Re: [RFC] Don't immediately SIGTERM the child of "target remote |".

Doug Evans
2011-11-24T00:09:33Z

Sorry for the followup.
Minor comment inline.

On Wed, Nov 23, 2011 at 4:00 PM, Doug Evans <dje@google.com> wrote:
> Hi.
>
> While testing a patch to allow gdbserver to communicate over stdio
> (e.g. target remote | gdbserver ...)
> I found that several tests from the testsuite were left running
> and traced it to gdb SIGTERMing gdbserver when the connection is closed.
>
> We could add a SIGTERM handler to gdbserver
> but I'm starting with this patch.
> I don't understand why gdb kills the child at all.
> In a properly written child connection, closing stdin should be a
> sufficient signal for the child to know to shutdown.
>
> AFAICT The kill() call was added in cvs revision 1.2 with the log entry:
> Replace ../include/wait.h with gdb_wait.h.
> Blech.
>
> Alas I can't remove this kill(SIGTERM) call
> for fear of breaking something so I'm trying a
> compromise and only killing the child after some timeout.
>
> Comments?
>
> I borrowed the SA_RESTART handling from other places in gdb.
> I don't actually know that this works (i.e. the alarm call
> will wake up waitpid) on systems without SA_RESTART,
> and I don't know if alarm() is portable enough
> (there are no current uses of it AFAICT).
>
> 2011-11-23  Doug Evans  <dje@google.com>
>
>        * ser-pipe.c (sigalrm_handler): New function.
>        (pipe_close): Don't immediately send SIGTERM to the child.
>        Give it awhile to cleanly shutdown, but don't wait forever.
>
> Index: ser-pipe.c
> ===================================================================
> RCS file: /cvs/src/src/gdb/ser-pipe.c,v
> retrieving revision 1.32
> diff -u -p -r1.32 ser-pipe.c
> --- ser-pipe.c  4 Mar 2011 19:23:42 -0000       1.32
> +++ ser-pipe.c  23 Nov 2011 23:46:55 -0000
> @@ -154,6 +154,12 @@ pipe_open (struct serial *scb, const cha
>  }
>
>  static void
> +sigalrm_handler (int signo)
> +{
> +  /* nothing to do */
> +}
> +
> +static void
>  pipe_close (struct serial *scb)
>  {
>   struct pipe_state *state = scb->state;
> @@ -163,14 +169,45 @@ pipe_close (struct serial *scb)
>
>   if (state != NULL)
>     {
> -      int status;
> -      kill (state->pid, SIGTERM);
> +      int rc, status;
> +      void (*ofunc) ();        /* Previous SIGALRM handler.  */
> +
> +      /* Don't kill the task right away, give it a chance to shut down cleanly.
> +        But don't wait forever though.  */
> +#if defined (HAVE_SIGACTION) && defined (SA_RESTART)
> +      {
> +       struct sigaction sa, osa;
> +       sa.sa_handler = sigalrm_handler;
> +       sigemptyset (&sa.sa_mask);
> +       sa.sa_flags = 0;
> +       sigaction (SIGALRM, &sa, &osa);
> +       ofunc = osa.sa_handler;
> +      }
> +#else
> +      ofunc = (void (*)()) signal (SIGALRM, sigalrm_handler);
> +#endif
> +
> +#define PIPE_CLOSE_TIMEOUT 5
> +      alarm (PIPE_CLOSE_TIMEOUT);
> +
> +      rc = -1;
>  #ifdef HAVE_WAITPID
>       /* Assume the program will exit after SIGTERM.  Might be
>         useful to print any remaining stderr output from
>         scb->error_fd while waiting.  */
> -      waitpid (state->pid, &status, 0);
> +      rc = waitpid (state->pid, &status, 0);
> +#endif
> +      if (rc != 0)
> +       {
> +         kill (state->pid, SIGTERM);
> +#ifdef HAVE_WAITPID

Hmmm, in order to be compatible with the previous code the alarm
should be cancelled before this call to waitpid.
OTOH, having an alarm could be a useful thing anyway.

> +         waitpid (state->pid, &status, 0);
>  #endif
> +       }
> +
> +      alarm (0);
> +      signal (SIGALRM, ofunc);
> +
>       if (scb->error_fd != -1)
>        close (scb->error_fd);
>       scb->error_fd = -1;
>