Race condition on @on_load callback

[josevalim]

Describe the bug

Initially reported here: elixir-lang/elixir#12777

This code:

https://github.com/swoosh/swoosh/blob/v1.11.3/lib/swoosh/mailer.ex#L236-L256

Uses Code.ensure_loaded/1 which is not sufficient if the adapter module is defined within the same application as the mailer module, as Code.ensure_loaded/1 does not wait for the module to be compiled, so depending on the order compilation goes, the check may fail.

Steps to Reproduce the Bug or Issue

It is hard to reproduce since it is a race condition but we have isolated the root case.

Expected behavior

One quick fix is to use Code.ensure_compiled/1. However, my suggestion is to move the @on_load check altogether to compile-time (via @before_compile/@after_compile callbacks) and use Code.ensure_compiled there. I don't see the need to check for this at runtime. The way Elixir manages your dependencies, you should assume that if a module is available at compile-time, it will be available at runtime.

Thank you!

Your reproducible example

No response

Screenshots or Videos

No response

Platform

It happens in any Erlang/OTP and Elixir version, but some versions may be more likely than others depending on how fast compilation goes.

Additional context

Thank you! ❤️

[princemaple]

I love that making the compiler faster exposes a race condition.

[princemaple]

Released 1.11.4 with Code.ensure_compiled/1. This should buy me time for the bigger refactor :)

[josevalim]

Closing as explained in #793. Thanks!