This is my best result attempting to answer my own question to date. It appears to work for JRuby (tested in Travis and on my local installation under RVM), which was my main goal. However, I would be very interested in confirmations of it working in other environments, and for any input on how to make it more generic and/or robust:
The gem installation code expects a Makefile
as output from extconf.rb
, but has no opinion on what that should contain. Therefore extconf.rb
can decide to create a do nothing Makefile
, instead of calling create_makefile
from mkmf
. In practice that might look like this:
ext/foo/extconf.rb
can_compile_extensions = false
want_extensions = true
begin
require 'mkmf'
can_compile_extensions = true
rescue Exception
# This will appear only in verbose mode.
$stderr.puts "Could not require 'mkmf'. Not fatal, the extensions are optional."
end
if can_compile_extensions && want_extensions
create_makefile( 'foo/foo' )
else
# Create a dummy Makefile, to satisfy Gem::Installer#install
mfile = open("Makefile", "wb")
mfile.puts '.PHONY: install'
mfile.puts 'install:'
mfile.puts "\t" + '@echo "Extensions not installed, falling back to pure Ruby version."'
mfile.close
end
As suggested in the question, this answer also requires the following logic to load the Ruby fallback code in the main library:
lib/foo.rb (excerpt)
begin
# Extension target, might not exist on some installations
require 'foo/foo'
rescue LoadError
# Pure Ruby fallback, should cover all methods that are otherwise in extension
require 'foo/foo_pure_ruby'
end
Following this route also requires some juggling of rake tasks, so that the default rake task doesn't attempt to compile on Rubies that we're testing on that don't have capability to compile extensions:
Rakefile (excerpts)
def can_compile_extensions
return false if RUBY_DESCRIPTION =~ /jruby/
return true
end
if can_compile_extensions
task :default => [:compile, :test]
else
task :default => [:test]
end
Note the Rakefile
part doesn't have to be completely generic, it just has to cover known environments we want to locally build and test the gem on (e.g. all the Travis targets).
I have noticed one annoyance. That is by default you will see Ruby Gems' message Building native extensions. This could take a while...
, and no indication that the extension compilation was skipped. However, if you invoke the installer with gem install foo --verbose
you do see the messages added to extconf.rb
, so it's not too bad.