bp::environment::native_handle() cannot be compiled on Linux

[AJIOB]

Code example to reproduce the issue:

#include <boost/process.hpp>

#include <iostream>

namespace bp = boost::process;

int main() {
    auto nativeEnv = boost::this_process::environment();
    auto nativeEnvHandle = nativeEnv.native_handle();
    std::cout << *nativeEnvHandle;

    bp::environment env = nativeEnv;
    // should also work, but don't
    auto envHandle = env.native_handle();
    std::cout << *envHandle;

    return 0;
}

GodBolt link to reproduce the issue: https://godbolt.org/z/cT7ovGv68

Affected library versions: boost 1.77.0, boost 1.82.0 (another versions were not tested).
Affected compilers: gcc 6.3.0, gcc 13.2, clang 16.0.0 (another versions were not tested).

Compiler flags: -std=c++11 -O2

Compiler output (gcc 13.2):

In file included from /opt/compiler-explorer/libs/boost_1_82_0/boost/process/environment.hpp:17,
                 from /opt/compiler-explorer/libs/boost_1_82_0/boost/process/env.hpp:10,
                 from /opt/compiler-explorer/libs/boost_1_82_0/boost/process.hpp:28,
                 from <source>:1:
/opt/compiler-explorer/libs/boost_1_82_0/boost/process/detail/posix/environment.hpp: In instantiation of 'Char** boost::process::detail::posix::basic_environment_impl<Char>::native_handle() const [with Char = char; native_handle_type = char**]':
<source>:14:39:   required from here
/opt/compiler-explorer/libs/boost_1_82_0/boost/process/detail/posix/environment.hpp:213:54: error: cannot convert 'const __gnu_cxx::__alloc_traits<std::allocator<std::__cxx11::basic_string<char> >, std::__cxx11::basic_string<char> >::value_type*' {aka 'const std::__cxx11::basic_string<char>*'} to 'boost::process::detail::posix::basic_environment_impl<char>::native_handle_type' {aka 'char**'} in return
  213 |     native_handle_type native_handle() const {return &_data.front();}
      |                                                      ^~~~~~~~~~~~~~
      |                                                      |
      |                                                      const __gnu_cxx::__alloc_traits<std::allocator<std::__cxx11::basic_string<char> >, std::__cxx11::basic_string<char> >::value_type* {aka const std::__cxx11::basic_string<char>*}
Compiler returned: 1

Proposed solution: store the duplicate vector<char*> _nativeData. Every char* should point on the same element on _data vector. _nativeData should reload elements on every _data rebuilding or element (stored std::string) changing.

[klemens-morgenstern]

Well that's embarrassing. Pretty amazing that you're the first one to catch it though. I'll have a fix in the next release.