Sync parallel test DBs to schema using SHA

Previously, every time we ran tests in parallel we would load the schema
for all N DBs.

Now that we have the ability to sync DBs with the schema file exactly
using metadata and a SHA, we can instead only load the schemas when they
change.

activerecord/lib/active_record/tasks/database_tasks.rb

@@ -347,6 +347,24 @@ def schema_up_to_date?(configuration, format = ActiveRecord::Base.schema_format,
         ActiveRecord::InternalMetadata[:schema_sha1] == schema_sha1(file)
       end
 
+      def reconstruct_from_schema(configuration, format = ActiveRecord::Base.schema_format, file = nil, environment = env, spec_name = "primary") # :nodoc:
+        file ||= dump_filename(spec_name, format)
+
+        check_schema_file(file)
+
+        ActiveRecord::Base.establish_connection(configuration)
+
+        if schema_up_to_date?(configuration, format, file, environment, spec_name)
+          truncate_tables(configuration)
+        else
+          purge(configuration)
+          load_schema(configuration, format, file, environment, spec_name)
+        end
+      rescue ActiveRecord::NoDatabaseError
+        create(configuration)
+        load_schema(configuration, format, file, environment, spec_name)
+      end
+
       def dump_schema(configuration, format = ActiveRecord::Base.schema_format, spec_name = "primary") # :nodoc:
         require "active_record/schema_dumper"
         filename = dump_filename(spec_name, format)

activerecord/lib/active_record/test_databases.rb

@@ -8,31 +8,16 @@ module TestDatabases # :nodoc:
       create_and_load_schema(i, env_name: Rails.env)
     end
 
-    ActiveSupport::Testing::Parallelization.run_cleanup_hook do
-      drop(env_name: Rails.env)
-    end
-
     def self.create_and_load_schema(i, env_name:)
       old, ENV["VERBOSE"] = ENV["VERBOSE"], "false"
 
       ActiveRecord::Base.configurations.configs_for(env_name: env_name).each do |db_config|
         db_config.config["database"] += "-#{i}"
-        ActiveRecord::Tasks::DatabaseTasks.create(db_config.config)
-        ActiveRecord::Tasks::DatabaseTasks.load_schema(db_config.config, ActiveRecord::Base.schema_format, nil, env_name, db_config.spec_name)
+        ActiveRecord::Tasks::DatabaseTasks.reconstruct_from_schema(db_config.config, ActiveRecord::Base.schema_format, nil, env_name, db_config.spec_name)
       end
     ensure
       ActiveRecord::Base.establish_connection(Rails.env.to_sym)
       ENV["VERBOSE"] = old
     end
-
-    def self.drop(env_name:)
-      old, ENV["VERBOSE"] = ENV["VERBOSE"], "false"
-
-      ActiveRecord::Base.configurations.configs_for(env_name: env_name).each do |db_config|
-        ActiveRecord::Tasks::DatabaseTasks.drop(db_config.config)
-      end
-    ensure
-      ENV["VERBOSE"] = old
-    end
   end
 end