[PATCH] Fix --disable-threading build

[Pedro Alves]

On 2022-05-09 19:08, Tom Tromey via Gdb-patches wrote:
> PR build/29110 points out that GDB fails to build on mingw when the
> "win32" thread model is in use.  It turns out that the Fedora cross
> tools using the "posix" thread model, which somehow manages to support
> std::future, whereas the win32 model does not.

I guess it depends on GCC version.  Ubuntu 20.04 ships both win32 and posix
models as alternatives:

"
$ sudo update-alternatives --config x86_64-w64-mingw32-gcc
There are 2 choices for the alternative x86_64-w64-mingw32-gcc (providing /usr/bin/x86_64-w64-mingw32-gcc).

  Selection    Path                                   Priority   Status
------------------------------------------------------------
  0            /usr/bin/x86_64-w64-mingw32-gcc-win32   60        auto mode
* 1            /usr/bin/x86_64-w64-mingw32-gcc-posix   30        manual mode
  2            /usr/bin/x86_64-w64-mingw32-gcc-win32   60        manual mode

Press <enter> to keep the current choice[*], or type selection number: 
"

and there the posix model doesn't have std::future either.  The version is:

 gcc version 9.3-posix 20200320 (GCC) 

Above I have posix selected, but the default is win32.

> 
> While looking into this, I found that the configuring with
> --disable-threading will also cause a build failure.
> 
> This patch fixes this build by introducing a compatibility wrapper for
> std::future.
> 
> I am not able to test the win32 thread model build, but I'm going to
> ask the reporter to try this patch.
> 
> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29110

I tested this on Ubuntu 20.04, two builds, one with posix model, and another
with win32 model.  Both worked (as in, gdb built).

>  thread_pool::thread_function ()
>  {
> diff --git a/gdbsupport/thread-pool.h b/gdbsupport/thread-pool.h
> index 5e203fd896c..fc35d61faf0 100644
> --- a/gdbsupport/thread-pool.h
> +++ b/gdbsupport/thread-pool.h
> @@ -27,13 +27,69 @@
>  #include <thread>
>  #include <mutex>
>  #include <condition_variable>
> -#endif
>  #include <future>
> +#endif
>  #include "gdbsupport/gdb_optional.h"
>  
>  namespace gdb
>  {
>  
> +#if CXX_STD_THREAD
> +
> +/* Simply use the standard future.  */
> +template<typename T>
> +using future = std::future<T>;
> +
> +#else /* CXX_STD_THREAD */
> +
> +/* A compatibility wrapper for std::future.  Once <thread> and
> +   <future> are available in all GCC builds -- should that ever happen
> +   -- this can be removed.

I think this should mention mingw.

Otherwise LGTM.

Thanks for fixing this.

Pedro Alves

> +
> +   Meanwhile, in this mode, there are no threads.  Tasks submitted to
> +   the thread pool are invoked immediately and their result is stored
> +   here.  The base template here simply wraps a T and provides some
> +   std::future compatibility methods.  The provided methods are chosen
> +   based on what GDB needs presently.  */
> +
> +template<typename T>
> +class future