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.

1 files changed, 42 insertions, 0 deletions

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