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Posted by Robert Cummings on 11/30/05 02:42
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
Do a grep for dynamic includes of the file script in question.
Alternatively do a grep for files that dynamically include the script in
question. Work your way to the root.
Cheers,
Rob.
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