Posted by Robert Cummings on 11/30/05 03:53

On Tue, 2005-11-29 at 19:42, Robert Cummings wrote:
> On Tue, 2005-11-29 at 19:22, Adam Atlas wrote:
> > So... I have this script (being called in a perfectly typical way by
> > PHP 4.4.1 through mod_php in Apache 2.0.55) which sometimes runs
> > perfectly, and sometimes chooses, totally haphazardly, to seemingly
> > run itself twice, which of course causes it to die with a fatal error
> > after it tries to redefine some functions.
> >
> > As mysterious as that is, it turns out it must be something
> > altogether more sinister. I tried putting die() at the end of the
> > script, on the assumption that it was for some reason executing
> > twice, yet behold: the problem is still present, and the PHP error
> > log still complains about constants and functions being redefined!
> > The problem, I therefore thought, cannot be that the script is being
> > run twice. (In retrospect, this was the most likely possibility,
> > because the page doesn't actually output anything if it dies with
> > this error.) So for debugging, I added a bit that logs one message to
> > a file immediately before the die() at the end of the file, and a
> > different message after the die(). The die() seems to be working
> > normally, in that it only logs the first message...
> >
> > But wait a second! WTF? If the PHP error log is to be believed, then
> > the script should be dying with a fatal error before it even *gets*
> > to that point in the script, isn't it?
> >
> > And the greater WTF is the fact that, as I mentioned above, every
> > time the page is requested, it unpredictably either does this or
> > works flawlessly. Oh my. How do I even *begin* to debug something
> > like this?
>
> I think you mean heisenbug...
>
> http://en.wikipedia.org/wiki/Heisenbug
> http://en.wikipedia.org/wiki/Schr%C3%B6dinger%27s_cat

Hmmm, you learn something new every day:

http://en.wikipedia.org/wiki/Schroedinbug

But the problem you're having seems better represented as a heisenbug :)

Cheers,
Rob.
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