Previously, we were using multiple threads to run migrations for
multisite setups in parallel. However, it was discovered that
`db:migrate` is
not threadsafe and was not designed to be. As a result,
`multisite:migrate` was hardcoded to only support spinning up a single
thread.
This new attempt Uses the `parallel` gem with `in_processes` to run
migrations concurrently using multiple processes. Concurrency defaults
to 2 but can be configured via the
`DISCOURSE_MULTISITE_MIGRATE_CONCURRENCY` env. Example:
`DISCOURSE_MULTISITE_MIGRATE_CONCURRENCY=5 bin/rails multisite:migrate`
Alan Guo Xiang Tan
790dc4a5ed