Expose link_type in REST API

Exposes issue link's type in REST API.

changelogs/unreleased/link-types-api-rest.yml

+---
+title: Expose issue link type in REST API
+merge_request: 21375
+author:
+type: added

doc/api/issue_links.md

@@ -48,6 +48,7 @@ Parameters:
     "web_url": "http://example.com/example/example/issues/14",
     "confidential": false,
     "weight": null,
+    "link_type": "relates_to",
   }
 ]
 ```
@@ -66,6 +67,7 @@ POST /projects/:id/issues/:issue_iid/links
 | `issue_iid` | integer | yes      | The internal ID of a project's issue |
 | `target_project_id` | integer/string | yes      | The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) of a target project  |
 | `target_issue_iid` | integer/string | yes      | The internal ID of a target project's issue |
+| `link_type` | string  | no | The type of the relation ("relates_to", "blocks", "is_blocked_by"), defaults to "relates_to"). Ignored unless `issue_link_types` feature flag is enabled. |
 
 ```bash
 curl --request POST --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/projects/4/issues/1/links?target_project_id=5&target_issue_iid=1"
@@ -134,7 +136,8 @@ Example response:
     "web_url": "http://example.com/example/example/issues/14",
     "confidential": false,
     "weight": null,
-  }
+  },
+  "link_type": "relates_to"
 }
 ```
 
@@ -151,6 +154,7 @@ DELETE /projects/:id/issues/:issue_iid/links/:issue_link_id
 | `id`        | integer/string | yes      | The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user  |
 | `issue_iid` | integer | yes      | The internal ID of a project's issue |
 | `issue_link_id` | integer/string | yes      | The ID of an issue relationship |
+| `link_type` | string  | no | The type of the relation ('relates_to', 'blocks', 'is_blocked_by'), defaults to 'relates_to' |
 
 ```json
 {
@@ -213,6 +217,7 @@ DELETE /projects/:id/issues/:issue_iid/links/:issue_link_id
     "web_url": "http://example.com/example/example/issues/14",
     "confidential": false,
     "weight": null,
-  }
+  },
+  "link_type": "relates_to"
 }
 ```

ee/lib/api/issue_links.rb

@@ -30,6 +30,8 @@ module API
       params do
         requires :target_project_id, type: String, desc: 'The ID of the target project'
         requires :target_issue_iid, type: Integer, desc: 'The IID of the target issue'
+        optional :link_type, type: String, values: IssueLink.link_types.keys,
+          desc: 'The type of the relation. Ignored unless `issue_link_types` feature flag is enabled.'
       end
       # rubocop: disable CodeReuse/ActiveRecord
       post ':id/issues/:issue_iid/links' do
@@ -37,7 +39,7 @@ module API
         target_issue = find_project_issue(declared_params[:target_issue_iid],
                                           declared_params[:target_project_id])
 
-        create_params = { target_issuable: target_issue }
+        create_params = { target_issuable: target_issue, link_type: params[:link_type] }
 
         result = ::IssueLinks::CreateService
                    .new(source_issue, current_user, create_params)

ee/lib/ee/api/entities.rb

@@ -269,6 +269,7 @@ module EE
 
       class RelatedIssue < ::API::Entities::Issue
         expose :issue_link_id
+        expose :issue_link_type, as: :link_type
       end
 
       class LinkedEpic < Grape::Entity
@@ -395,6 +396,7 @@ module EE
       class IssueLink < Grape::Entity
         expose :source, as: :source_issue, using: ::API::Entities::IssueBasic
         expose :target, as: :target_issue, using: ::API::Entities::IssueBasic
+        expose :link_type
       end
 
       class SpecialBoardFilter < Grape::Entity

ee/spec/requests/api/issue_links_spec.rb

@@ -3,9 +3,9 @@
 require 'spec_helper'
 
 describe API::IssueLinks do
-  let(:user) { create(:user) }
-  let(:project) { create(:project) }
-  let(:issue) { create(:issue, project: project) }
+  let_it_be(:user) { create(:user) }
+  let_it_be(:project) { create(:project) }
+  let_it_be(:issue) { create(:issue, project: project) }
 
   before do
     project.add_guest(user)
@@ -30,7 +30,7 @@ describe API::IssueLinks do
         expect(response).to have_gitlab_http_status(200)
         expect(json_response).to be_an Array
         expect(json_response.length).to eq(1)
-        expect(json_response.first).to include('iid', 'title', 'issue_link_id')
+        expect(json_response.first).to include('iid', 'title', 'issue_link_id', 'link_type')
       end
     end
   end
@@ -113,26 +113,43 @@ describe API::IssueLinks do
       end
 
       context 'when user has ability to create an issue link' do
-        it 'returns 201' do
-          target_issue = create(:issue, project: project)
+        let_it_be(:target_issue) { create(:issue, project: project) }
+
+        before do
           project.add_reporter(user)
+        end
 
+        it 'returns 201' do
           post api("/projects/#{project.id}/issues/#{issue.iid}/links", user),
-               params: { target_project_id: project.id, target_issue_iid: target_issue.iid }
+               params: { target_project_id: project.id, target_issue_iid: target_issue.iid, link_type: 'blocks' }
 
-          expect(response).to have_gitlab_http_status(201)
-          expect(json_response).to include('source_issue', 'target_issue')
+          expect_link_response(link_type: 'blocks')
         end
 
-        it 'returns 201 when sending full path of target project' do
-          target_issue = create(:issue, project: project)
-          project.add_reporter(user)
+        context 'when `issue_link_type` is disabled' do
+          before do
+            stub_feature_flags(issue_link_types: false)
+          end
+
+          it 'ignores `link_type` parameter' do
+            post api("/projects/#{project.id}/issues/#{issue.iid}/links", user),
+                 params: { target_project_id: project.id, target_issue_iid: target_issue.iid, link_type: 'blocks' }
+
+            expect_link_response
+          end
+        end
 
+        it 'returns 201 when sending full path of target project' do
           post api("/projects/#{project.id}/issues/#{issue.iid}/links", user),
                params: { target_project_id: project.to_reference(full: true), target_issue_iid: target_issue.iid }
 
+          expect_link_response
+        end
+
+        def expect_link_response(link_type: 'relates_to')
           expect(response).to have_gitlab_http_status(201)
-          expect(json_response).to include('source_issue', 'target_issue')
+          expect(json_response).to include('source_issue', 'target_issue', 'link_type')
+          expect(json_response['link_type']).to eq(link_type)
         end
       end
     end
@@ -195,7 +212,7 @@ describe API::IssueLinks do
           delete api("/projects/#{project.id}/issues/#{issue.iid}/links/#{issue_link.id}", user)
 
           expect(response).to have_gitlab_http_status(200)
-          expect(json_response).to include('source_issue', 'target_issue')
+          expect(json_response).to include('source_issue', 'target_issue', 'link_type')
         end
       end
     end