Limit ucontext linking to arm32* and s390x arches

[yosifkit]

And keep libucontext-dev on those arches to ensure rubygems work. (~249kb)

Fixes #365

[keithlayne]

This looks good, but this seems like it's pretty easy to test with just gem install <something small with native extensions>. I confess I found your test infra in the oi repo a little confusing. Should this be easy to write?

[tianon]

@keithlayne yes, adding that would be really easy, and I was about to go do exactly that, but got a little stuck on which native extension we should use (it'd be awesome if there was a "hello world" of native extensions we could use that we know won't ever be pre-compiled, but I couldn't find anything -- maybe you know better 👀)

[keithlayne]

I'm mostly out of the ruby world, and I never wrote a single native extension, so no. But immediately after I wrote that I was wondering basically the same thing. I thought of several ways it could be goofy. Maybe it's just a matter of generating the makefile or whatever and trying to run it for a no-op extension? I wonder if that could be bundled up into a very few files...

[tianon]

Looks like the magic incantation is --platform ruby to force it to build from source. I'll cook up a basic integration test for this real fast. 😅

(The gem I ran into this problem on was eventmachine, which seems like a pretty lightweight one to use -- if someone reading this later knows a better lightweight "hello-world" native extension example, please @ me! 😄)

[bpo]

@tianon I think you'll find bcrypt lighter/simpler than eventmachine for a quick test like this.

[tianon]

Indeed; just tested and it builds way faster!! Thanks @bpo! ❤️ 👀

Edit: it also appears to be easier to use/debug because it isn't hiding the compiler errors in a log file like eventmachine does 👀

Edit 2x: and it doesn't need g++!! 🤘 🥳

[keithlayne]

@tianon because I was already down this path...if you run this on ruby:2.7.4-alpine3.1.4 on amd64, it fails as expected.

Possibly the bundle command would be fiddly and want to be interactive on version changes, but this is very much the "hello world":

bundle gem --ext  --no-coc --no-mit --test=none  test
cd test
sed -i''  '/none/d' Gemfile
sed -i'' -e 's/spec\.summary.*$/spec.summary = "bogus"/' \
  -e 's/spec\.authors.*$/spec.authors = ["bogus"]/'  \
  -e '/TODO/d' -e '/metadata/d' \
  test.gemspec
apk add make gcc musl-dev git
bundle
rake build

[keithlayne]

Downside of the above is that a) you already have your test 😁 and b) it doesn't cover jruby.

[tianon]

Yeah, bcrypt does just fine (and feels less fiddly and less likely to be broken by future changes to Bundler -- at most maybe we'll have to update the version at some point).

I've got docker-library/official-images#11192 merged now with that new test and restarted the builds here so it'll run it and validate that @yosifkit's fix is solid. 👍