Add issues/:iid/closed_by api endpoint

changelogs/unreleased/26437-closed-by.yml

+---
+title: Add issues/:iid/closed_by api endpoint
+merge_request:
+author: mhasbini

doc/api/issues.md

@@ -823,6 +823,67 @@ Example response:
 }
 ```
 
+## List merge requests that will close issue on merge
+
+Get all the merge requests that will close issue when merged.
+
+```
+GET /projects/:id/issues/:issue_iid/closed_by
+```
+
+| Attribute   | Type    | Required | Description                          |
+| ---------   | ----    | -------- | -----------                          |
+| `id`        | integer | yes      | The ID of a project                  |
+| `issue_iid` | integer | yes      | The internal ID of a project issue   |
+
+```bash
+curl --header "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" https://gitlab.example.com/api/v4/projects/1/issues/11/closed_by
+```
+
+Example response:
+
+```json
+[
+  {
+    "id": 6471,
+    "iid": 6432,
+    "project_id": 1,
+    "title": "add a test for cgi lexer options",
+    "description": "closes #11",
+    "state": "opened",
+    "created_at": "2017-04-06T18:33:34.168Z",
+    "updated_at": "2017-04-09T20:10:24.983Z",
+    "target_branch": "master",
+    "source_branch": "feature.custom-highlighting",
+    "upvotes": 0,
+    "downvotes": 0,
+    "author": {
+      "name": "Administrator",
+      "username": "root",
+      "id": 1,
+      "state": "active",
+      "avatar_url": "http://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=80&d=identicon",
+      "web_url": "https://gitlab.example.com/root"
+    },
+    "assignee": null,
+    "source_project_id": 1,
+    "target_project_id": 1,
+    "labels": [],
+    "work_in_progress": false,
+    "milestone": null,
+    "merge_when_pipeline_succeeds": false,
+    "merge_status": "unchecked",
+    "sha": "5a62481d563af92b8e32d735f2fa63b94e806835",
+    "merge_commit_sha": null,
+    "user_notes_count": 1,
+    "should_remove_source_branch": null,
+    "force_remove_source_branch": false,
+    "web_url": "https://gitlab.example.com/gitlab-org/gitlab-test/merge_requests/6432"
+  }
+]
+```
+
+
 ## Comments on issues
 
 Comments are done via the [notes](notes.md) resource.

lib/api/issues.rb

@@ -215,6 +215,21 @@ module API
         authorize!(:destroy_issue, issue)
         issue.destroy
       end
+
+      desc 'List merge requests closing issue'  do
+        success Entities::MergeRequestBasic
+      end
+      params do
+        requires :issue_iid, type: Integer, desc: 'The internal ID of a project issue'
+      end
+      get ':id/issues/:issue_iid/closed_by' do
+        issue = find_project_issue(params[:issue_iid])
+
+        merge_request_ids = MergeRequestsClosingIssues.where(issue_id: issue).select(:merge_request_id)
+        merge_requests = MergeRequestsFinder.new(current_user, project_id: user_project.id).execute.where(id: merge_request_ids)
+
+        present paginate(merge_requests), with: Entities::MergeRequestBasic, current_user: current_user, project: user_project
+      end
     end
   end
 end

spec/factories/merge_requests_closing_issues.rb

+FactoryGirl.define do
+  factory :merge_requests_closing_issues do
+    issue
+    merge_request
+  end
+end

spec/features/issuables/issuable_list_spec.rb

@@ -68,7 +68,7 @@ describe 'issuable list', feature: true do
                               source_project: project,
                               source_branch: generate(:branch))
 
-      MergeRequestsClosingIssues.create!(issue: issue, merge_request: merge_request)
+      create(:merge_requests_closing_issues, issue: issue, merge_request: merge_request)
     end
   end
 end

spec/requests/api/issues_spec.rb

@@ -1345,6 +1345,41 @@ describe API::Issues, api: true  do
     include_examples 'time tracking endpoints', 'issue'
   end
 
+  describe 'GET :id/issues/:issue_iid/closed_by' do
+    let(:merge_request) do
+      create(:merge_request,
+             :simple,
+             author: user,
+             source_project: project,
+             target_project: project,
+             description: "closes #{issue.to_reference}")
+    end
+
+    before do
+      create(:merge_requests_closing_issues, issue: issue, merge_request: merge_request)
+    end
+
+    it 'returns merge requests that will close issue on merge' do
+      get api("/projects/#{project.id}/issues/#{issue.iid}/closed_by", user)
+
+      expect_paginated_array_response(size: 1)
+    end
+
+    context 'when no merge requests will close issue' do
+      it 'returns empty array' do
+        get api("/projects/#{project.id}/issues/#{closed_issue.iid}/closed_by", user)
+
+        expect_paginated_array_response(size: 0)
+      end
+    end
+
+    it "returns 404 when issue doesn't exists" do
+      get api("/projects/#{project.id}/issues/9999/closed_by", user)
+
+      expect(response).to have_http_status(404)
+    end
+  end
+
   def expect_paginated_array_response(size: nil)
     expect(response).to have_http_status(200)
     expect(response).to include_pagination_headers