fix: merge session mode pool config with main config, preserving options

[gf3]

When providing a session mode pool config via session_mode_url, it's not clear that session_mode_url is parsed into a keyword list and replaces the parent config.

For example, we use the following configuration to connect to Digital Ocean's managed Postgres:

config :our_app, OurApp.EventStore,
  default_database: "defaultdb",
  serializer: Commanded.Serialization.JsonSerializer,
  url: event_store_database_url,
  session_mode_url: event_store_session_database_url,
  ssl: true,
  ssl_opts: [cacertfile: ~c"/etc/ssl/certs/db.ca-certificate.crt", verify: :verify_peer],
  prepare: :unnamed,
  pool_size: String.to_integer(System.get_env("POOL_SIZE") || "10"),
  socket_options: maybe_ipv6

The options ssl, ssl_opts, and prepare are particularly important to our connection and are not included when generating the config (session mode pool config) from the session_mode_url.

This PR merges the session mode pool config into the parent config before passing it along. So that all the specified configuration options are used for the session mode pool, too. Which, I think, is what people probably expect when looking at the above event store configuration.

[drteeth]

Hi @gf3, thanks so much for your contribution.

I can see that you are fixing a real world issue, and I'm sure it's correct, but I'm having trouble seeing that from the description of the issue and the code changes.

Can I ask you to include a test that fails without your change and passes with it? This way we won't break it again and the reason for the new implementation will be clear.

Let me know if you need any help with that.