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