Ruby: run irb from trap context? - ruby

Question

The following example is given in the book Programming Ruby 1.9 & 2.0:
require 'irb'
trap "INT" do
IRB.start
end
count = 0
loop do
count += 1
puts count
puts "Value = #{#value}" if defined? #value
sleep 1
end
Unfortunately, if I run this code with my Ruby 2.5.1 setup I get the following ThreadError (run_irb.rb is the name of the file with the code above):
24: from run_irb.rb:8:in `<main>'
23: from run_irb.rb:8:in `loop'
22: from run_irb.rb:12:in `block in <main>'
21: from run_irb.rb:12:in `sleep'
20: from run_irb.rb:4:in `block in <main>'
19: from /usr/lib/ruby/2.5.0/irb.rb:376:in `start'
18: from /usr/lib/ruby/2.5.0/irb/init.rb:17:in `setup'
17: from /usr/lib/ruby/2.5.0/irb/init.rb:112:in `init_config'
16: from /usr/lib/ruby/2.5.0/irb/init.rb:112:in `new'
15: from /usr/lib/ruby/2.5.0/irb/locale.rb:32:in `initialize'
14: from /usr/lib/ruby/2.5.0/irb/locale.rb:108:in `load'
13: from /usr/lib/ruby/2.5.0/irb/locale.rb:124:in `find'
12: from /usr/lib/ruby/2.5.0/irb/locale.rb:145:in `search_file'
11: from /usr/lib/ruby/2.5.0/irb/locale.rb:157:in `each_localized_path'
10: from /usr/lib/ruby/2.5.0/irb/locale.rb:167:in `each_sublocale'
9: from /usr/lib/ruby/2.5.0/irb/locale.rb:158:in `block in each_localized_path'
8: from /usr/lib/ruby/2.5.0/irb/locale.rb:150:in `block in search_file'
7: from /usr/lib/ruby/2.5.0/rubygems.rb:213:in `try_activate'
6: from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1063:in `find_by_path'
5: from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1063:in `find'
4: from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1063:in `each'
3: from /usr/lib/ruby/2.5.0/rubygems/specification.rb:1064:in `block in find_by_path'
2: from /usr/lib/ruby/2.5.0/rubygems/core_ext/kernel_require.rb:40:in `require'
1: from /usr/lib/ruby/2.5.0/monitor.rb:185:in `mon_enter'
/usr/lib/ruby/2.5.0/monitor.rb:185:in `lock': can't be called from trap context (ThreadError)
Is there a way to get the irb run from the trap context with Ruby 2.5.1?

Answer 1

Ruby 2.0 does not allow Mutex#lock within a signal handler
According to this bug 7917 in ruby lang this is the "expected" behaviour. So you've basically found a language bug and a book bug because the author didn't test this example in ruby 2.0 (confirmed that it works in 1.9.)
An alternative method is to use an exception/rescue and execute an IRB session from an interrupt like this:
require 'irb'
count = 0
loop do
count += 1
puts count
puts "Value = #{#value}" if defined? #value
sleep 1
rescue Interrupt => e
IRB.start
break
end
The above will correctly trap an interrupt and allow an IRB session to start. I did notice that IRB.start won't give you a local execution context, so you may want to look at binding.irb that let's you start a more useful debugging session like this:
$ ruby thread-test.rb
1
2
^C
[1] irb(main)>
From: /work/_scratch/thread-test/thread-test.rb # line 11 :
6: puts count
7: puts "Value = #{#value}" if defined? #value
8: sleep 1
9: rescue Interrupt => e
10: binding.irb
=> 11: break
12: end
[2] irb(main)> e
=> Interrupt
[3] irb(main)> count
=> 2
I found a few blog posts on handling Signals, Traps, and Rescues and Graceful Shutdown that may help you further with your problem.