multithreading - Handling exceptions raised in a Ruby thread

Question

I am looking for a solution of classic problem of exception handling. Consider following piece of code:

def foo(n)
  puts " for #{n}"
  sleep n
  raise "after #{n}"
end

begin
  threads = []
  [5, 15, 20, 3].each do |i|
    threads << Thread.new do
      foo(i)
    end
  end

  threads.each(&:join)      
rescue Exception => e
  puts "EXCEPTION: #{e.inspect}"
  puts "MESSAGE: #{e.message}"
end

This code catches the exception after 5 seconds.

But if I change the array as [15, 5, 20, 3], above code catch the exception after 15 seconds. In short, it always catch the exception raised in first thread.

Any idea, why so. Why doesn't it catch the exception after 3 seconds each time? How do I catch the first raised exception by any thread?

Answer 1

If you want any unhandled exception in any thread to cause the interpreter to exit, you need to set Thread::abort_on_exception= to true. Unhandled exception cause the thread to stop running. If you don't set this variable to true, exception will only be raised when you call Thread#join or Thread#value for the thread. If set to true it will be raised when it occurs and will propagate to the main thread.

Thread.abort_on_exception=true # add this

def foo(n)
    puts " for #{n}"
    sleep n
    raise "after #{n}"
end

begin
    threads = []
    [15, 5, 20, 3].each do |i|
        threads << Thread.new do
            foo(i)
        end
    end
    threads.each(&:join)

rescue Exception => e

    puts "EXCEPTION: #{e.inspect}"
    puts "MESSAGE: #{e.message}"
end

Output:

 for 5
 for 20
 for 3
 for 15
EXCEPTION: #<RuntimeError: after 3>
MESSAGE: after 3

Note: but if you want any particular thread instance to raise exception this way there are similar abort_on_exception= Thread instance method:

t = Thread.new {
   # do something and raise exception
}
t.abort_on_exception = true