Allow merge requests related to a commit to be found via API

- Original work for the web interface here:
 - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13713

changelogs/unreleased/expose-commits-mr-api.yml

+---
+title: Allow merge requests related to a commit to be found via API
+merge_request:
+author:
+type: added

doc/api/commits.md

@@ -536,6 +536,74 @@ Example response:
 }
 ```
 
+## List Merge Requests associated with a commit
+
+Get a list of Merge Requests related to the specified commit.
+
+```
+GET /projects/:id/repository/commits/:sha/merge_requests
+```
+
+| Attribute | Type | Required | Description |
+| --------- | ---- | -------- | ----------- |
+| `id`      | integer/string | yes | The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user
+| `sha`     | string  | yes   | The commit SHA
+
+
+```bash
+curl --header "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" "https://gitlab.example.com/api/v4/projects/5/repository/commits/af5b13261899fb2c0db30abdd0af8b07cb44fdc5/merge_requests"
+```
+
+Example response:
+
+```json
+[
+   {
+      "id":45,
+      "iid":1,
+      "project_id":35,
+      "title":"Add new file",
+      "description":"",
+      "state":"opened",
+      "created_at":"2018-03-26T17:26:30.916Z",
+      "updated_at":"2018-03-26T17:26:30.916Z",
+      "target_branch":"master",
+      "source_branch":"test-branch",
+      "upvotes":0,
+      "downvotes":0,
+      "author" : {
+        "web_url" : "https://gitlab.example.com/thedude",
+        "name" : "Jeff Lebowski",
+        "avatar_url" : "https://gitlab.example.com/uploads/user/avatar/28/The-Big-Lebowski-400-400.png",
+        "username" : "thedude",
+        "state" : "active",
+        "id" : 28
+      },
+      "assignee":null,
+      "source_project_id":35,
+      "target_project_id":35,
+      "labels":[ ],
+      "work_in_progress":false,
+      "milestone":null,
+      "merge_when_pipeline_succeeds":false,
+      "merge_status":"can_be_merged",
+      "sha":"af5b13261899fb2c0db30abdd0af8b07cb44fdc5",
+      "merge_commit_sha":null,
+      "user_notes_count":0,
+      "discussion_locked":null,
+      "should_remove_source_branch":null,
+      "force_remove_source_branch":false,
+      "web_url":"http://https://gitlab.example.com/root/test-project/merge_requests/1",
+      "time_stats":{
+         "time_estimate":0,
+         "total_time_spent":0,
+         "human_time_estimate":null,
+         "human_total_time_spent":null
+      }
+   }
+]
+```
+
 [ce-6096]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6096 "Multi-file commit"
 [ce-8047]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8047
 [ce-15026]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15026

lib/api/commits.rb

@@ -231,6 +231,20 @@ module API
           render_api_error!("Failed to save note #{note.errors.messages}", 400)
         end
       end
+
+      desc 'Get Merge Requests associated with a commit' do
+        success Entities::MergeRequestBasic
+      end
+      params do
+        requires :sha, type: String, desc: 'A commit sha, or the name of a branch or tag on which to find Merge Requests'
+        use :pagination
+      end
+      get ':id/repository/commits/:sha/merge_requests', requirements: API::COMMIT_ENDPOINT_REQUIREMENTS do
+        commit = user_project.commit(params[:sha])
+        not_found! 'Commit' unless commit
+
+        present paginate(commit.merge_requests), with: Entities::MergeRequestBasic
+      end
     end
   end
 end

spec/requests/api/commits_spec.rb

@@ -1141,4 +1141,33 @@ describe API::Commits do
       end
     end
   end
+
+  describe 'GET /projects/:id/repository/commits/:sha/merge_requests' do
+    let!(:project) { create(:project, :repository, :private) }
+    let!(:merged_mr) { create(:merge_request, source_project: project, source_branch: 'master', target_branch: 'feature') }
+    let(:commit) { merged_mr.merge_request_diff.commits.last }
+
+    it 'returns the correct merge request' do
+      get api("/projects/#{project.id}/repository/commits/#{commit.id}/merge_requests", user)
+
+      expect(response).to have_gitlab_http_status(200)
+      expect(response).to include_pagination_headers
+      expect(json_response.length).to eq(1)
+      expect(json_response[0]['id']).to eq(merged_mr.id)
+    end
+
+    it 'returns 403 for an unauthorized user' do
+      project.add_guest(user)
+
+      get api("/projects/#{project.id}/repository/commits/#{commit.id}/merge_requests", user)
+
+      expect(response).to have_gitlab_http_status(403)
+    end
+
+    it 'responds 404 when the commit does not exist' do
+      get api("/projects/#{project.id}/repository/commits/a7d26f00c35b/merge_requests", user)
+
+      expect(response).to have_gitlab_http_status(404)
+    end
+  end
 end