[cpp-threads] Re: Exception propogation across threads

[Kevlin Henney]

Paul E. McKenney <paulmck at linux.vnet.ibm.com> writes
>
>I am uncharacteristically with Hans on this one.  ;-)
>
>Here is the scenario that I am concerned about:
>
>o      A threaded "server" application with indefinite lifetime.
>
>o      One of the threads encounters a bug that results in an
>       exception.
>
>o      The rest of the server sort of runs along, but with strange
>       behavior due to the dead thread.
>
>Seems like it would be better to just kill the whole process.

Agreed. Therefore, register a handler that does this. Problem solved, 
clearly and explicitly :-)

>If the
>developer would prefer to make the failure known at join() time, it is
>pretty easy to put in top-level exception handlers, right?

Actually, no it isn't: a top-level exception handler in the executing 
thread will stop an exception in the thread, but it won't propagate it 
across a join. You need plumbing to do this, hence the reason to favour 
a model that takes this into account rather than one that is oblivious 
to it.

The bias to date in considering a programming model for threads has been 
to treat thread executors as launching functions and function objects, 
all of which are reasonably entitled to throw exceptions. This point of 
view more closely aligns with the idea that an exception that leaves a 
function should not be in the business of killing a process by default, 
which makes more sense for coarse-grained threading whose failure you 
consider critical, and therefore need to make a special point of 
highlighting.

I am concerned that making total process death the chosen behaviour 
shows a default bias towards coarse-grained threading, just as we are 
moving into the era of fine-grained threading. As I said before, killing 
the whole process seems unnecessarily severe: feel free to consider it 
an option, but we should avoid imposing what some (myself included) 
might consider to be unreasonable defaults.

Kevlin
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