Re: [PATCH] Fix setting watchpoints when current thread is running (PR gdb/31521)

[Eli Zaretskii <eliz@gnu.org>]

> From: Pedro Alves <pedro@palves.net>
> Date: Thu, 21 Mar 2024 16:43:31 +0000
> 
> Currently, when the current thread is running, you can print global
> variables.  However, if you try to set a watchpoint on the same
> globals, GDB errors out, complaining that the selected thread is
> running.  Like so:
> 
>  (gdb) c&
>  Continuing.
>  (gdb) p global
>  $1 = 1098377287
>  (gdb) watch global
>  Selected thread is running.
> 
> This patch makes setting the watchpoint work.  The problem is that
> update_watchpoint calls get_selected_frame unconditionally.  We can
> skip it if the watchpoint expression is only watching globals.

What does this mean in practice?  E.g., suppose GDB sets the
watchpoint when the selected thread is changing the variable to be
watched -- will the thread stop or won't it?  IOW, I don't understand
what happens with programming the debug registers while some thread is
running.

> New NEWS blurb added.  I don't think we need to change anything in the
> manual.

If this is deemed a bugfix, then we don't need to say anything in
NEWS, either, IMO.

> --- a/gdb/NEWS
> +++ b/gdb/NEWS
> @@ -28,6 +28,11 @@ maintenance info line-table
>    if the line is considered the start of the epilgoue, and thus a point at
>    which the frame can be considered destroyed.
>  
> +watch
> +awatch
> +rwatch
> +  GDB now lets you set a watchpoint even if the selected thread is running.
> +

Reviewed-By: Eli Zaretskii <eliz@gnu.org>