Re: [PATCH 2/2] [gdb] Initialize main_thread_id earlier

[Pedro Alves <pedro@palves.net>]

On 2023-07-28 11:59, Tom de Vries via Gdb-patches wrote:
> I wrote a patch using is_main_thread (), and found it returning false in the
> main thread due to main_thread_id not being initialized yet.
> 
> Initialization currently takes place in _initialize_run_on_main_thread, but
> that's too late for earlier uses.
> 
> Fix this by also initializing on first use.
> 
> Tested on x86_64-linux.
> ---
>  gdb/run-on-main-thread.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/gdb/run-on-main-thread.c b/gdb/run-on-main-thread.c
> index 91d25dae28f..bee5885d9a6 100644
> --- a/gdb/run-on-main-thread.c
> +++ b/gdb/run-on-main-thread.c
> @@ -94,12 +94,27 @@ run_on_main_thread (std::function<void ()> &&func)
>    serial_event_set (runnable_event);
>  }
>  
> +#if CXX_STD_THREAD
> +static void
> +initialize_main_thread_id (void)
> +{
> +  static bool initialized = false;
> +
> +  if (initialized)
> +    return;
> +  initialized = true;
> +
> +  main_thread_id = std::this_thread::get_id ();
> +}
> +#endif
> +
>  /* See run-on-main-thread.h.  */
>  
>  bool
>  is_main_thread ()
>  {
>  #if CXX_STD_THREAD
> +  initialize_main_thread_id ();

This is assuming that is_main_thread() will always be called once by
the main thread, before any other thread could call it.  Otherwise if
is_main_thread is called by some other thread first,
is_main_thread -> initialize_main_thread_id will record the wrong thread
id in the main_thread_id.  At least a comment somewhere would
be warranted.  I think putting:

  gdb_assert (is_main_thread ());

somewhere in the early main code would be good.  It would prevent such
situation from ever happening undetected.