How to simulate "Mysql2::Error: MySQL client is not connected" - ruby-on-rails

Question

Can anyone suggest a way of forcing the above exception to occur, in the context of a Rails app?
I have a particular situation where it arises (involving scheduled database maintenance) and I need to be able to trigger it locally so I can test my application handles it correctly.
I would guess there's either something I could do to the DB itself, or else some method I could call on the ActiveRecord connection that would trigger this, but I haven't been able to figure it out.

Answer 1

You are probably getting this error because the MySQL connection is killed during maintenance while a SQL query is being made. (Here is a test case of this scenario https://github.com/brianmario/mysql2/blob/a8c96fbe277723e53985983415f9875e759e1d47/spec/mysql2/client_spec.rb#L597)
To reproduce this locally, you can run a long running SQL query in rails. E.g.
ActiveRecord::Base.connection.execute("select sleep(100)")
While that is running, find and kill the rails SQL connections by running
SELECT id FROM INFORMATION_SCHEMA.PROCESSLIST WHERE `db` = '<your-database-name>';
kill <id>; -- Run for each id listed in the previous query.

Answer 2

Find connection ID
ActiveRecord::Base.connection.raw_connection.thread_id
# or
ActiveRecord::Base.connection_pool.connections.map { |conn| conn.raw_connection.thread_id }
or by SQL like mentioned in Cameron's answer
By mysql client invoke
KILL <ID>; -- which you have got by #thread_id
Future attempts to query via this connection will fail with "Mysql2::Error: MySQL client is not connected"
Notes:
Option reconnect: true in database.yml will lead to immediate reconnect after a KILL. You able observe it by calling #thread_id again, it will return new ID.
ActiveRecord::Base.connection uses separate connection for thread where it have been called. While we have killed single connection, another threads will be able to query mysql without error.
You able to access all process connections by
ActiveRecord::Base.connection_pool.connections
You may wonder why in console for pool size, for say, 5 (ActiveRecord::Base.connection_pool.size) you have got ActiveRecord::Base.connection_pool.connections.count == 1. In this case you may checkout more connections by
ActiveRecord::Base.connection_pool.checkout