TravisCI caching and Bundler use the same path for C extensions for different TruffleRuby versions

[eregon]

This leads to errors such as:

RuntimeError:
  Global variable rb_tr_true is declared but not defined.

Example build: https://travis-ci.org/socketry/async-websocket/jobs/554414769

We see msgpack is not recompiled (Using msgpack 1.3.0 in the log), even though it was compiled with TruffleRuby 19.0.0 and now reused on TruffleRuby 19.1.0, which doesn't work.

TravisCI caching seems to simply ignore the TruffleRuby version when using:

rvm:
- truffleruby

The cache URLs seem to just consider the value of rvm::

fetching master/cache-linux-xenial-e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855--rvm-2.6--gemfile-Gemfile.tgz
fetching master/cache-linux-xenial-e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855--rvm-truffleruby--gemfile-Gemfile.tgz

This seems like a bug that needs to be fixed in TravisCI.
If RUBY_ENGINE_VERSION was used, then it should work out of the box.

A workaround until this is fixed in the next release is to manually delete the caches on TravisCI.
This is necessary only if having a cache against 19.0.0 and needs to be done only once, as after the cache will be rebuilt against 19.1.0 and the next release will include a fix this.

On the TruffleRuby side, maybe we could try to have a better error in such a case, but that's probably quite difficult to achieve, because we'd need a check before starting to load the C extension.

cc @ioquatix

[eregon]

From that log I see that paths like these are used:

/home/travis/build/socketry/async-websocket/vendor/bundle/truffleruby/2.6.0/gems/rspec-core-3.8.2/exe/rspec

2.6.0 is most likely RbConfig::CONFIG['ruby_version'], which seems to be considered as the binary compatibility/ABI version.
Maybe we should just set RbConfig::CONFIG['ruby_version'] to the TruffleRuby version.

[eregon]

I can reproduce the problem locally, and confirm setting RbConfig::CONFIG['ruby_version'] to the TruffleRuby version solves it for bundle install --path .... I'll do a fix with that.

[eregon]

Fixed by 5ed7adc, at least for the Bundler case.

chruby needs a fix to not reuse the same GEM_HOME for multiple TruffleRuby releases.

[eregon]

Let's track the fix for chruby separately: #1723, and close this issue since it will be fixed in the upcoming release.

[eregon]

TruffleRuby 19.2.0 was released and fixes this bug, please try it.