Bootsnap breaks Zeitwerk on Ruby 2.5.x

[casperisfine]

There's a bug on Ruby 2.5, when code is loaded a ISeq TracePoint callbacks are not fired:

iseq = RubyVM::InstructionSequence.compile(<<-RUBY)
  class A
    class B
    end
  end
RUBY

iseq_bin = iseq.to_binary
TracePoint.new(:end){|tp|
  p tp.lineno
}.enable{
  RubyVM::InstructionSequence.load_from_binary(iseq_bin).eval
}

TracePoint.new(:class){|tp|
  p "class #{tp.lineno}"
}.enable{
  RubyVM::InstructionSequence.load_from_binary(iseq_bin).eval
}

This snippet prints nothing on MRI 2.5, but works as expected on 2.6.

The issue with this is that Zeitwerk, that's the default autoloader on Rails 6 rely on TracePoint to work properly. Since Rails also include bootsnap as default, 2.5 users trying to use Rails 6 might encounter very weird load issues.

I'm not quite sure what can be done, but It would probably be best to figure something out before Rails 6 is released.

@burke @fxn @rafaelfranca thoughts on this?

Rails load bootsnap/setup: https://github.com/Shopify/bootsnap/blob/master/lib/bootsnap/setup.rb

I suppose we could check the Ruby version and set compile_cache_iseq: false for MRI 2.5 ?

The perf hit is quite significant, but if it can save headaches for users, it might be best.

[rafaelfranca]

This bug is fixed already in Ruby, it is just not released. They are planning a release next week. Related with rails/rails#35475.

I think we can just wait the new Ruby release. It will happen before Rails 6 final is released.

[casperisfine]

Oh great. Well closing this then.

[fxn]

Ruby 2.5.5 still has this problem.

[casperisfine]

Yeah we discovered this the hard way. The TracePoint patch was supposed to be part of 2.5.5, but the release was borked and that fix wasn't properly included...

[fxn]

For the archives: after #257 the default setup has this cache disabled.

The issue is probably worth being open still, however, because it affects people enabling the cache by hand.

[casperisfine]

Seems like the fix was never properly backported to 2.5, however bootsnap no longer enable ISeq caching by default on 2.5.x, and I added a warning if users enable it manually: eab8d8d

I think we can close this now.