Fix uninitialized variable

[dzlaus]

Have run into a very peculiar error on GCC 15.1 which only manifests when using with the following flags
'-Wall Werror=uninitialized -fhardened' and only under Release mode compilation.

The -fhardened flag is critical, as without it the compiler will happily compile the code. No idea what trips the compiler, but the fix is trivial.

The issue is reproducible on the WebSocket server, asynchronous example.

The full compiler output is as follows:

ninja
[1/2] Building CXX object CMakeFiles/unitialized_variable.dir/main.cpp.o
FAILED: CMakeFiles/unitialized_variable.dir/main.cpp.o 
/usr/lib/ccache/bin/g++   -O3 -DNDEBUG -std=gnu++26 -fdiagnostics-color=always -Wall -Werror=uninitialized -fhardened -MD -MT CMakeFiles/unitialized_variable.dir/main.cpp.o -MF CMakeFiles/unitialized_variable.dir/main.cpp.o.d -o CMakeFiles/unitialized_variable.dir/main.cpp.o -c /home/user/workspace/unitialized_variable/main.cpp
In file included from /usr/include/boost/endian/detail/endian_reverse.hpp:9,
                 from /usr/include/boost/endian/conversion.hpp:11,
                 from /usr/include/boost/beast/websocket/detail/frame.hpp:20,
                 from /usr/include/boost/beast/websocket/detail/impl_base.hpp:14,
                 from /usr/include/boost/beast/websocket/stream.hpp:20,
                 from /usr/include/boost/beast/websocket.hpp:18,
                 from /home/user/workspace/unitialized_variable/main.cpp:17:
In function ‘constexpr uint16_t boost::endian::detail::endian_reverse_impl(uint16_t)’,
    inlined from ‘constexpr typename std::enable_if<(! std::is_class<_Tp>::value), T>::type boost::endian::endian_reverse(T) [with T = short unsigned int]’ at /usr/include/boost/endian/detail/endian_reverse.hpp:131:55,
    inlined from ‘constexpr EndianReversible boost::endian::detail::conditional_reverse_impl(EndianReversible, std::false_type) [with EndianReversible = short unsigned int]’ at /usr/include/boost/endian/conversion.hpp:181:26,
    inlined from ‘constexpr EndianReversible boost::endian::conditional_reverse(EndianReversible) [with order From = boost::endian::order::big; order To = boost::endian::order::little; EndianReversible = short unsigned int]’ at /usr/include/boost/endian/conversion.hpp:191:44,
    inlined from ‘constexpr EndianReversible boost::endian::big_to_native(EndianReversible) [with EndianReversible = short unsigned int]’ at /usr/include/boost/endian/conversion.hpp:148:73,
    inlined from ‘void boost::beast::websocket::detail::read_close(boost::beast::websocket::close_reason&, const Buffers&, boost::beast::error_code&) [with Buffers = boost::beast::buffers_prefix_view<boost::beast::detail::buffers_pair<true> >]’ at /usr/include/boost/beast/websocket/detail/frame.hpp:224:36:
/usr/include/boost/endian/detail/endian_reverse.hpp:60:12: error: ‘code_be’ may be used uninitialized [-Werror=maybe-uninitialized]
   60 |     return BOOST_ENDIAN_INTRINSIC_BYTE_SWAP_2(x);
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/beast/websocket/detail/frame.hpp: In function ‘void boost::beast::websocket::detail::read_close(boost::beast::websocket::close_reason&, const Buffers&, boost::beast::error_code&) [with Buffers = boost::beast::buffers_prefix_view<boost::beast::detail::buffers_pair<true> >]’:
/usr/include/boost/beast/websocket/detail/frame.hpp:216:19: note: ‘code_be’ was declared here
  216 |     std::uint16_t code_be;
      |                   ^~~~~~~
cc1plus: some warnings being treated as errors
ninja: build stopped: subcommand failed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.11%. Comparing base (03f0ff3) to head (44bc18a).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3019   +/-   ##
========================================
  Coverage    93.11%   93.11%           
========================================
  Files          176      176           
  Lines        13643    13647    +4     
========================================
+ Hits         12704    12708    +4     
  Misses         939      939           

Files with missing lines	Coverage Δ
include/boost/beast/websocket/detail/frame.hpp	100.00% <100.00%> (ø)
include/boost/beast/websocket/impl/stream_impl.hpp	96.21% <100.00%> (+0.02%)	⬆️

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[ashtum]

The following is minimal reproducible example of the same warning:
https://godbolt.org/z/9axKqv1PP

#include <boost/beast.hpp>

using namespace boost;

std::uint16_t
fun(
    asio::const_buffer const& cb,
    system::error_code& ec)
{
    if(cb.size() < 2)
        return 0;

    ec = {}; // commenting this line resolves warning

    std::uint16_t result;
    asio::buffer_copy(
        asio::mutable_buffer(&result, sizeof(result)),
        cb);
    return endian::big_to_native(result);
}

It seems the compiler thinks assigning to ec might change the buffer size.
There are a few other places in the code with the same issue. Zero initializing the variable is a workaround, but ideally, we want the compiler to reason about the code, since logically a zero initialized variable is not what we want either.

[ashtum]

For the record, I couldn't get the compiler to reason correctly about asio::buffer_copy. I used statements similar to if (buffer_bytes(bs) < 2) __builtin_unreachable(); to provide additional information to the compiler. While this worked in a few cases, it failed in two cases in stream_impl.hpp. Therefore, it seems more consistent to zero-initialize all of them instead.
https://godbolt.org/z/54GbYbr4a

#include <boost/beast.hpp>

using namespace boost;

std::uint16_t
fun(
    asio::const_buffer const& cb,
    system::error_code& ec)
{
    if(cb.size() < 2)
        return 0;

    ec = {};
    if(beast::buffer_bytes(cb) < 2) __builtin_unreachable();

    std::uint16_t result;
    asio::buffer_copy(
        asio::mutable_buffer(&result, sizeof(result)),
        cb);
    return endian::big_to_native(result);
}

[cppalliance-bot]

An automated preview of the documentation is available at https://3019.beast.prtest.cppalliance.org/libs/beast/doc/html/index.html