### Summary While testing [automatic labelling on incident issues](https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14705) I discovered that users can create duplicate labels on a project. In this case we utilize [`Labels::FindOrCreateService`](https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14705/diffs#3b92485756f7a3178af55f82581d87e5bf8b8d5f_45_62) to find or create a label for each incoming Prometheus alert. If the alerts come in simultaneously duplicate labels are created. Using these duplicate labels in issue fail due to validation constraints (`Label is invalid`). Refs https://gitlab.com/gitlab-org/gitlab-ce/issues/34953 ### Steps to reproduce On your local GDK run Rails console: ```ruby user = User.first # mostly root project = Project.last project.group # => #<Group id:99 @gitlab-qa-sandbox-group/qa-test-2019-07-14-12-49-15-42529f648751dc6d> # Simulate concurrency threads = 5.times.map { Thread.new { Labels::FindOrCreateService.new(user, project, title: "not uniq").execute } } threads.map(&:join) label = Label.where(project_id: project, title: "not uniq").first # Does really matter which one issue = Issues::CreateService.new(project, user, title: "fail", label_ids: [label.id]).execute issue.persisted? # => false issue.errors # => #<ActiveModel::Errors:0x000055a336637200 # @base=#<Issue id: manual-prometheus/autodevops-deploy#192>, # @details={:labels=>[{:error=>:invalid}]}, # @messages={:labels=>["is invalid"]}> ``` ### What is the current *bug* behavior? Users can create duplicate labels. ### What is the expected *correct* behavior? Labels should be unique. ### Relevant logs and/or screenshots  ### Possible fixes #### 1. Database index does not work properly Not sure as we already have an `UNIQUE` index on `labels` table: ```sql ... "index_labels_on_group_id_and_project_id_and_title" UNIQUE, btree (group_id, project_id, title) ... ``` <details> ```sql gitlabhq_development=# \d labels Table "public.labels" Column | Type | Modifiers -------------------------+-----------------------------+----------------------------------------------------- id | integer | not null default nextval('labels_id_seq'::regclass) title | character varying | color | character varying | project_id | integer | created_at | timestamp without time zone | updated_at | timestamp without time zone | template | boolean | default false description | character varying | description_html | text | type | character varying | group_id | integer | cached_markdown_version | integer | Indexes: "labels_pkey" PRIMARY KEY, btree (id) "index_labels_on_group_id_and_project_id_and_title" UNIQUE, btree (group_id, project_id, title) "index_labels_on_project_id" btree (project_id) "index_labels_on_template" btree (template) WHERE template "index_labels_on_title" btree (title) "index_labels_on_type_and_project_id" btree (type, project_id) Foreign-key constraints: "fk_7de4989a69" FOREIGN KEY (project_id) REFERENCES projects(id) ON DELETE CASCADE "fk_rails_c1ac5161d8" FOREIGN KEY (group_id) REFERENCES namespaces(id) ON DELETE CASCADE Referenced by: TABLE "lists" CONSTRAINT "fk_7a5553d60f" FOREIGN KEY (label_id) REFERENCES labels(id) ON DELETE CASCADE TABLE "label_links" CONSTRAINT "fk_d97dd08678" FOREIGN KEY (label_id) REFERENCES labels(id) ON DELETE CASCADE TABLE "board_labels" CONSTRAINT "fk_rails_362b0600a3" FOREIGN KEY (label_id) REFERENCES labels(id) ON DELETE CA SCADE TABLE "resource_label_events" CONSTRAINT "fk_rails_b126799f57" FOREIGN KEY (label_id) REFERENCES labels(id) ON DELETE SET NULL TABLE "label_priorities" CONSTRAINT "fk_rails_e161058b0f" FOREIGN KEY (label_id) REFERENCES labels(id) ON DELET E CASCADE ``` </details> This index does not work properly: ```sql gitlabhq_development=# select project_id, title, group_id from labels where project_id = 61; project_id | title | group_id ------------+-----------+---------- 61 | not uniq | 61 | not uniq | 61 | not uniq | 61 | not uniq | 61 | not uniq | ``` #### 2. Handle duplicate label in service `Labels::FindOrCreateService` should handle the case where the label was not persisted [after creation](https://gitlab.com/gitlab-org/gitlab-ce/blob/9de60d6d607c9a52589fe442cbefc5cc5bc0262c/app/services/labels/find_or_create_service.rb#L38).