hashtable exception safety patch

[Paolo Carlini]

Hi,
>     Here is the patch regarding exception safety issues. Writing 
> dedicated tests I even found an other situation where the 
> _M_next_resize was not correctly restored on exception. It was in 
> _M_insert_bucket method if the node allocation fails. I also 
> considered using some kind of guard object that would automatically 
> restore hash policy previous resize value on exception but as it is 
> not the case when the exception is raised by the hash functor the code 
> wouldn't have been simpler.
>
> 2011-09-07  François Dumont <fdumont@gcc.gnu.org>
>
>         * include/bits/hashtable.h (_Hashtable<>::_M_rehash): Take and 
> restore
>         hash policy _M_prev_resize on bad_alloc exception.
>         (_Hashtable<>::_M_insert_bucket): Capture hash policy next resize
>         before using it and use latter method to have it restore on 
> exception.
>         (_Hashtable<>::_M_insert(_Arg&& __v, std::false_type): Likewise.
>         (_Hashtable<>::insert(_InputIterator, _InputIterator): Likewise.
>         (_Hashtable<>::rehash): Likewise.
>         * testsuite/util/testsuite_allocators.h: Add throw_allocator
>         std::allocator implementation that offer controls on 
> generation of
>         std::bad_alloc exceptions.
>         * testsuite/23_containers/unordered_set/insert/hash_policy.cc: 
> New.
>         * 
> testsuite/23_containers/unordered_multiset/insert/hash_policy.cc:
>         Likewise.
The patch is largely Ok, I'm attaching below what I have been testing 
here (formatting fixes in the testcases; resolved a diffing error in 
testsuite_allocator.h which had removed ExplicitConsAlloc; naming fixes 
(__prev_resize is not ok because wrongly gives the impression to be the 
opposite of next_resize), etc).

However, before going ahead, can you see whether you can use instead the 
existing ext/throw_allocator.h? Or extend / adapt it, I see redundancy here.

Thanks,
Paolo.
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