Fix Error 500s when attempting to destroy a protected tag

Due to a missing `on_delete: :cascade`, users would hit the error that
looked like:

```
PG::ForeignKeyViolation: ERROR: update or delete on table "protected_tags"
violates foreign key constraint "fk_rails_f7dfda8c51" on table
"protected_tag_create_access_levels" DETAIL: Key (id)=(1385) is still
referenced from table "protected_tag_create_access_levels". : DELETE FROM
"protected_tags" WHERE "protected_tags"."id" = 1385
```

Closes #36013

db/migrate/20170820100558_correct_protected_tags_foreign_keys.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class CorrectProtectedTagsForeignKeys < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    remove_foreign_key_without_error(:protected_tag_create_access_levels,
+                                     column: :protected_tag_id)
+
+    execute <<-EOF
+    DELETE FROM protected_tag_create_access_levels
+    WHERE NOT EXISTS (
+      SELECT true
+      FROM protected_tags
+      WHERE protected_tag_create_access_levels.protected_tag_id = protected_tags.id
+    )
+    AND protected_tag_id IS NOT NULL
+    EOF
+
+    add_concurrent_foreign_key(:protected_tag_create_access_levels,
+                               :protected_tags,
+                               column: :protected_tag_id)
+  end
+
+  def down
+    # Previously there was a foreign key without a CASCADING DELETE, so we'll
+    # just leave the foreign key in place.
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20170815060945) do
+ActiveRecord::Schema.define(version: 20170820100558) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -1726,7 +1726,7 @@ ActiveRecord::Schema.define(version: 20170815060945) do
   add_foreign_key "protected_branch_push_access_levels", "protected_branches", name: "fk_9ffc86a3d9", on_delete: :cascade
   add_foreign_key "protected_branches", "projects", name: "fk_7a9c6d93e7", on_delete: :cascade
   add_foreign_key "protected_tag_create_access_levels", "namespaces", column: "group_id"
-  add_foreign_key "protected_tag_create_access_levels", "protected_tags"
+  add_foreign_key "protected_tag_create_access_levels", "protected_tags", name: "fk_f7dfda8c51", on_delete: :cascade
   add_foreign_key "protected_tag_create_access_levels", "users"
   add_foreign_key "protected_tags", "projects", name: "fk_8e4af87648", on_delete: :cascade
   add_foreign_key "push_event_payloads", "events_for_migration", column: "event_id", name: "fk_36c74129da", on_delete: :cascade

lib/gitlab/database/migration_helpers.rb

@@ -606,6 +606,11 @@ module Gitlab
           Arel::Nodes::SqlLiteral.new(replace.to_sql)
         end
       end
+
+      def remove_foreign_key_without_error(*args)
+        remove_foreign_key(*args)
+      rescue ArgumentError
+      end
     end
   end
 end