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Clean up connection pool in database_replica specs

Simon Tomlinsonrequested to merge
replica-connection-leak-in-tests into master

What does this MR do and why?

Fixes a connection pool leak in the :database_replica rspec decorator.

Before this change, the connection pools created for each test were never disconnected, so each test wrapped in :database_replica would permenantly reserve 1 connection per database for the lifetime of the rspec session.

References

I found this while working on Extract Load Balancer code to a gem (#544027).

How to set up and validate locally

I found this while running spec/lib/gitlab/database/load_balancing_spec.rb, since that spec runs a lot of :database_replica specs.

  1. Open a psql console with gdk psql
  2. In the psql console, count how many connections your database has: select count(*) from pg_stat_activity where backend_type = 'client backend';
  3. Start psql running the previous command in a loop: \watch 1 (no semicolon)
  4. Run spec/lib/gitlab/database/load_balancing_spec.rb, either with or without this change. Observe the connection counts growing without bound without this change, while they level out with this change.

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