Workers::TaskGroup::run is not signal safe

[jlfwong]

It's possible for the TaskGroup.run call to terminate before all the tasks in the group have completed.

Repro:

require 'workers'

# test.rb
Signal.trap("USR1") do
  puts "RECEIVED SIGNAL"
end

group = Workers::TaskGroup.new
group.add { sleep(10000000) }
puts group.run
puts group.tasks[0].state

In one shell: ruby test.rb

In another shell, run:

$ ps aux | grep test.rb
jlfwong          42699   0.0  0.1  2511824  14252 s004  S+    2:16PM   0:00.36 ruby test.rb
$ kill -30 

Then switching back to the original shell, I see the following:

RECEIVED SIGNAL
false
running

My expectation in this case would be that run would just not terminate until much much later.

After 10000000 seconds, I'd expect the output to be:

RECEIVED SIGNAL
true
succeeded

From digging around in ruby source, it seems to me that ConditionVariable::wait is not signal safe. http://ruby-doc.org/stdlib-2.0.0/libdoc/thread/rdoc/ConditionVariable.html

::wait calls mutex.sleep, which in turn calls rb_mutex_sleep, which eventually ends up doing this: https://github.com/ruby/ruby/blob/55f93cb63f350c7705733f86923561363a297e00/thread.c#L1084

Which will exit as soon as the native_sleep call terminates, which is I believe upon receiving any handled signal.

I arrived here trying to debug why running my code under stackprof was causing failures (commentary on this issue here: tmm1/stackprof#91 (comment))