[PATCH] libstdc++: Fix division by zero in std::sample

[Patrick Palka]

On Fri, 18 Sep 2020, Patrick Palka wrote:

> This fixes a division by zero in the selection-sampling std::__search

Whoops, this line say std::__sample, not std::__search.

> overload when the input range is empty (and hence __unsampled_sz is 0).
> 
> Tested on x86_64-pc-linux-gnu.
> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/bits/stl_algo.h (__sample): Exit early when the
> 	input range is empty.
> 	* testsuite/25_algorithms/sample/3.cc: New test.
> ---
>  libstdc++-v3/include/bits/stl_algo.h          |  3 ++
>  .../testsuite/25_algorithms/sample/3.cc       | 50 +++++++++++++++++++
>  2 files changed, 53 insertions(+)
>  create mode 100644 libstdc++-v3/testsuite/25_algorithms/sample/3.cc
> 
> diff --git a/libstdc++-v3/include/bits/stl_algo.h b/libstdc++-v3/include/bits/stl_algo.h
> index a0b96c61798..2478b5857c1 100644
> --- a/libstdc++-v3/include/bits/stl_algo.h
> +++ b/libstdc++-v3/include/bits/stl_algo.h
> @@ -5775,6 +5775,9 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
>        using _Gen = remove_reference_t<_UniformRandomBitGenerator>;
>        using __uc_type = common_type_t<typename _Gen::result_type, _USize>;
>  
> +      if (__first == __last)
> +	return __out;
> +
>        __distrib_type __d{};
>        _Size __unsampled_sz = std::distance(__first, __last);
>        __n = std::min(__n, __unsampled_sz);
> diff --git a/libstdc++-v3/testsuite/25_algorithms/sample/3.cc b/libstdc++-v3/testsuite/25_algorithms/sample/3.cc
> new file mode 100644
> index 00000000000..e89c40e27ee
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/25_algorithms/sample/3.cc
> @@ -0,0 +1,50 @@
> +// Copyright (C) 2020 Free Software Foundation, Inc.
> +//
> +// This file is part of the GNU ISO C++ Library.  This library is free
> +// software; you can redistribute it and/or modify it under the
> +// terms of the GNU General Public License as published by the
> +// Free Software Foundation; either version 3, or (at your option)
> +// any later version.
> +
> +// This library is distributed in the hope that it will be useful,
> +// but WITHOUT ANY WARRANTY; without even the implied warranty of
> +// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +// GNU General Public License for more details.
> +
> +// You should have received a copy of the GNU General Public License along
> +// with this library; see the file COPYING3.  If not see
> +// <http://www.gnu.org/licenses/>.
> +
> +// { dg-options "-std=gnu++17" }
> +// { dg-do run { target c++17 } }
> +// { dg-require-cstdint "" }
> +
> +#include <algorithm>
> +#include <random>
> +#include <testsuite_hooks.h>
> +#include <testsuite_iterators.h>
> +
> +std::mt19937 rng;
> +
> +using std::sample;
> +using __gnu_test::test_container;
> +using __gnu_test::output_iterator_wrapper;
> +using __gnu_test::forward_iterator_wrapper;
> +
> +void
> +test01()
> +{
> +  const int in = 0;
> +  test_container<const int, forward_iterator_wrapper> pop(&in, &in);
> +  int out;
> +  test_container<int, output_iterator_wrapper> samp(&out, &out + 1);
> +
> +  auto it = sample(pop.begin(), pop.end(), samp.begin(), 1, rng);
> +  VERIFY( it.ptr == &out );
> +}
> +
> +int
> +main()
> +{
> +  test01();
> +}
> -- 
> 2.28.0.497.g54e85e7af1
> 
>