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Re: [libvirt] [PATCH] don't test "res == NULL" after we've already dereferenced it



On Thu, Jan 07, 2010 at 11:03:23AM +0100, Daniel Veillard wrote:
> On Thu, Jan 07, 2010 at 09:37:57AM +0100, Jim Meyering wrote:
> > 
> > However, the point is still valid, so I'll wait for confirmation.
> > This is still about defensive coding, i.e., ensuring that
> > maintenance doesn't violate invariants in harder-to-diagnose ways.
> > If you get a bug report, which would you rather hear?
> > "libvirt sometimes segfaults" or
> > "I get an assertion failure on file.c:999"
> 
>   I guess it's mostly a matter of coding style, I'm not sure it makes
> such a difference in practice, libvirt output will likely be burried
> in a log file, unless virsh or similar CLI tool is used.
>   We have only 4 asserts in the code currently, I think it shows that
> so far we are not relying on it. On one hand this mean calling exit()
> and I prefer a good old core dump for debugging than a one line message,
> on the other hand if you managed to catch that message, well this can
> help pinpoint if the person reporting has no debugging skills.
> 
>   I think there is pros and cons and that the current status quo is
> that we don't use asserts but more defensing coding by erroring out
> when this happen. The best way is not to assert() when we may
> dereference a NULL pointer but check for NULL at the point where
> we get that pointer, that's closer to the current code practice of
> libvirt (or well I expect so :-) and allow useful reporting (we
> failed to do a given operation) and a graceful error handling without
> exit'ing. So in general if we think we need an assert somewhere that
> mean that we failed to do the check at the right place, and I would
> rather not start to get into the practice of just asserting in a zillion
> place instead of checking at the correct place.
> 
>   So in my opinion, I'm still not fond of assert(), and I would prefer
> to catch up problem earlier, like parameter checking on function entry
> points or checking return value for functions producing pointers.

The other reason for adding assert(), is if the code leading upto a 
particular point is too complex to reliably understand whether a
variable is NULL. I think that applies in this case. I don't think
adding an assert() is the right way to deal with that complexity
though. I think it is better to make the code clearer/easier to
follow & understand


Daniel
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