Fix API to remove deploy key from project instead of deleting it entirely

changelogs/unreleased/security-dm-delete-deploy-key.yml

+---
+title: Fix API to remove deploy key from project instead of deleting it entirely
+merge_request:
+author:
+type: security

lib/api/deploy_keys.rb

@@ -148,10 +148,10 @@ module API
         requires :key_id, type: Integer, desc: 'The ID of the deploy key'
       end
       delete ":id/deploy_keys/:key_id" do
-        key = user_project.deploy_keys.find(params[:key_id])
-        not_found!('Deploy Key') unless key
+        deploy_key_project = user_project.deploy_keys_projects.find_by(deploy_key_id: params[:key_id])
+        not_found!('Deploy Key') unless deploy_key_project
 
-        destroy_conditionally!(key)
+        destroy_conditionally!(deploy_key_project)
       end
     end
   end

spec/requests/api/deploy_keys_spec.rb

@@ -171,7 +171,7 @@ describe API::DeployKeys do
       deploy_key
     end
 
-    it 'deletes existing key' do
+    it 'removes existing key from project' do
       expect do
         delete api("/projects/#{project.id}/deploy_keys/#{deploy_key.id}", admin)
 
@@ -179,6 +179,44 @@ describe API::DeployKeys do
       end.to change { project.deploy_keys.count }.by(-1)
     end
 
+    context 'when the deploy key is public' do
+      it 'does not delete the deploy key' do
+        expect do
+          delete api("/projects/#{project.id}/deploy_keys/#{deploy_key.id}", admin)
+
+          expect(response).to have_gitlab_http_status(204)
+        end.not_to change { DeployKey.count }
+      end
+    end
+
+    context 'when the deploy key is not public' do
+      let!(:deploy_key) { create(:deploy_key, public: false) }
+
+      context 'when the deploy key is only used by this project' do
+        it 'deletes the deploy key' do
+          expect do
+            delete api("/projects/#{project.id}/deploy_keys/#{deploy_key.id}", admin)
+
+            expect(response).to have_gitlab_http_status(204)
+          end.to change { DeployKey.count }.by(-1)
+        end
+      end
+
+      context 'when the deploy key is used by other projects' do
+        before do
+          create(:deploy_keys_project, project: project2, deploy_key: deploy_key)
+        end
+
+        it 'does not delete the deploy key' do
+          expect do
+            delete api("/projects/#{project.id}/deploy_keys/#{deploy_key.id}", admin)
+
+            expect(response).to have_gitlab_http_status(204)
+          end.not_to change { DeployKey.count }
+        end
+      end
+    end
+
     it 'returns 404 Not Found with invalid ID' do
       delete api("/projects/#{project.id}/deploy_keys/404", admin)