Validate not null file_store

And remove an index which was added only to facilitate populating NULL
values.

changelogs/unreleased/mk-enforce-not-null-file-store-on-package-files.yml

+---
+title: Validate not null file_store field on packages_package_files to maintain data
+  integrity
+merge_request: 42400
+author:
+type: added

db/post_migrate/20200915191156_validate_not_null_file_store_on_package_files.rb

+# frozen_string_literal: true
+
+class ValidateNotNullFileStoreOnPackageFiles < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  # Remove index which was only added to fill file_store
+  INDEX_NAME = 'index_packages_package_files_file_store_is_null'
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    validate_not_null_constraint :packages_package_files, :file_store
+
+    remove_concurrent_index_by_name :packages_package_files, INDEX_NAME
+  end
+
+  def down
+    add_concurrent_index :packages_package_files, :id, where: 'file_store IS NULL', name: INDEX_NAME
+  end
+end

db/schema_migrations/20200915191156

+d8ddec6b234d59b3b85705dfa7b724d3be4974bfa57fae70aa5c2dbdd2e73cfa
\ No newline at end of file

db/structure.sql

@@ -14123,7 +14123,8 @@ CREATE TABLE packages_package_files (
     verified_at timestamp with time zone,
     verification_failure character varying(255),
     verification_retry_count integer,
-    verification_checksum bytea
+    verification_checksum bytea,
+    CONSTRAINT check_4c5e6bb0b3 CHECK ((file_store IS NOT NULL))
 );
 
 CREATE SEQUENCE packages_package_files_id_seq
@@ -18083,9 +18084,6 @@ ALTER TABLE design_management_designs
 ALTER TABLE vulnerability_scanners
     ADD CONSTRAINT check_37608c9db5 CHECK ((char_length(vendor) <= 255)) NOT VALID;
 
-ALTER TABLE packages_package_files
-    ADD CONSTRAINT check_4c5e6bb0b3 CHECK ((file_store IS NOT NULL)) NOT VALID;
-
 ALTER TABLE group_import_states
     ADD CONSTRAINT check_cda75c7c3f CHECK ((user_id IS NOT NULL)) NOT VALID;
 
@@ -20683,8 +20681,6 @@ CREATE INDEX index_packages_nuget_dl_metadata_on_dependency_link_id ON packages_
 
 CREATE UNIQUE INDEX index_packages_on_project_id_name_version_unique_when_generic ON packages_packages USING btree (project_id, name, version) WHERE (package_type = 7);
 
-CREATE INDEX index_packages_package_files_file_store_is_null ON packages_package_files USING btree (id) WHERE (file_store IS NULL);
-
 CREATE INDEX index_packages_package_files_on_file_store ON packages_package_files USING btree (file_store);
 
 CREATE INDEX index_packages_package_files_on_package_id_and_file_name ON packages_package_files USING btree (package_id, file_name);