Allow extconf.rb to cancel building the extension

[casperisfine]

Ref: https://bugs.ruby-lang.org/issues/20152

There are various gems that ship with a native extension as a way to speedup part of the gem, but also ship with a pure Ruby version of these methods as a fallback. So they only want to compile the extension if the platform supports it, and if not, just fallback Ruby implementation.

Right now users rely on one of two hacks to do this. Either they create an empty Makefile, but then still depend on make being available and it feels very hacky, or they publish platform specific packages without any extension in them.

Examples include bootnsap, erb, hiredis-client, ddtrace, prism.

This changes attempt to make this use case a first class citizen by checking if the extconf.rb did generate a gem.build_skipped file. If it did, make isn't invoked at all.

[segiddins]

Seems reasonable to me, probably needs to be documented somewhere though 👍🏻

[casperisfine]

probably needs to be documented somewhere though

Is there an existing documentation for extconf.rb in Rubygems? I can't find it.

Also my plan is to add some skip_compilation method to mkmf on the Ruby side, that would be documented.

[eregon]

One problem with this is the one I explained in https://bugs.ruby-lang.org/issues/20152#note-11:

There could be a skip-make/skip-compilation or so file which does not include RUBY_ENGINE in the filename and is created dynamically by the extconf.rb, but this will take a lot longer to get adopted, as it means each gem skipping compilation would need to use that.
But it is error-prone, because e.g. then if one builds (e.g. rake compile) a gem locally on JRuby and then on CRuby then compilation will be unexpectedly skipped on CRuby, unless the code also takes care of removing that file (I guess mkmf create_makefile could remove it, it seems too annoying if extconf.rb or Rakefile needs to remove it manually).

i.e. specifically it is very error-prone to use this feature until the oldest Ruby a gem supports would have this skip_compilation in mkmf, which will be in a while.
In the meantime, doing things like File.write("gem.build_skipped", "") would cause this problem.

[deivid-rodriguez]

In principle the idea of not trying to make if there's nothing to make sounds more appealing to me? Is there any downside to that? Could it potentially hinder bugs in extconf.rb files?

[casperisfine]

Could it potentially hinder bugs in extconf.rb files?

Not that I can think of. I mean if you generate an empty make file by accident today, running a noop make or not running anything shouldn't make a difference.

I went with a different file because it seemed more deliberate, but I'm fine with trying to detect empty Makefile too.