libstdc++: Fix deadlock in debug iterator increment [PR108288]

[François Dumont]

On 12/01/23 13:00, Jonathan Wakely wrote:
> On Thu, 12 Jan 2023 at 05:52, François Dumont wrote:
>> Small update for an obvious compilation issue and to review new test
>> case that could have lead to an infinite loop if the increment issue was
>> not detected.
>>
>> I also forgot to ask if there is more chance for the instantiation to be
>> elided when it is implemented like in the _Safe_local_iterator:
>> return { __cur, this->_M_sequence };
> No, that doesn't make any difference.
>
>> than in the _Safe_iterator:
>> return _Safe_iterator(__cur, this->_M_sequence);
>>
>> In the case where the user code do not use it ?
>>
>> Fully tested now, ok to commit ?
>>
>> François
>>
>> On 11/01/23 07:03, François Dumont wrote:
>>> Thanks for fixing this.
>>>
>>> Here is the extension of the fix to all post-increment/decrement
>>> operators we have on _GLIBCXX_DEBUG iterator.
> Thanks, I completely forgot we have other partial specializations, I
> just fixed the one that showed a deadlock in the user's example!
>
>>> I prefer to restore somehow previous implementation to continue to
>>> have _GLIBCXX_DEBUG post operators implemented in terms of normal post
>>> operators.
> Why?
>
> Implementing post-increment as:
>
>      auto tmp = *this;
>      ++*this;
>      return tmp;
>
> is the idiomatic way to write it, and it works fine in this case. I
> don't think it performs any more work than your version, does it?
> Why not use the idiomatic form?
>
> Is it just so that post-inc of a debug iterator uses post-inc of the
> underlying iterator? Why does that matter?
>
A little yes, but that's a minor reason that is just making me happy.

Main reason is that this form could produce a __msg_init_copy_singular 
before the __msg_bad_inc.

And moreover I plan to propose a patch later to skip any check in the 
call to _Safe_iterator(__cur, _M_sequence) as we already know that __cur 
is ok here like anywhere else in the lib.

There will still be one in the constructor normally elided unless 
--no-elide-constructors but there is not much I can do about it.