Bug in environment::key_char_traits::move on Windows

[js-nano]

There's a bug in environment::key_char_traits::move() on Windows:

process/include/boost/process/v2/detail/environment_win.hpp

Line 88 in bb375f5

return std::move_backward(s2, s2 + n, s1);

The final parameter should point one-past-the-end of the range to be moved to -- https://en.cppreference.com/w/cpp/algorithm/move_backward

The correct code is:

    return std::move_backward(s2, s2 + n, s1 + n);

Analysis

I saw segfaults when adding elements to a std::unordered_map<env::key, env::value> from an env::key_view/env::value_view

While investigating, I noticed that, occasionally, a newly constructed key would be empty, despite all the parameters looking plausible. I eventually narrowed it down to the use of move_backward when moving the string contents -- the final parameter should point to the end of the range to be moved to.

A minimal demonstration of the underlying problem:

#include <boost/process/v2/environment.hpp>
#include <array>
#include <cassert>

int main() {
    std::array<boost::process::v2::environment::key::string_type, 2> arr;
    arr[0] = L"def";
    arr[1].assign(arr[0].data(), arr[0].size());
    assert(arr[1] == L"def");
}

In this demonstration:

• The source string at arr[0] has an address before the destination string arr[1]
• The small string optimization is in effect, so the addresses of the buffers are in the same order as the strings themselves
• Therefore, in char_traits::move(), s1 (dest) is after s2 (src)
• So the move_backward() branch is taken
• move_backward() moves the chars before the start of the destination string, due to the 3rd parameter
• Either you get a segfault (due to writing before the start of the string memory), or the assert fails (due to the contents being written before the target string buffer)

[klemens-morgenstern]

Do you want to create a PR?

[js-nano]

Very happy to if that makes things easier for you; equally happy for you to make the change if that works for you