Setup initial cross compilation workflow

[ianks]

This is a PR to setup cross gem compilation. Right now, it just adds the ability to cross compile gems via Github UI. We'll figure out publishing later.

[ianks]

cha-ching!

[jbourassa]

This is great, thank you so much Ian.

I see the cross-compile's smoke test fails:

ERROR:  Error installing pkg/wasmtime-rb-0.1.0-x86_64-linux.gem:
  wasmtime-rb-0.1.0-x86_64-linux requires Ruby version >= 3.0, < > 3.1.dev. The current ruby version is 3.1.2.20.

Looks like we're only building for 3.0.0:

no configuration section for specified version of Ruby (rbconfig-x86_64-linux-"3.1.0)
no configuration section for specified version of Ruby (rbconfig-x86_64-linux-2.7.0")

Searching for that error message led me to rake-compiler. Looks like config.yml doesn't have entries for all rubies. Is it because xrubies bundles 1 ruby version per image, unlike rake-compiler-docker?

Edit: also no idea where 3.0.0 is coming from. I understand the intent to be "build for all RUBY_CC_VERSION", not only 3.0.0 🤔.

[jbourassa]

Nit: the config is optional now, this should also work:

Suggested change

	script="puts Wasmtime::Engine.new(Wasmtime::Config.new).precompile_module('(module)')"
	script="puts Wasmtime::Engine.new.precompile_module('(module)')"

[ianks]

@jbourassa I am very confused about this as well. Not sure where the bug is… Gonna contact rake-compiler team

[flavorjones]

The current error, I think, is because rake-compiler will put the cross-compiled shared object file into a subdirectory named after the ruby minor version (e.g., lib/wasmtime/3.1/ext.so instead of lib/wasmtime/ext.so).

You can see this in the build log: https://github.com/bytecodealliance/wasmtime-rb/actions/runs/3340442663/jobs/5530540697#step:4:280

You should look first for a ruby-specific precompiled .so, and fall back to lib/wasmtime/ext.so if you can't find it (which means the user compiled upon installation). A lot of this is explained in https://github.com/flavorjones/ruby-c-extensions-explained/tree/main/precompiled

An example of how to do this is

• https://github.com/flavorjones/ruby-c-extensions-explained/tree/main/precompiled
• also https://github.com/sparklemotion/nokogiri/blob/main/lib/nokogiri/extension.rb

Hope this helps!