From: "rklemme (Robert Klemme)" <shortcutter@...>
Date: 2012-04-16T21:07:09+09:00
Subject: [ruby-core:44387] [ruby-trunk - Bug #6288] Change error message for thread block to be less misleading


Issue #6288 has been updated by rklemme (Robert Klemme).


mame (Yusuke Endoh) wrote:
> Hello,
> 
> 2012/4/14 rklemme (Robert Klemme) <shortcutter@googlemail.com>:
> > mame (Yusuke Endoh) wrote:
> >> Is "possible deadlock detected" better?
> >
> > If I understand properly what the deadlock check does (see also #5258) it merely verifies that there is at least one thread left which could wake up this thread. ��So I'd rather have something like "No live threads left. Deadlock?", but then again my understanding of the code in question is not too thorough.
> 
> Looks reasonable to me.  I'll commit unless there is no objection.

Great, thank you!

> >> Please elaborate "a more complex scenario" with small example.
> >
> > $ ruby19 -r thread -e 'q=SizedQueue.new(100);r=Thread.new {until (x=q.deq).nil?; raise "SilentError";end};1000.times {|i| q.enq i};q.enq nil'
> > /opt/lib/ruby/1.9.1/thread.rb:301:in `sleep': deadlock detected (fatal)
> > �� �� �� ��from /opt/lib/ruby/1.9.1/thread.rb:301:in `block in push'
> > �� �� �� ��from <internal:prelude>:10:in `synchronize'
> > �� �� �� ��from /opt/lib/ruby/1.9.1/thread.rb:297:in `push'
> > �� �� �� ��from -e:1:in `block in <main>'
> > �� �� �� ��from -e:1:in `times'
> > �� �� �� ��from -e:1:in `<main>'
> >
> > Basically what happens is that the reader thread silently dies as you can see if you set Thread.abort_on_exception:
> 
> It does not make sense.  Did you write the code to wait forever?

I am not sure what you mean.  The real code was more complex and the exception was thrown from another method - unintentionally of course.  This is just a simplified example to illustrate the situation.

> If so, you should write "sleep" simply.  If not, your code is
> actually "deadlocked", in a common sense.

There is no deadlock because there are no two threads (or processes) accessing resources in a bad order.  I am not aware of any deadlock which can be caused by a single thread only.  If you find a definition of deadlock which needs only a single thread / action / process please let us know.

"A deadlock is a situation wherein two or more competing actions are each waiting for the other to finish, and thus neither ever does."
http://en.wikipedia.org/wiki/Deadlock

> Why didn't you insist that Thread.abort_on_exception be true by
> default?  I can't understand why you blame deadlock detection.

Well, others have done already before. :-)  Also, regardless of abort_on_exception the error message for this particular situation is at least misleading if not plain wrong (according to definition of "deadlock").  The default value of abort_on_exception does not change that fact a bit.  I mean, the same would happen with a different default of abort_on_exception and someone setting it explicitly to false.

Kind regards

robert
----------------------------------------
Bug #6288: Change error message for thread block to be less misleading
https://bugs.ruby-lang.org/issues/6288#change-25932

Author: rklemme (Robert Klemme)
Status: Assigned
Priority: Normal
Assignee: mame (Yusuke Endoh)
Category: core
Target version: 
ruby -v: ruby 1.9.3p125 (2012-02-16) [i386-cygwin]


Test case:

11:50:07 ~$ ruby19 -r thread -e 'q=SizedQueue.new 10;1_000_000.times {|i| p i;q.enq i}'
0
1
2
3
4
5
6
7
8
9
10
/opt/lib/ruby/1.9.1/thread.rb:301:in `sleep': deadlock detected (fatal)
        from /opt/lib/ruby/1.9.1/thread.rb:301:in `block in push'
        from <internal:prelude>:10:in `synchronize'
        from /opt/lib/ruby/1.9.1/thread.rb:297:in `push'
        from -e:1:in `block in <main>'
        from -e:1:in `times'
        from -e:1:in `<main>'


This is not a deadlock, but there is no other thread which could wake up main thread.  Deadlock is misleading because in a more complex scenario where I had the error initially it wasn't obvious that the other thread had died and I looked for the wrong error in my code.


-- 
http://bugs.ruby-lang.org/