Add API endpoint to delete topics

Refs https://gitlab.com/gitlab-org/gitlab/-/issues/350743

doc/api/topics.md

@@ -203,3 +203,26 @@ curl --request PUT \
      --header "PRIVATE-TOKEN: <your_access_token>" \
      "https://gitlab.example.com/api/v4/topics/1"
 ```
+
+## Delete a project topic
+
+You must be an administrator to delete a project.
+When you delete a project topic, you also delete the topic assignment for projects.
+
+```plaintext
+DELETE /topics/:id
+```
+
+Supported attributes:
+
+| Attribute     | Type    | Required               | Description         |
+| ------------- | ------- | ---------------------- | ------------------- |
+| `id`          | integer | **{check-circle}** Yes | ID of project topic |
+
+Example request:
+
+```shell
+curl --request DELETE \
+     --header "PRIVATE-TOKEN: <your_access_token>" \
+     "https://gitlab.example.com/api/v4/topics/1"
+```

lib/api/topics.rb

@@ -77,5 +77,19 @@ module API
         render_validation_error!(topic)
       end
     end
+
+    desc 'Delete a topic' do
+      detail 'This feature was introduced in GitLab 14.8.'
+    end
+    params do
+      requires :id, type: Integer, desc: 'ID of project topic'
+    end
+    delete 'topics/:id' do
+      authenticated_as_admin!
+
+      topic = ::Projects::Topic.find(params[:id])
+
+      destroy_conditionally!(topic)
+    end
   end
 end

spec/requests/api/topics_spec.rb

@@ -248,4 +248,43 @@ RSpec.describe API::Topics do
       end
     end
   end
+
+  describe 'DELETE /topics', :aggregate_failures do
+    context 'as administrator' do
+      it 'deletes a topic' do
+        delete api("/topics/#{topic_3.id}", admin), params: { name: 'my-topic' }
+
+        expect(response).to have_gitlab_http_status(:no_content)
+      end
+
+      it 'returns 404 for non existing id' do
+        delete api("/topics/#{non_existing_record_id}", admin), params: { name: 'my-topic' }
+
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+
+      it 'returns 400 for invalid `id` parameter' do
+        delete api('/topics/invalid', admin), params: { name: 'my-topic' }
+
+        expect(response).to have_gitlab_http_status(:bad_request)
+        expect(json_response['error']).to eql('id is invalid')
+      end
+    end
+
+    context 'as normal user' do
+      it 'returns 403 Forbidden' do
+        delete api("/topics/#{topic_3.id}", user), params: { name: 'my-topic' }
+
+        expect(response).to have_gitlab_http_status(:forbidden)
+      end
+    end
+
+    context 'as anonymous' do
+      it 'returns 401 Unauthorized' do
+        delete api("/topics/#{topic_3.id}"), params: { name: 'my-topic' }
+
+        expect(response).to have_gitlab_http_status(:unauthorized)
+      end
+    end
+  end
 end