Add mutant DB parallelism

Author: mbj

core/spec/mutant_hooks.rb

@@ -2,9 +2,93 @@
 
 # Mutant hooks (see core/.mutant.yml).
 #
-# The DummyApp boots with `config.eager_load = false`, so Zeitwerk loads classes
-# lazily. Mutant discovers subjects from loaded constants, so without forcing
-# eager loading the subject expressions can silently match nothing.
+# Per the mutant Rails guide (docs/rails.md), parallel workers must each get
+# their own database or they corrupt each other's fixtures. This is the
+# PostgreSQL pattern: each worker clones the migrated test database via
+# `CREATE DATABASE ... TEMPLATE`, named "<db>_mutant_worker_<index>".
+#
+# The guide's `with_root_connection` uses `ActiveRecord::Base.postgresql_connection`,
+# which was removed in Rails 8.x; we open the maintenance connection with the pg
+# gem directly instead. The rest follows the guide.
+require "pg"
+
+# Eager load so subjects are discoverable (Zeitwerk lazy-loads otherwise).
 hooks.register(:env_infection_post) do
   Rails.application.eager_load!
 end
+
+# Disconnect the parent before workers clone the template database.
+hooks.register(:setup_integration_post) do
+  base_records.each do |base|
+    disconnect_pool(base:)
+  end
+end
+
+# Both registrations are required to isolate in both modes:
+# mutation_worker_process_start for `mutant run`, test_worker_process_start for `mutant test`.
+hooks.register(:test_worker_process_start)     { |index:| isolate_index(index:) }
+hooks.register(:mutation_worker_process_start) { |index:| isolate_index(index:) }
+
+def self.base_records
+  [
+    ActiveRecord::Base,
+  ]
+end
+
+def self.isolate_index(index:)
+  base_records.each do |base|
+    disconnect_pool(base:)
+    isolate_database(base:, index:)
+  end
+end
+
+def self.isolate_database(base:, index:)
+  db_config = base
+    .connection_handler
+    .retrieve_connection_pool(base.connection_specification_name)
+    .db_config
+
+  raw_template_database = db_config.database
+  raw_isolated_database = "#{raw_template_database}_mutant_worker_#{index}"
+
+  with_root_connection do |connection|
+    template_database = PG::Connection.quote_ident(raw_template_database)
+    isolated_database = PG::Connection.quote_ident(raw_isolated_database)
+
+    connection.exec("DROP DATABASE IF EXISTS #{isolated_database}")
+    connection.exec("CREATE DATABASE #{isolated_database} TEMPLATE #{template_database}")
+  end
+
+  db_config._database = raw_isolated_database
+end
+
+def self.disconnect_pool(base:)
+  base
+    .connection_handler
+    .retrieve_connection_pool(base.connection_specification_name)
+    .disconnect
+end
+
+# Open a connection to the "postgres" maintenance database so we can issue
+# CREATE/DROP DATABASE. (Replaces the guide's removed Base.postgresql_connection.)
+def self.with_root_connection
+  base = ActiveRecord::Base
+
+  config = base
+    .connection_handler
+    .retrieve_connection_pool(base.connection_specification_name)
+    .db_config
+    .configuration_hash
+
+  connection = PG.connect(
+    host:     config[:host],
+    port:     config[:port] || 5432,
+    user:     config[:username],
+    password: config[:password],
+    dbname:   "postgres"
+  )
+
+  yield connection
+ensure
+  connection&.close
+end