How do I determine whether ruby can fork without excessive use of regexp?

Question

Is it possible to determine whether the implementation of ruby you're running on is capable of supporting fork, without running a regex against RUBY_PLATFORM that'll expand until it summons Cthulhu?

(Related question: Ruby - How can I find out on which system my program is running?)

Edit: I tried Marc-Andre's suggestion. It doesn't work for jruby with fork disabled by default:

192-168-1-7:~ agrimm$ jruby --1.9 -S jirb 
irb(main):001:0> RUBY_VERSION
=> "1.9.2dev"
irb(main):002:0> Process.respond_to?(:fork)
=> true
irb(main):003:0> Process.fork
NotImplementedError: fork is unsafe and disabled by default on JRuby

Update: From Marc-Andre's link, it seems wiser heads than I have grappled with this problem from the perspective of creating ruby implementations, and failed.

From the perspective of someone who's writing a ruby library, what would be the most comprehensive incantation, short of running fork and seeing if it raises an exception?

Answer 1

Instead of error-prone testing against RUBY_PLATFORM or other things you could test fork itself:

def can_fork?
  pid = fork
  exit unless pid # exit the child immediately
  true
rescue NotImplementedError
  false
end

One downside would be if fork is somehow emulated which could make this check expensive.

Answer 2

In Ruby 1.9:

Process.respond_to?(:fork)  # => true if fork is supported, false otherwise

For Ruby 1.8, or JRuby (which doesn't implement this currently) you'll have to actually test it.

See also this long discussion on ruby-core.

Answer 3

A recent commit to rails uses

Config::CONFIG['host_os'] !~ /mswin|mingw/)

to test whether it's not on Windows, and

RUBY_PLATFORM !~ /java/

to test whether it's not on JRuby. However, I know it's possible to enable fork as a command-line switch. I'd have to look into whether there's any way of telling whether it's been enabled.

I suspect macruby doesn't support, or strongly discourages, forking, and from the thread Marc-Andre refers to, you can test for it with

 RUBY_ENGINE != "macruby"

Now for all the other platforms out there... (work in progress, marking as community wiki, please feel free to edit)