Add delete endpoint for related epics

Allows deleting related epic links through REST API, similar to
issue_links endpoint for issues.

Changelog: added
EE: true

doc/api/linked_epics.md

@@ -91,6 +91,8 @@ Example response:
 
 ## Create a related epic link
 
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/352840) in GitLab 14.10.
+
 Create a two-way relation between two epics. The user must be allowed to
 update both epics to succeed.
 
@@ -205,3 +207,120 @@ Example response:
   "link_type": "relates_to"
 }
 ```
+
+## Delete a related epic link
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/352840) in GitLab 14.10.
+
+Delete a two-way relation between two epics. The user must be allowed to
+update both epics to succeed.
+
+```plaintext
+DELETE /groups/:id/epics/:epic_iid/related_epics/:related_epic_link_id
+```
+
+Supported attributes:
+
+| Attribute                | Type           | Required                    | Description                           |
+|--------------------------|----------------|-----------------------------|---------------------------------------|
+| `epic_iid`               | integer        | **{check-circle}** Yes      | Internal ID of a group's epic.        |
+| `id`                     | integer/string | **{check-circle}** Yes      | ID or [URL-encoded path of the group](index.md#namespaced-path-encoding) owned by the authenticated user. |
+| `related_epic_link_id`   | integer/string | **{check-circle}** Yes      | Internal ID of a related epic link. |
+
+Example request:
+
+```shell
+curl --request DELETE --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/groups/26/epics/1/related_epics/1"
+```
+
+Example response:
+
+```json
+{
+  "source_epic": {
+    "id": 21,
+    "iid": 1,
+    "color": "#1068bf",
+    "text_color": "#FFFFFF",
+    "group_id": 26,
+    "parent_id": null,
+    "parent_iid": null,
+    "title": "Aspernatur recusandae distinctio omnis et qui est iste.",
+    "description": "some description",
+    "confidential": false,
+    "author": {
+      "id": 15,
+      "username": "trina",
+      "name": "Theresia Robel",
+      "state": "active",
+      "avatar_url": "https://www.gravatar.com/avatar/085e28df717e16484cbf6ceca75e9a93?s=80&d=identicon",
+      "web_url": "http://gitlab.example.com/trina"
+    },
+    "start_date": null,
+    "end_date": null,
+    "due_date": null,
+    "state": "opened",
+    "web_url": "http://gitlab.example.com/groups/flightjs/-/epics/1",
+    "references": {
+      "short": "&1",
+      "relative": "&1",
+      "full": "flightjs&1"
+    },
+    "created_at": "2022-01-31T15:10:44.988Z",
+    "updated_at": "2022-03-16T09:32:35.712Z",
+    "closed_at": null,
+    "labels": [],
+    "upvotes": 0,
+    "downvotes": 0,
+    "_links": {
+      "self": "http://gitlab.example.com/api/v4/groups/26/epics/1",
+      "epic_issues": "http://gitlab.example.com/api/v4/groups/26/epics/1/issues",
+      "group": "http://gitlab.example.com/api/v4/groups/26",
+      "parent": null
+    }
+  },
+  "target_epic": {
+    "id": 25,
+    "iid": 5,
+    "color": "#1068bf",
+    "text_color": "#FFFFFF",
+    "group_id": 26,
+    "parent_id": null,
+    "parent_iid": null,
+    "title": "Aut assumenda id nihil distinctio fugiat vel numquam est.",
+    "description": "some description",
+    "confidential": false,
+    "author": {
+      "id": 3,
+      "username": "valerie",
+      "name": "Erika Wolf",
+      "state": "active",
+      "avatar_url": "https://www.gravatar.com/avatar/9ef7666abb101418a4716a8ed4dded80?s=80&d=identicon",
+      "web_url": "http://gitlab.example.com/valerie"
+    },
+    "start_date": null,
+    "end_date": null,
+    "due_date": null,
+    "state": "opened",
+    "web_url": "http://gitlab.example.com/groups/flightjs/-/epics/5",
+    "references": {
+      "short": "&5",
+      "relative": "&5",
+      "full": "flightjs&5"
+    },
+    "created_at": "2022-01-31T15:10:45.080Z",
+    "updated_at": "2022-03-16T09:32:35.842Z",
+    "closed_at": null,
+    "labels": [],
+    "upvotes": 0,
+    "downvotes": 0,
+    "_links": {
+      "self": "http://gitlab.example.com/api/v4/groups/26/epics/5",
+      "epic_issues": "http://gitlab.example.com/api/v4/groups/26/epics/5/issues",
+      "group": "http://gitlab.example.com/api/v4/groups/26",
+      "parent": null
+    }
+  },
+  "link_type": "relates_to"
+}
+```

ee/lib/api/related_epic_links.rb

@@ -84,6 +84,28 @@ module API
           render_api_error!(result[:message], result[:http_status])
         end
       end
+
+      desc 'Remove epics relation' do
+        success Entities::RelatedEpicLink
+      end
+      params do
+        requires :related_epic_link_id, type: Integer, desc: 'The ID of a related epic link'
+      end
+      delete ':id/epics/:epic_iid/related_epics/:related_epic_link_id' do
+        find_permissioned_epic!(params[:epic_iid])
+
+        epic_link = ::Epic::RelatedEpicLink.find(declared_params[:related_epic_link_id])
+
+        result = ::Epics::RelatedEpicLinks::DestroyService
+                   .new(epic_link, current_user)
+                   .execute
+
+        if result[:status] == :success
+          present epic_link, with: Entities::RelatedEpicLink
+        else
+          render_api_error!(result[:message], result[:http_status])
+        end
+      end
     end
   end
 end

ee/spec/requests/api/related_epic_links_spec.rb

@@ -189,13 +189,97 @@ RSpec.describe API::RelatedEpicLinks do
 
           it_behaves_like 'not found resource', '404 Not found'
         end
+      end
+    end
+  end
+
+  describe 'DELETE /related_epics' do
+    let_it_be(:target_group) { create(:group, :private) }
+    let_it_be(:target_epic) { create(:epic, group: target_group) }
+    let_it_be_with_reload(:related_epic_link) { create(:related_epic_link, source: epic, target: target_epic) }
+
+    subject { perform_request(user) }
+
+    def perform_request(user = nil)
+      delete api("/groups/#{group.id}/epics/#{epic.iid}/related_epics/#{related_epic_link.id}", user)
+    end
+
+    shared_examples 'not found resource' do |message|
+      it 'returns 404' do
+        subject
+
+        expect(response).to have_gitlab_http_status(:not_found)
+        expect(json_response['message']).to eq(message)
+      end
+    end
+
+    shared_examples 'forbidden resource' do |message|
+      it 'returns 403' do
+        subject
+
+        expect(response).to have_gitlab_http_status(:forbidden)
+      end
+    end
+
+    context 'when unauthenticated' do
+      it 'returns 401' do
+        perform_request
+
+        expect(response).to have_gitlab_http_status(:unauthorized)
+      end
+    end
+
+    context 'when user can not access source epic' do
+      before do
+        target_group.add_reporter(user)
+      end
+
+      it_behaves_like 'not found resource', '404 Group Not Found'
+    end
+
+    context 'when user can only read source epic' do
+      before do
+        group.add_guest(user)
+        target_group.add_reporter(user)
+      end
+
+      it_behaves_like 'forbidden resource'
+    end
+
+    context 'when user can manage source epic' do
+      before do
+        group.add_reporter(user)
+      end
 
-        def expect_link_response(link_type: 'relates_to')
-          expect(response).to have_gitlab_http_status(:created)
-          expect(response).to match_response_schema('public_api/v4/related_epic_link')
-          expect(json_response['link_type']).to eq(link_type)
+      it_behaves_like 'not found resource', 'No Related Epic Link found'
+
+      context 'when user is guest in target group' do
+        before do
+          target_group.add_guest(user)
+        end
+
+        it_behaves_like 'not found resource', 'No Related Epic Link found'
+      end
+
+      context 'when user can relate epics' do
+        before do
+          target_group.add_reporter(user)
+        end
+
+        it_behaves_like 'a not available endpoint'
+
+        it 'returns 200 status and contains the expected link response' do
+          subject
+
+          expect_link_response(status: :ok)
         end
       end
     end
   end
+
+  def expect_link_response(link_type: 'relates_to', status: :created)
+    expect(response).to have_gitlab_http_status(status)
+    expect(response).to match_response_schema('public_api/v4/related_epic_link')
+    expect(json_response['link_type']).to eq(link_type)
+  end
 end