Add test parallelization to Rails

Provides both a forked process and threaded parallelization options. To
use add `parallelize` to your test suite.

Takes a `workers` argument that controls how many times the process
is forked. For each process a new database will be created suffixed
with the worker number; test-database-0 and test-database-1
respectively.

If `ENV["PARALLEL_WORKERS"]` is set the workers argument will be ignored
and the environment variable will be used instead. This is useful for CI
environments, or other environments where you may need more workers than
you do for local testing.

If the number of workers is set to `1` or fewer, the tests will not be
parallelized.

The default parallelization method is to fork processes. If you'd like to
use threads instead you can pass `with: :threads` to the `parallelize`
method. Note the threaded parallelization does not create multiple
database and will not work with system tests at this time.

parallelize(workers: 2, with: :threads)

The threaded parallelization uses Minitest's parallel exector directly.
The processes paralleliztion uses a Ruby Drb server.

For parallelization via threads a setup hook and cleanup hook are
provided.

```
class ActiveSupport::TestCase
  parallelize_setup do |worker|
    # setup databases
  end

  parallelize_teardown do |worker|
    # cleanup database
  end

  parallelize(workers: 2)
end
```

[Eileen M. Uchitelle, Aaron Patterson]

3 files changed, 42 insertions, 2 deletions

diff --git a/activerecord/lib/active_record.rb b/activerecord/lib/active_record.rb
index b4377ad6be..d43378c64f 100644
--- a/activerecord/lib/active_record.rb
+++ b/activerecord/lib/active_record.rb
@@ -163,6 +163,7 @@ module ActiveRecord
       "active_record/tasks/postgresql_database_tasks"
   end
 
+  autoload :TestDatabases, "active_record/test_databases"
   autoload :TestFixtures, "active_record/fixtures"
 
   def self.eager_load!
diff --git a/activerecord/lib/active_record/fixtures.rb b/activerecord/lib/active_record/fixtures.rb
index d9a75d9ad6..8f022ff7a7 100644
--- a/activerecord/lib/active_record/fixtures.rb
+++ b/activerecord/lib/active_record/fixtures.rb
@@ -874,6 +874,7 @@ module ActiveRecord
       class_attribute :use_instantiated_fixtures, default: false # true, false, or :no_instances
       class_attribute :pre_loaded_fixtures, default: false
       class_attribute :config, default: ActiveRecord::Base
+      class_attribute :lock_threads, default: true
     end
 
     module ClassMethods
@@ -973,7 +974,7 @@ module ActiveRecord
         @fixture_connections = enlist_fixture_connections
         @fixture_connections.each do |connection|
           connection.begin_transaction joinable: false
-          connection.pool.lock_thread = true
+          connection.pool.lock_thread = true if lock_threads
         end
 
         # When connections are established in the future, begin a transaction too
@@ -989,7 +990,7 @@ module ActiveRecord
 
             if connection && !@fixture_connections.include?(connection)
               connection.begin_transaction joinable: false
-              connection.pool.lock_thread = true
+              connection.pool.lock_thread = true if lock_threads
               @fixture_connections << connection
             end
           end
diff --git a/activerecord/lib/active_record/test_databases.rb b/activerecord/lib/active_record/test_databases.rb
new file mode 100644
index 0000000000..606a3b0fb5
--- /dev/null
+++ b/activerecord/lib/active_record/test_databases.rb
@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+require "active_support/testing/parallelization"
+
+module ActiveRecord
+  module TestDatabases # :nodoc:
+    ActiveSupport::Testing::Parallelization.after_fork_hook do |i|
+      create_and_migrate(i, spec_name: Rails.env)
+    end
+
+    ActiveSupport::Testing::Parallelization.run_cleanup_hook do |i|
+      drop(i, spec_name: Rails.env)
+    end
+
+    def self.create_and_migrate(i, spec_name:)
+      old, ENV["VERBOSE"] = ENV["VERBOSE"], "false"
+
+      connection_spec = ActiveRecord::Base.configurations[spec_name]
+
+      connection_spec["database"] += "-#{i}"
+      ActiveRecord::Tasks::DatabaseTasks.create(connection_spec)
+      ActiveRecord::Base.establish_connection(connection_spec)
+      ActiveRecord::Tasks::DatabaseTasks.migrate
+    ensure
+      ActiveRecord::Base.establish_connection(ActiveRecord::Base.configurations[Rails.env])
+      ENV["VERBOSE"] = old
+    end
+
+    def self.drop(i, spec_name:)
+      old, ENV["VERBOSE"] = ENV["VERBOSE"], "false"
+      connection_spec = ActiveRecord::Base.configurations[spec_name]
+
+      ActiveRecord::Tasks::DatabaseTasks.drop(connection_spec)
+    ensure
+      ENV["VERBOSE"] = old
+    end
+  end
+end