Print repo name in connection pool errors

[guisehn]

Adds the pool name to connection pool errors, helping to find the database with problems when an application uses multiple repos:

  ** (EXIT from #PID<0.933.0>) shell process exited with reason: an exception was raised:
-     ** (DBConnection.ConnectionError) connection not available and request was dropped from queue after 4324ms. This means requests are coming in and your connection pool cannot serve them fast enough. You can address this by:
+     ** (DBConnection.ConnectionError) [Elixir.MyApp.Repo] connection not available and request was dropped from queue after 4324ms. This means requests are coming in and your connection pool cannot serve them fast enough. You can address this by:

    1. Ensuring your database is available and that you can connect to it
    2. Tracking down slow queries and making sure they are running fast enough
    3. Increasing the pool_size (although this increases resource consumption)
    4. Allowing requests to wait longer by increasing :queue_target and :queue_interval

  See DBConnection.start_link/2 for more information

          (db_connection 2.8.1) lib/db_connection.ex:986: DBConnection.run/3
          (elixir 1.15.7) src/elixir.erl:396: :elixir.eval_external_handler/3
          (stdlib 5.1.1) erl_eval.erl:750: :erl_eval.do_apply/7
          (stdlib 5.1.1) erl_eval.erl:136: :erl_eval.exprs/6
          (elixir 1.15.7) lib/task/supervised.ex:101: Task.Supervised.invoke_mfa/2

Solves #333

[josevalim]

Hi @guisehn I would prefer to not depend on repo, because it is really not a db_connection configuration or concern. Looking at the connection pool, maybe you can rely on $ancestors?

Something like this:

List.first(Process.get(:"$ancestors, []))

Or perhaps this:

Enum.find(Process.get(:"$ancestors, []), &is_atom/1)

This would also simplify the PR as you won't need to pass all state around. Thanks!

[guisehn]

@josevalim nice trick! Touching all the function signatures was really giving me the creeps. :)

Implemented on 343dec5

[josevalim]

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