libstdc++: Implement std::formatter<std::thread::id> without <sstream> [PR115099]

Does anybody see any issue with the drive-by fixes to constraint
std::formatter<thread::id> to only work for pointers and integers (since
we don't know how to format pthread_t if it's an arbitrary struct, for
example), and to cast pointers to const void* for output (because if
pthread_t is char* then writing it to a stream would be bad! and we
don't want to allow users to overload operator<< for pointers to opaque
structs, for example). I don't think this will change anything in
practice for common targets, where pthread_t is either an integer or
void*.

Tested x86_64-linux.

-- >8 --

The std::thread::id formatter uses std::basic_ostringstream without
including <sstream>, which went unnoticed because the test for it uses
a stringstream to check the output is correct.

The fix implemented here is to stop using basic_ostringstream for
formatting thread::id and just use std::format instead.

As a drive-by fix, the formatter specialization is constrained to
require that the thread::id::native_handle_type can be formatted, to
avoid making the formatter ill-formed if the pthread_t type is not a
pointer or integer. Since non-void pointers can't be formatted, ensure
that we convert pointers to const void* for formatting. Make a similar
change to the existing operator<< overload so that in the unlikely case
that pthread_t is a typedef for char* we don't treat it as a
null-terminated string when inserting into a stream.

libstdc++-v3/ChangeLog:

	PR libstdc++/115099
	* include/bits/std_thread.h: Declare formatter as friend of
	thread::id.
	* include/std/thread (operator<<): Convert non-void pointers to
	void pointers for output.
	(formatter): Add constraint that thread::native_handle_type is a
	pointer or integer.
	(formatter::format): Reimplement without basic_ostringstream.
	* testsuite/30_threads/thread/id/output.cc: Check output
	compiles before <sstream> has been included.
---
 libstdc++-v3/include/bits/std_thread.h        | 11 ++++-
 libstdc++-v3/include/std/thread               | 43 ++++++++++++++-----
 .../testsuite/30_threads/thread/id/output.cc  | 21 ++++++++-
 3 files changed, 63 insertions(+), 12 deletions(-)

Pushed to trunk. Backport to gcc-14 to follow.

On Fri, 17 May 2024 at 14:45, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> Does anybody see any issue with the drive-by fixes to constraint
> std::formatter<thread::id> to only work for pointers and integers (since
> we don't know how to format pthread_t if it's an arbitrary struct, for
> example), and to cast pointers to const void* for output (because if
> pthread_t is char* then writing it to a stream would be bad! and we
> don't want to allow users to overload operator<< for pointers to opaque
> structs, for example). I don't think this will change anything in
> practice for common targets, where pthread_t is either an integer or
> void*.
>
> Tested x86_64-linux.
>
> -- >8 --
>
> The std::thread::id formatter uses std::basic_ostringstream without
> including <sstream>, which went unnoticed because the test for it uses
> a stringstream to check the output is correct.
>
> The fix implemented here is to stop using basic_ostringstream for
> formatting thread::id and just use std::format instead.
>
> As a drive-by fix, the formatter specialization is constrained to
> require that the thread::id::native_handle_type can be formatted, to
> avoid making the formatter ill-formed if the pthread_t type is not a
> pointer or integer. Since non-void pointers can't be formatted, ensure
> that we convert pointers to const void* for formatting. Make a similar
> change to the existing operator<< overload so that in the unlikely case
> that pthread_t is a typedef for char* we don't treat it as a
> null-terminated string when inserting into a stream.
>
> libstdc++-v3/ChangeLog:
>
>         PR libstdc++/115099
>         * include/bits/std_thread.h: Declare formatter as friend of
>         thread::id.
>         * include/std/thread (operator<<): Convert non-void pointers to
>         void pointers for output.
>         (formatter): Add constraint that thread::native_handle_type is a
>         pointer or integer.
>         (formatter::format): Reimplement without basic_ostringstream.
>         * testsuite/30_threads/thread/id/output.cc: Check output
>         compiles before <sstream> has been included.
> ---
>  libstdc++-v3/include/bits/std_thread.h        | 11 ++++-
>  libstdc++-v3/include/std/thread               | 43 ++++++++++++++-----
>  .../testsuite/30_threads/thread/id/output.cc  | 21 ++++++++-
>  3 files changed, 63 insertions(+), 12 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/std_thread.h b/libstdc++-v3/include/bits/std_thread.h
> index 2d7df12650d..5817bfb29dd 100644
> --- a/libstdc++-v3/include/bits/std_thread.h
> +++ b/libstdc++-v3/include/bits/std_thread.h
> @@ -53,6 +53,10 @@ namespace std _GLIBCXX_VISIBILITY(default)
>  {
>  _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> +#if __glibcxx_formatters
> +  template<typename, typename> class formatter;
> +#endif
> +
>    /** @addtogroup threads
>     *  @{
>     */
> @@ -117,13 +121,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<class _CharT, class _Traits>
>         friend basic_ostream<_CharT, _Traits>&
>         operator<<(basic_ostream<_CharT, _Traits>& __out, id __id);
> +
> +#if __glibcxx_formatters
> +      friend formatter<id, char>;
> +      friend formatter<id, wchar_t>;
> +#endif
>      };
>
>    private:
>      id                         _M_id;
>
>      // _GLIBCXX_RESOLVE_LIB_DEFECTS
> -    // 2097.  packaged_task constructors should be constrained
> +    // 2097. packaged_task constructors should be constrained
>      // 3039. Unnecessary decay in thread and packaged_task
>      template<typename _Tp>
>        using __not_same = __not_<is_same<__remove_cvref_t<_Tp>, thread>>;
> diff --git a/libstdc++-v3/include/std/thread b/libstdc++-v3/include/std/thread
> index 09ca3116e7f..e994d683bff 100644
> --- a/libstdc++-v3/include/std/thread
> +++ b/libstdc++-v3/include/std/thread
> @@ -42,10 +42,6 @@
>  # include <stop_token> // std::stop_source, std::stop_token, std::nostopstate
>  #endif
>
> -#if __cplusplus > 202002L
> -# include <format>
> -#endif
> -
>  #include <bits/std_thread.h> // std::thread, get_id, yield
>  #include <bits/this_thread_sleep.h> // std::this_thread::sleep_for, sleep_until
>
> @@ -53,6 +49,10 @@
>  #define __glibcxx_want_formatters
>  #include <bits/version.h>
>
> +#if __cpp_lib_formatters
> +# include <format>
> +#endif
> +
>  namespace std _GLIBCXX_VISIBILITY(default)
>  {
>  _GLIBCXX_BEGIN_NAMESPACE_VERSION
> @@ -104,10 +104,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      inline basic_ostream<_CharT, _Traits>&
>      operator<<(basic_ostream<_CharT, _Traits>& __out, thread::id __id)
>      {
> +      // Convert non-void pointers to const void* for formatted output.
> +      using __output_type
> +       = __conditional_t<is_pointer<thread::native_handle_type>::value,
> +                         const void*,
> +                         thread::native_handle_type>;
> +
>        if (__id == thread::id())
>         return __out << "thread::id of a non-executing thread";
>        else
> -       return __out << __id._M_thread;
> +       return __out << static_cast<__output_type>(__id._M_thread);
>      }
>    /// @}
>
> @@ -287,8 +293,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  #endif // __cpp_lib_jthread
>
>  #ifdef __cpp_lib_formatters // C++ >= 23
> -
>    template<typename _CharT>
> +    requires is_pointer_v<thread::native_handle_type>
> +      || is_integral_v<thread::native_handle_type>
>      class formatter<thread::id, _CharT>
>      {
>      public:
> @@ -331,10 +338,26 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>         typename basic_format_context<_Out, _CharT>::iterator
>         format(thread::id __id, basic_format_context<_Out, _CharT>& __fc) const
>         {
> -         std::basic_ostringstream<_CharT> __os;
> -         __os << __id;
> -         auto __str = __os.view();
> -         return __format::__write_padded_as_spec(__str, __str.size(),
> +         basic_string_view<_CharT> __sv;
> +         if constexpr (is_same_v<_CharT, char>)
> +           __sv = "{}thread::id of a non-executing thread";
> +         else
> +           __sv = L"{}thread::id of a non-executing thread";
> +         basic_string<_CharT> __str;
> +         if (__id == thread::id())
> +           __sv.remove_prefix(2);
> +         else
> +           {
> +             using _FmtStr = __format::_Runtime_format_string<_CharT>;
> +             // Convert non-void pointers to const void* for formatted output.
> +             using __output_type
> +               = __conditional_t<is_pointer_v<thread::native_handle_type>,
> +                                 const void*,
> +                                 thread::native_handle_type>;
> +             auto __o = static_cast<__output_type>(__id._M_thread);
> +             __sv = __str = std::format(_FmtStr(__sv.substr(0, 2)), __o);
> +           }
> +         return __format::__write_padded_as_spec(__sv, __sv.size(),
>                                                   __fc, _M_spec,
>                                                   __format::_Align_right);
>         }
> diff --git a/libstdc++-v3/testsuite/30_threads/thread/id/output.cc b/libstdc++-v3/testsuite/30_threads/thread/id/output.cc
> index 3c167202b02..94a6ff0e2a1 100644
> --- a/libstdc++-v3/testsuite/30_threads/thread/id/output.cc
> +++ b/libstdc++-v3/testsuite/30_threads/thread/id/output.cc
> @@ -3,8 +3,27 @@
>  // { dg-require-gthreads "" }
>
>  #include <thread>
> -#include <sstream>
> +
> +#include <ostream>
>  #include <format>
> +
> +void
> +test_no_includes(std::ostream& out)
> +{
> +  std::thread::id i{};
> +  // Check stream insertion works without including <sstream>:
> +  out << i;
> +#if __cpp_lib_formatters >= 202302
> +  // PR libstdc++/115099 - compilation error: format thread::id
> +  // Verify we can use std::thread::id with std::format without <sstream>:
> +  (void) std::format("{}", i);
> +#ifdef _GLIBCXX_USE_WCHAR_T
> +  (void) std::format(L"{}", i);
> +#endif
> +#endif
> +}
> +
> +#include <sstream>
>  #include <testsuite_hooks.h>
>
>  void
> --
> 2.45.0
>