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, 19 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
index 402159ac13..f5b2e9fa9d 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
@@ -214,6 +214,11 @@ module ActiveRecord
         false
       end
 
+      # Does this adapter support application-enforced advisory locking?
+      def supports_advisory_locks?
+        false
+      end
+
       # Should primary key values be selected from their corresponding
       # sequence before the insert statement? If true, next_sequence_value
       # is called before each insert to set the record's primary key.
@@ -280,6 +285,20 @@ module ActiveRecord
       def enable_extension(name)
       end
 
+      # This is meant to be implemented by the adapters that support advisory
+      # locks
+      #
+      # Return true if we got the lock, otherwise false
+      def get_advisory_lock(key) # :nodoc:
+      end
+
+      # This is meant to be implemented by the adapters that support advisory
+      # locks.
+      #
+      # Return true if we released the lock, otherwise false
+      def release_advisory_lock(key) # :nodoc:
+      end
+
       # A list of extensions, to be filled in by adapters that support them.
       def extensions
         []