Compile task modifies RbConfig values

[bjfish]

The compile task currently modifies RbConfig values which should be avoided since these are intended to be read only and can cause issues for other users:

rake-compiler/lib/rake/extensiontask.rb

Lines 148 to 151 in 2b14662

	%w[sitearchdir sitelibdir].each do |dir|
	siteconf.puts "RbConfig::MAKEFILE_CONFIG['#{dir}'] = dest_path"
	siteconf.puts "RbConfig::CONFIG['#{dir}'] = dest_path"
	end

[eregon]

For context, this caused a fairly complex issue in TruffleRuby: oracle/truffleruby#2586
RubyGems uses RbConfig::CONFIG['sitelibdir'] to find out the initial load path (https://github.com/rubygems/rubygems/blob/c98464e8e859157375177d2b88d9ee01c1355d3f/lib/rubygems.rb#L573-L581), and because that path is changed by rake compiler it doesn't find it anymore, and insert incorrectly in the $LOAD_PATH, loading incorrect gem versions.
This could happen on CRuby too with --disable-gems but luckily it does not thanks to CRuby setting the @gem_prelude_index, a pretty well hidden "hack" which saves CRuby here.
TruffleRuby will do the same for compatibility, but I believe no gem should mutate RbConfig as it can cause issue by anything else using it.

[eregon]

This issue was introduced in f48d669 / #191, cc @larskanis

[eregon]

In particular this breaks using Bundler (except the Bundler shipped with TruffleRuby, that works fine because there is then a single Bundler version) + the latest TruffleRuby release (22.0), and that's only fixable here.

[kou]

@larskanis Can we use make install sitearchdir=... instead of rewriting RbConfig::CONFIG["sitearchdir"]?

[larskanis]

As written in #191, rubygems is using the same mechanism of changing RbConfig values when installing gems. That's why I didn't had a bad conscience proposing this.

Can we use make install sitearchdir=... instead of rewriting RbConfig::CONFIG["sitearchdir"]?

I think so. mkmf.rb seems to not use the variables for anything but passing it into the Makefile. This could make the siteconf file obsolete.

[eregon]

Right, it is unfortunate RubyGems uses that, and it seems from a long time ago.
The only "advantage" is RubyGems is fully loaded by the time it uses that code so it doesn't trigger the issue of RubyGems not finding where to insert in $LOAD_PATH.
But of course it could break anything else depending on RbConfig::CONFIG values being correct.
I'll file an issue to RubyGems about that.

I think so. mkmf.rb seems to not use the variables for anything but passing it into the Makefile. This could make the siteconf file obsolete.

That would be great :)

[kou]

Could @bjfish , @eregon or someone try implementing #202 (comment) approach and confirm whether the approach works with TruffleRuby?

[eregon]

PR to fix it in RubyGems: rubygems/rubygems#5626
@larskanis Could you do the same here?