From 2c2a8755460ec3d32ece91c9766dbd0304ece028 Mon Sep 17 00:00:00 2001 From: Sam Davies Date: Thu, 29 Oct 2015 18:26:54 -0300 Subject: Use advisory locks to prevent concurrent migrations - Addresses issue #22092 - Works on Postgres and MySQL - Uses advisory locks because of two important properties: 1. The can be obtained outside of the context of a transaction 2. They are automatically released when the session ends, so if a migration process crashed for whatever reason the lock is not left open perpetually - Adds get_advisory_lock and release_advisory_lock methods to database adapters - Attempting to run a migration while another one is in process will raise a ConcurrentMigrationError instead of attempting to run in parallel with undefined behavior. This could be rescued and the migration could exit cleanly instead. Perhaps as a configuration option? Technical Notes ============== The Migrator uses generate_migrator_advisory_lock_key to build the key for the lock. In order to be compatible across multiple adapters there are some constraints on this key. - Postgres limits us to 64 bit signed integers - MySQL advisory locks are server-wide so we have to scope to the database - To fulfil these requirements we use a Migrator salt (a randomly chosen signed integer with max length of 31 bits) that identifies the Rails migration process as the owner of the lock. We multiply this salt with a CRC32 unsigned integer hash of the database name to get a signed 64 bit integer that can also be converted to a string to act as a lock key in MySQL databases. - It is important for subsequent versions of the Migrator to use the same salt, otherwise different versions of the Migrator will not see each other's locks. --- .../cases/adapters/postgresql/connection_test.rb | 42 ++++++++++++++++++++++ 1 file changed, 42 insertions(+) (limited to 'activerecord/test/cases/adapters/postgresql/connection_test.rb') diff --git a/activerecord/test/cases/adapters/postgresql/connection_test.rb b/activerecord/test/cases/adapters/postgresql/connection_test.rb index 722e2377c1..b12beb91de 100644 --- a/activerecord/test/cases/adapters/postgresql/connection_test.rb +++ b/activerecord/test/cases/adapters/postgresql/connection_test.rb @@ -209,5 +209,47 @@ module ActiveRecord ActiveRecord::Base.establish_connection(orig_connection.deep_merge({:variables => {:debug_print_plan => :default}})) end end + + def test_get_and_release_advisory_lock + key = 5295901941911233559 + list_advisory_locks = <<-SQL + SELECT locktype, + (classid::bigint << 32) | objid::bigint AS lock_key + FROM pg_locks + WHERE locktype = 'advisory' + SQL + + got_lock = @connection.get_advisory_lock(key) + assert got_lock, "get_advisory_lock should have returned true but it didn't" + + advisory_lock = @connection.query(list_advisory_locks).find {|l| l[1] == key} + assert advisory_lock, + "expected to find an advisory lock with key #{key} but there wasn't one" + + released_lock = @connection.release_advisory_lock(key) + assert released_lock, "expected release_advisory_lock to return true but it didn't" + + advisory_locks = @connection.query(list_advisory_locks).select {|l| l[1] == key} + assert_empty advisory_locks, + "expected to have released advisory lock with key #{key} but it was still held" + end + + def test_release_non_existent_advisory_lock + fake_key = 2940075057017742022 + with_warning_suppression do + released_non_existent_lock = @connection.release_advisory_lock(fake_key) + assert_equal released_non_existent_lock, false, + 'expected release_advisory_lock to return false when there was no lock to release' + end + end + + protected + + def with_warning_suppression + log_level = @connection.client_min_messages + @connection.client_min_messages = 'error' + yield + @connection.client_min_messages = log_level + end end end -- cgit v1.2.3