Create unique index on projects namespace_id and name

Changelog: added

db/post_migrate/20220325155953_steal_background_job_for_fixing_conflicting_project_names_and_paths.rb

+# frozen_string_literal: true
+
+class StealBackgroundJobForFixingConflictingProjectNamesAndPaths < Gitlab::Database::Migration[1.0]
+  def up
+    Gitlab::BackgroundMigration.steal('FixDuplicateProjectNameAndPath')
+  end
+
+  def down
+    # no-op
+  end
+end

db/post_migrate/20220325160153_create_unique_index_on_projects_name_and_namespace_id.rb

+# frozen_string_literal: true
+
+class CreateUniqueIndexOnProjectsNameAndNamespaceId < Gitlab::Database::Migration[1.0]
+  INDEX_NAME = 'unique_projects_on_name_namespace_id'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :projects, [:name, :namespace_id], unique: true, name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :projects, INDEX_NAME
+  end
+end

db/schema_migrations/20220325155953

+bbcd27340d9d684b6134292772dad6f523ce6e1af9853ce12284a0c138d6b7bd
\ No newline at end of file

db/schema_migrations/20220325160153

+7f33fc1c6b47e3ff732caa62d15f0d725c03d993eacb24bc603992fc2579e63c
\ No newline at end of file

db/structure.sql

@@ -29684,6 +29684,8 @@ CREATE UNIQUE INDEX uniq_pkgs_debian_project_distributions_project_id_and_suite
 
 CREATE UNIQUE INDEX unique_merge_request_metrics_by_merge_request_id ON merge_request_metrics USING btree (merge_request_id);
 
+CREATE UNIQUE INDEX unique_projects_on_name_namespace_id ON projects USING btree (name, namespace_id);
+
 CREATE INDEX user_follow_users_followee_id_idx ON user_follow_users USING btree (followee_id);
 
 CREATE INDEX users_forbidden_state_idx ON users USING btree (id) WHERE ((confirmed_at IS NOT NULL) AND ((state)::text <> ALL (ARRAY['blocked'::text, 'banned'::text, 'ldap_blocked'::text])));

spec/lib/gitlab/background_migration/fix_duplicate_project_name_and_path_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-RSpec.describe Gitlab::BackgroundMigration::FixDuplicateProjectNameAndPath, :migration do
+RSpec.describe Gitlab::BackgroundMigration::FixDuplicateProjectNameAndPath, :migration, schema: 20220325155953 do
   let(:migration) { described_class.new }
   let(:namespaces) { table(:namespaces) }
   let(:projects) { table(:projects) }