Re: [patch][google/integration] Add lightweight checks to vector::begin et. al. (issue4973065)

[Paul Pluzhnikov <ppluzhnikov at google dot com>]

On Tue, Sep 6, 2011 at 12:44 PM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> On 6 September 2011 20:23, Jonathan Wakely wrote:
>>
>> What's a dangling vector anyway? ?One that has been moved from?
>
> Apparently not, since a moved-from vector would pass __valid() (as
> indeed it should)
>
> So I'm quite curious what bugs this catches.

The garden variety "use after free":

  vector<> *v = new vector<>;
...
  delete v;
...

  for (it = v->begin(); it != v->end(); ++it)  // Oops!

> The existing debug mode
> catches some fairly subtle cases of user error such as comparing
> iterators from different containers, or dereferencing past-the-end
> iterators. ?This patch seems to catch user errors related to ... what?
> ?Heap corruption? ?Using a vector after its destructor runs? ?Those
> are pretty serious, un-subtle errors and not specific to vector, so
> why add code to vector (code which isn't 100% reliable if it only
> catches corruption after the memory is reused and new data written to
> it)

It is 100% percent reliable for us, due to use of a debugging allocator
which scribbles on all free()d memory.

But yes, that's one more reason why this patch is not really good for
upstream.

> to detect more general problems that can happen to any type?

We can't (easily) catch the general problem. This patch allows us to easily
catch this particular instance of it.

> The fact the patch did catch real bugs doesn't mean it's a good idea,
> as you say, those bugs probably could have been caught in other ways.

Sure -- we have other ways to catch the these bugs. They are not very
practical at the moment due to their runtime overhead.

As for your other suggestion: enabling _GLIBCXX_DEBUG just for vector,
that didn't occur to me and is something I'd like to explore.

Thanks,
-- 
Paul Pluzhnikov