Skip validation for concurrent foreign key

Add `validate: false` option for adding concurrent foreign key.
This allows the table lock to be held for a minimal amount of time
while still performing the checks that `add_concurrent_foreign_key`
performs for adding a foreign key to a table that may contain invalid
data, allowing us to put off any validation to a different time.

lib/gitlab/database/migration_helpers.rb

@@ -158,7 +158,7 @@ module Gitlab
       # name - The name of the foreign key.
       #
       # rubocop:disable Gitlab/RailsLogger
-      def add_concurrent_foreign_key(source, target, column:, on_delete: :cascade, name: nil)
+      def add_concurrent_foreign_key(source, target, column:, on_delete: :cascade, name: nil, validate: true)
         # Transactions would result in ALTER TABLE locks being held for the
         # duration of the transaction, defeating the purpose of this method.
         if transaction_open?
@@ -197,10 +197,14 @@ module Gitlab
         # Validate the existing constraint. This can potentially take a very
         # long time to complete, but fortunately does not lock the source table
         # while running.
+        # Disable this check by passing `validate: false` to the method call
         #
         # Note this is a no-op in case the constraint is VALID already
-        disable_statement_timeout do
-          execute("ALTER TABLE #{source} VALIDATE CONSTRAINT #{options[:name]};")
+
+        if validate
+          disable_statement_timeout do
+            execute("ALTER TABLE #{source} VALIDATE CONSTRAINT #{options[:name]};")
+          end
         end
       end
       # rubocop:enable Gitlab/RailsLogger

spec/lib/gitlab/database/migration_helpers_spec.rb

@@ -325,6 +325,25 @@ describe Gitlab::Database::MigrationHelpers do
           end
         end
       end
+
+      describe 'validate option' do
+        args = [:projects, :users]
+        options = { column: :user_id, on_delete: nil }
+
+        context 'when validate is supplied with a falsey value' do
+          it_behaves_like 'skips validation', args, options.merge(validate: false)
+          it_behaves_like 'skips validation', args, options.merge(validate: nil)
+        end
+
+        context 'when validate is supplied with a truthy value' do
+          it_behaves_like 'performs validation', args, options.merge(validate: true)
+          it_behaves_like 'performs validation', args, options.merge(validate: :yes)
+        end
+
+        context 'when validate is not supplied' do
+          it_behaves_like 'performs validation', args, options
+        end
+      end
     end
   end
 

spec/support/shared_examples/migration_helpers_examples.rb

+# frozen_string_literal: true
+
+shared_examples 'skips validation' do |args, options|
+  it 'skips validation' do
+    expect(model).not_to receive(:disable_statement_timeout)
+    expect(model).to receive(:execute).with(/ADD CONSTRAINT/)
+    expect(model).not_to receive(:execute).with(/VALIDATE CONSTRAINT/)
+
+    model.add_concurrent_foreign_key(*args, **options)
+  end
+end
+
+shared_examples 'performs validation' do |args, options|
+  it 'performs validation' do
+    expect(model).to receive(:disable_statement_timeout).and_call_original
+    expect(model).to receive(:execute).with(/statement_timeout/)
+    expect(model).to receive(:execute).ordered.with(/NOT VALID/)
+    expect(model).to receive(:execute).ordered.with(/VALIDATE CONSTRAINT/)
+    expect(model).to receive(:execute).with(/RESET ALL/)
+
+    model.add_concurrent_foreign_key(*args, **options)
+  end
+end