From: Robert Klemme <shortcutter@...>
Date: 2007-03-30T17:00:06+09:00
Subject: Re: Exceptions occassionally lost by child threads

On 30.03.2007 09:19, Sylvain Joyeux wrote:
>> Should exceptions automatically propagate from children up to the
>> parent?  And if so, why should the positioning of something as innocuous
>> as sleep 1 matter?
> Because you have a timeout on the join (a.join(1))
> 
> In the first case it sleeps one second, and waits for 'a' to finish. It 
> works if 'a' raises within two seconds. Note that it could also fail 
> since 'sleep' is not perfect and it is possible for the main thread to 
> reach the timeout on 'join' before 'a' raises.
> 
> In the second case, the main thread only waits 'a' for one second. It is 
> useless because of the sleep(2) in 'a'.
> 
> It should work as expected if you replace a.join(1) by a.join

It seems to be a property of Thread#join and #value that they will 
automatically propagate any uncaught exceptions of the the joined thread:

irb(main):003:0> t=Thread.new { sleep 20; raise "Foo" }
=> #<Thread:0x7ef7ef6c sleep>
irb(main):004:0> begin; t.join; rescue Exception => e; p e; end
#<RuntimeError: Foo>
=> nil

I think this behavior is documented, although a bit cryptic IMHO:

"If thr had previously raised an exception and the abort_on_exception 
and $DEBUG flags are not set (so the exception has not yet been 
processed) it will be processed at this time."
http://www.ruby-doc.org/core/classes/Thread.html#M000474

Kind regards

	robert