Re: [PATCH] [gdb/server] Emit warning for SIGINT failure

[Simon Marchi <simark@simark.ca>]

On 11/9/22 15:32, Tom de Vries via Gdb-patches wrote:
> Consider the executable from test-case gdb.base/interrupt-daemon.exp.
> 
> When starting it using gdbserver:
> ...
> $ ./build/gdbserver/gdbserver localhost:2345 \
>   ./outputs/gdb.base/interrupt-daemon/interrupt-daemon
> ...
> and connecting to it using gdb:
> ...
> $ gdb -q -ex "target remote localhost:2345" \
>     -ex "set follow-fork-mode child" \
>     -ex "break daemon_main" -ex cont
> ...
> we are setup to do the same as in the test-case: interrupt a running inferior
> using ^C.
> 
> So let's try:
> ...
> (gdb) continue
> Continuing.
> ^C
> ...
> After pressing ^C, nothing happens.  This a known problem, filed as
> PR remote/18772.
> 
> The problem is that in linux_process_target::request_interrupt, a kill is used
> to send a SIGINT, but it fails.  And it fails silently.
> 
> Make the failure verbose by adding a warning, such that the gdbserver output
> becomes more helpful:
> ...
> Process interrupt-daemon created; pid = 15068
> Listening on port 2345
> Remote debugging from host ::1, port 35148
> Detaching from process 15068
> Detaching from process 15085
> gdbserver: Sending SIGINT to process group of pid 15068 failed: \
>   No such process
> ...
> 
> Note that the failure can easily be reproduced using the test-case and target
> board native-gdbserver:
> ...
> (gdb) continue^M
> Continuing.^M
> PASS: gdb.base/interrupt-daemon.exp: fg: continue
> ^CFAIL: gdb.base/interrupt-daemon.exp: fg: ctrl-c stops process (timeout)
> ...
> as reported in PR server/23382.
> 
> Tested on x86_64-linux.
> ---
>  gdbserver/linux-low.cc | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/gdbserver/linux-low.cc b/gdbserver/linux-low.cc
> index 0cbfeb99086..f4df02d3ac9 100644
> --- a/gdbserver/linux-low.cc
> +++ b/gdbserver/linux-low.cc
> @@ -5467,7 +5467,10 @@ linux_process_target::request_interrupt ()
>  {
>    /* Send a SIGINT to the process group.  This acts just like the user
>       typed a ^C on the controlling terminal.  */
> -  ::kill (-signal_pid, SIGINT);
> +  int res = ::kill (-signal_pid, SIGINT);
> +  if (res == -1)
> +    warning (_("Sending SIGINT to process group of pid %ld failed: %s"),
> +	     signal_pid, safe_strerror (errno));

I don't fully understand the issue about the separate process group and
all that (and I just have a few minutes so I won't dig into that right
now), but I don't see any downside to your patch.  Therefore:

Approved-By: Simon Marchi <simon.marchi@efficios.com>

On a tangential note, at first glance, this use of the signal_pid global
seems bogus.  If you run two inferiors, signal_pid will have the pid of
the second one.  Then, if you only resume the first one and press
ctrl-c, I guess we'll send a SIGINT to the second one, that is not
resumed, and then wait for it to report a stop, which won't happen?

Simon