLab 13.9.
 
@@ -373,26 +373,26 @@ If the last tag is `v0.9.0` and the default branch is `main`, the range of commi
 included in this example is `v0.9.0..main`:
 
 ```shell
-curl --header "PRIVATE-TOKEN: token" --data "version=1.0.0" "https://gitlab.com/api/v4/projects/42/repository/changelog"
+curl --request POST --header "PRIVATE-TOKEN: token" --data "version=1.0.0" "https://gitlab.com/api/v4/projects/42/repository/changelog"
 ```
 
 To generate the data on a different branch, specify the `branch` parameter. This
 command generates data from the `foo` branch:
 
 ```shell
-curl --header "PRIVATE-TOKEN: token" --data "version=1.0.0&branch=foo" "https://gitlab.com/api/v4/projects/42/repository/changelog"
+curl --request POST --header "PRIVATE-TOKEN: token" --data "version=1.0.0&branch=foo" "https://gitlab.com/api/v4/projects/42/repository/changelog"
 ```
 
 To use a different trailer, use the `trailer` parameter:
 
 ```shell
-curl --header "PRIVATE-TOKEN: token" --data "version=1.0.0&trailer=Type" "https://gitlab.com/api/v4/projects/42/repository/changelog"
+curl --request POST --header "PRIVATE-TOKEN: token" --data "version=1.0.0&trailer=Type" "https://gitlab.com/api/v4/projects/42/repository/changelog"
 ```
 
 To store the results in a different file, use the `file` parameter:
 
 ```shell
-curl --header "PRIVATE-TOKEN: token" --data "version=1.0.0&file=NEWS" "https://gitlab.com/api/v4/projects/42/repository/changelog"
+curl --request POST --header "PRIVATE-TOKEN: token" --data "version=1.0.0&file=NEWS" "https://gitlab.com/api/v4/projects/42/repository/changelog"
 ```
 
 ### How it works
@@ -707,3 +707,39 @@ tag_regex: '^version-(?P<major>\d+)\.(?P<minor>\d+)\.(?P<patch>\d+)$'
 To test if your regular expression is working, you can use websites such as
 [regex101](https://regex101.com/). If the regular expression syntax is invalid,
 an error is produced when generating a changelog.
+
+## Generate changelog data
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/345934) in GitLab 14.6.
+
+Generate changelog data based on commits in a repository, without committing
+them to a changelog file.
+
+Works exactly like `POST /projects/:id/repository/changelog`, except the changelog
+data isn't committed to any changelog file.
+
+```plaintext
+GET /projects/:id/repository/changelog
+```
+
+Supported attributes:
+
+| Attribute | Type     | Required   | Description |
+| :-------- | :------- | :--------- | :---------- |
+| `version` | string   | yes | The version to generate the changelog for. The format must follow [semantic versioning](https://semver.org/). |
+| `from`    | string   | no | The start of the range of commits (as a SHA) to use for generating the changelog. This commit itself isn't included in the list. |
+| `to`      | string   | no | The end of the range of commits (as a SHA) to use for the changelog. This commit _is_ included in the list. Defaults to the branch specified in the `branch` attribute. |
+| `date`    | datetime | no | The date and time of the release, ISO 8601 formatted. Example: `2016-03-11T03:45:40Z`. Defaults to the current time. |
+| `trailer` | string   | no | The Git trailer to use for including commits, defaults to `Changelog`. |
+
+```shell
+curl --header "PRIVATE-TOKEN: token" --data "version=1.0.0" "https://gitlab.com/api/v4/projects/42/repository/changelog"
+```
+
+Example Response:
+
+```json
+{
+  "notes": "## 1.0.0 (2021-11-17)\n\n### feature (2 changes)\n\n- [Title 2](namespace13/project13@ad608eb642124f5b3944ac0ac772fecaf570a6bf) ([merge request](namespace13/project13!2))\n- [Title 1](namespace13/project13@3c6b80ff7034fa0d585314e1571cc780596ce3c8) ([merge request](namespace13/project13!1))\n"
+}
+```

lib/api/entities/changelog.rb

+# frozen_string_literal: true
+
+module API
+  module Entities
+    class Changelog < Grape::Entity
+      expose :to_s, as: :notes
+    end
+  end
+end

lib/api/repositories.rb

@@ -10,6 +10,32 @@ module API
 
     helpers ::API::Helpers::HeadersHelpers
 
+    helpers do
+      params :release_params do
+        requires :version,
+          type: String,
+          regexp: Gitlab::Regex.unbounded_semver_regex,
+          desc: 'The version of the release, using the semantic versioning format'
+
+        optional :from,
+          type: String,
+          desc: 'The first commit in the range of commits to use for the changelog'
+
+        optional :to,
+          type: String,
+          desc: 'The last commit in the range of commits to use for the changelog'
+
+        optional :date,
+          type: DateTime,
+          desc: 'The date and time of the release'
+
+        optional :trailer,
+          type: String,
+          desc: 'The Git trailer to use for determining if commits are to be included in the changelog',
+          default: ::Repositories::ChangelogService::DEFAULT_TRAILER
+      end
+    end
+
     before { authorize! :download_code, user_project }
 
     feature_category :source_code_management
@@ -208,36 +234,33 @@ module API
         end
       end
 
-      desc 'Generates a changelog section for a release' do
-        detail 'This feature was introduced in GitLab 13.9'
+      desc 'Generates a changelog section for a release and returns it' do
+        detail 'This feature was introduced in GitLab 14.6'
       end
       params do
-        requires :version,
-          type: String,
-          regexp: Gitlab::Regex.unbounded_semver_regex,
-          desc: 'The version of the release, using the semantic versioning format'
-
-        optional :from,
-          type: String,
-          desc: 'The first commit in the range of commits to use for the changelog'
+        use :release_params
+      end
+      get ':id/repository/changelog' do
+        service = ::Repositories::ChangelogService.new(
+          user_project,
+          current_user,
+          **declared_params(include_missing: false)
+        )
+        changelog = service.execute(commit_to_changelog: false)
 
-        optional :to,
-          type: String,
-          desc: 'The last commit in the range of commits to use for the changelog'
+        present changelog, with: Entities::Changelog
+      end
 
-        optional :date,
-          type: DateTime,
-          desc: 'The date and time of the release'
+      desc 'Generates a changelog section for a release and commits it in a changelog file' do
+        detail 'This feature was introduced in GitLab 13.9'
+      end
+      params do
+        use :release_params
 
         optional :branch,
           type: String,
           desc: 'The branch to commit the changelog changes to'
 
-        optional :trailer,
-          type: String,
-          desc: 'The Git trailer to use for determining if commits are to be included in the changelog',
-          default: ::Repositories::ChangelogService::DEFAULT_TRAILER
-
         optional :file,
           type: String,
           desc: 'The file to commit the changelog changes to',
@@ -261,7 +284,7 @@ module API
           **declared_params(include_missing: false)
         )
 
-        service.execute
+        service.execute(commit_to_changelog: true)
         status(200)
       rescue Gitlab::Changelog::Error => ex
         render_api_error!("Failed to generate the changelog: #{ex.message}", 422)

spec/lib/api/entities/changelog_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe API::Entities::Changelog do
+  let(:changelog) { "This is a changelog" }
+
+  subject { described_class.new(changelog).as_json }
+
+  it 'exposes correct attributes' do
+    expect(subject).to include(:notes)
+  end
+
+  it 'exposes correct notes' do
+    expect(subject[:notes]).to eq(changelog)
+  end
+end

spec/requests/api/repositories_spec.rb

@@ -731,6 +731,71 @@ RSpec.describe API::Repositories do
     end
   end
 
+  describe 'GET /projects/:id/repository/changelog' do
+    it 'generates the changelog for a version' do
+      spy = instance_spy(Repositories::ChangelogService)
+      release_notes = 'Release notes'
+
+      allow(Repositories::ChangelogService)
+        .to receive(:new)
+        .with(
+          project,
+          user,
+          version: '1.0.0',
+          from: 'foo',
+          to: 'bar',
+          date: DateTime.new(2020, 1, 1),
+          trailer: 'Foo'
+        )
+        .and_return(spy)
+
+      expect(spy).to receive(:execute).with(commit_to_changelog: false).and_return(release_notes)
+
+      get(
+        api("/projects/#{project.id}/repository/changelog", user),
+        params: {
+          version: '1.0.0',
+          from: 'foo',
+          to: 'bar',
+          date: '2020-01-01',
+          trailer: 'Foo'
+        }
+      )
+
+      expect(response).to have_gitlab_http_status(:ok)
+      expect(json_response['notes']).to eq(release_notes)
+    end
+
+    it 'supports leaving out the from and to attribute' do
+      spy = instance_spy(Repositories::ChangelogService)
+
+      allow(Repositories::ChangelogService)
+        .to receive(:new)
+        .with(
+          project,
+          user,
+          version: '1.0.0',
+          date: DateTime.new(2020, 1, 1),
+          trailer: 'Foo'
+        )
+        .and_return(spy)
+
+      expect(spy).to receive(:execute).with(commit_to_changelog: false)
+
+      get(
+        api("/projects/#{project.id}/repository/changelog", user),
+        params: {
+          version: '1.0.0',
+          date: '2020-01-01',
+          trailer: 'Foo'
+        }
+      )
+
+      expect(response).to have_gitlab_http_status(:ok)
+      expect(json_response['notes']).to be_present
+    end
+  end
+
   describe 'POST /projects/:id/repository/changelog' do
     it 'generates the changelog for a version' do
       spy = instance_spy(Repositories::ChangelogService)
@@ -751,7 +816,7 @@ RSpec.describe API::Repositories do
         )
         .and_return(spy)
 
-      allow(spy).to receive(:execute)
+      allow(spy).to receive(:execute).with(commit_to_changelog: true)
 
       post(
         api("/projects/#{project.id}/repository/changelog", user),
@@ -787,7 +852,7 @@ RSpec.describe API::Repositories do
         )
         .and_return(spy)
 
-      expect(spy).to receive(:execute)
+      expect(spy).to receive(:execute).with(commit_to_changelog: true)
 
       post(
         api("/projects/#{project.id}/repository/changelog", user),

spec/services/repositories/changelog_service_spec.rb

@@ -61,6 +61,8 @@ RSpec.describe Repositories::ChangelogService do
     let!(:commit2) { project.commit(sha3) }
     let!(:commit3) { project.commit(sha4) }
 
+    let(:commit_to_changelog) { true }
+
     it 'generates and commits a changelog section' do
       allow(MergeRequestDiffCommit)
         .to receive(:oldest_merge_request_id_per_commit)
@@ -73,7 +75,7 @@ RSpec.describe Repositories::ChangelogService do
       service = described_class
         .new(project, creator, version: '1.0.0', from: sha1, to: sha3)
 
-      recorder = ActiveRecord::QueryRecorder.new { service.execute }
+      recorder = ActiveRecord::QueryRecorder.new { service.execute(commit_to_changelog: commit_to_changelog) }
       changelog = project.repository.blob_at('master', 'CHANGELOG.md')&.data
 
       expect(recorder.count).to eq(9)
@@ -90,7 +92,7 @@ RSpec.describe Repositories::ChangelogService do
 
       described_class
         .new(project, creator, version: '1.0.0', from: sha1)
-        .execute
+        .execute(commit_to_changelog: commit_to_changelog)
 
       changelog = project.repository.blob_at('master', 'CHANGELOG.md')&.data
 
@@ -108,7 +110,7 @@ RSpec.describe Repositories::ChangelogService do
 
       described_class
         .new(project, creator, version: '1.0.0', from: sha1)
-        .execute
+        .execute(commit_to_changelog: commit_to_changelog)
 
       changelog = project.repository.blob_at('master', 'CHANGELOG.md')&.data
 
@@ -119,12 +121,33 @@ RSpec.describe Repositories::ChangelogService do
     it 'uses the target branch when "to" is unspecified' do
       described_class
         .new(project, creator, version: '1.0.0', from: sha1)
-        .execute
+        .execute(commit_to_changelog: commit_to_changelog)
 
       changelog = project.repository.blob_at('master', 'CHANGELOG.md')&.data
 
       expect(changelog).to include('Title 1', 'Title 2', 'Title 3')
     end
+
+    describe 'with commit_to_changelog: false' do
+      let(:commit_to_changelog) { false }
+
+      it 'generates changelog section' do
+        allow(MergeRequestDiffCommit)
+          .to receive(:oldest_merge_request_id_per_commit)
+          .with(project.id, [commit2.id, commit1.id])
+          .and_return([
+            { sha: sha2, merge_request_id: mr1.id },
+            { sha: sha3, merge_request_id: mr2.id }
+          ])
+
+        service = described_class
+          .new(project, creator, version: '1.0.0', from: sha1, to: sha3)
+
+        changelog = service.execute(commit_to_changelog: commit_to_changelog)
+
+        expect(changelog).to include('Title 1', 'Title 2')
+      end
+    end
   end
 
   describe '#start_of_commit_range' do