How to kill a thread?

Fuzzyman fuzzyman at gmail.com
Tue Jun 10 23:41:28 CEST 2008


On Jun 10, 2:03 am, Rhamphoryncus <rha... at gmail.com> wrote:
> On Jun 9, 2:52 pm, Fuzzyman <fuzzy... at gmail.com> wrote:
>
>
>
> > On Jun 9, 9:20 pm, Rhamphoryncus <rha... at gmail.com> wrote:
>
> > > On Jun 9, 5:33 am, Antoon Pardon <apar... at forel.vub.ac.be> wrote:
>
> > > > On 2008-06-07, Rhamphoryncus <rha... at gmail.com> wrote:
>
> > > > > On Jun 6, 12:44 pm, The Pythonista <n... at this.time> wrote:
> > > > >> It's always been my understanding that you can't forcibly kill a thread
> > > > >> in Python (at least not in a portable way).  The best you can do is
> > > > >> politely ask it to die, IIRC.
>
> > > > > Inherently, the best you can do in most languages is ask them politely
> > > > > to die.  Otherwise you'll leave locks and various other datastructures
> > > > > in an inconvenient state, which is too complex to handle correctly.
> > > > > The exception is certain functional languages, which aren't capable of
> > > > > having threads and complex state in the same sense.
>
> > > > Well it would of course depend on what is considered asking politely?
>
> > > > If one thread could cause an exception being thrown in an other thread,
> > > > would this be considered a polite way to ask? Would it be considered
> > > > an acceptable way?
>
> > > The exception must not be raised until a point explicitly designed as
> > > safe is hit.  Otherwise, any function that manipulates data you'll
> > > still use will potentially be buggered.  Consider sys.stdout: codecs,
> > > buffering, lots to go wrong.
>
> > Java and .NET both have ways of killing threads. They both work by
> > raising a 'ThreadAbort' (or similar) exception in the target thread.
> > In early implementations they both suffered from a similar problem.
> > You could protect locks (etc) by having a finally block that would
> > release all resources as needed - but what happens if the thread abort
> > exception is raised *inside* the finally block?
>
> > Java responded by deprecating thread aborting. .NET responded by
> > ensuring that a thread abort exception would never be raised inside a
> > finally block - if that happened the exception would only be raised
> > once the code has left the finally block.
>
> > Aborting threads in .NET can be extremely useful. Politely asking a
> > thread to die is no good if the task the thread is executing is
> > extremely coarse grained - it may not be able to respond to the
> > request for some time. If your code is structured correctly
> > (performing a long running background calculation for example) then
> > you may *know* that you can kill it without problems, but Python has
> > no native API to do this.
>
> So how does .NET deal with the sys.stdout corruption?  Does it?
>

That has never been an issue for us.

> If you've carefully written your code to use only safe primitives and
> local state (discarded if interrupted) then yes, it could be
> interruptible.  At this point you could specially mark that block of
> code as safe, leaving the vast majority of other code unsafe by
> default.  Then again, since you're going to the trouble of carefully
> designing and auditing your code you could just make it cancellable
> like blocking I/O should be - just by polling a flag at key points
> (and you're CPU-bound anyway, so it's not expensive.)
>
> The only place I know of that you *need* arbitrary interruption is
> hitting CTRL-C in the interactive interpreter.  At this point it's a
> debugging tool though, so the risk of weirdness is acceptable.

We use background threads for long running calculations that we know
are safe to abort. Resources that need protecting we do with finally
blocks which the thread abort honours.

The calculation is 'coarse grained' (it can call into .NET APIs that
can take a relatively long time to return) - so polling for exit
wouldn't work anyway. We also run user code and can't expect their
code to poll for exit conditions.

This system works fine for us. We had fun syncing results back when
the calculation terminates (race conditions if a new one needs to
start just as the old one ends) - but that is the fun of threads in
the first place and nothing to do with how we terminate calculations
early.

Michael Foord
http://www.ironpythoninaction.com/



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