bug#63084: 30.0.50; gud: set breakpoint while program is running

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 63084@debbugs.gnu.org
> Date: Wed, 26 Apr 2023 12:44:29 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> > From: TatriX <tatrics@gmail.com>
> > Date: Wed, 26 Apr 2023 10:48:53 +0200
> > Cc: 63084@debbugs.gnu.org
> > 
> > Oh, sorry. I tried on a different machine, and realized one have to use "run&".
> > 
> > So, here's what I did:
> > 
> > $ cat main.c
> >     #include <stdio.h>
> >     #include <unistd.h>
> > 
> >     int main(void) {
> >         for (int i = 0; ; i++) {
> >             printf("%d\n", i);
> >             sleep(1);
> >         }
> >     }
> > 
> > $ gcc -g -o break main.c
> > $ emacs -Q main.c
> > M-x gdb RET
> > # in *gud-break*, NOTE it's "run&"
> > (gdb) run&
> > # in main.c
> > (goto-line 6)
> > (gud-break 1) ; or C-x C-a C-b
> > # nothing happens
> > M-: (gud-call "break 6") RET
> > # breakpoint is set and process execution is paused on hitting that breakpoint
> 
> What is the value of gdb-running and of gdb-non-stop after you "run&"
> the program?

Ken, could you please take a look at this bug report?  AFAICT, it has
something to do with the code you changed some 11 years ago (see
bug#9878).

Basically, what "M-x gdb" now does is send the "-gdb-set non-stop 1"
command, then, when we get a valid response for it, it sends the
"-gdb-set target-async 1" command.  So far so good, but when we get
the response for the latter, we send the "-list-target-features"
command and expect it to report "async" as one of the features, and if
not, we decide that non-stop mode is not supported.

My testing indicates that -list-target-features will only report
"async" after we run the program or attach to a process.  So we are
(almost) always disabling the non-stop mode, which doesn't seem right
to me.

So I'm interested to know how you tested this particular addition of
the -list-target-features command back then (if you remember).  Also,
what happens today when you start "M-x gdb" with a modern version of
GDB that does support target-async and non-stop mode.

The root cause that "C-x C-a C-b" doesn't work in the OP is that
gud-break (and any other command defined via gud-def) does nothing
when gud-running is non-nil.  This needs to be changed if we are
running the program in the background, but the question is how to know
that reliably, and that is related -list-target-features, among other
things.

TIA