bug#16745: Can't send SIGTSTP from Emacs through GDB

[Barry OReilly]

When running GDB in Emacs and sending SIGTSTP via C-c C-z to another
Emacs ran under GDB, the Emacs under GDB does not appear to receive
the SIGTSTP.

Recipe:

  ./src/emacs -Q
  M-x gdb
  gdb -i=mi src/emacs
  run -Q
  C-c C-z
  [The Emacs under GDB is not stopped.]

However, when I run GDB from the shell rather than from Emacs:

  gdb src/emacs
  run -Q
  C-z
  [The Emacs under GDB stopped.]

I noticed src/.gdbinit has line:

  handle SIGTSTP nopass

So I tried with and without this to no effect.

When I run with these debug statements:

diff --git a/src/process.c b/src/process.c
index 85470b6..176de75 100644
--- a/src/process.c
+++ b/src/process.c
@@ -5753,6 +5753,9 @@ process_send_signal (Lisp_Object process, int signo, Lisp_Object current_group,
 #else
          sig_char = &t.c_cc[VSUSP];
 #endif
+          { struct timespec debug_ts; char debug_dateStr[20]; { clock_gettime(CLOCK_REALTIME, &debug_ts); struct tm mytm; localtime_r(&debug_ts.tv_sec, &mytm); strftime(debug_dateStr, 20, "%Y-%m-%dT%H:%M:%S", &mytm); }
+               printf( "%s.%09ld|pid:%d|tid:%ld|%s|%d| DEBUG: sig_char=%02x\n", // TODO: debugging
+                                 debug_dateStr, debug_ts.tv_nsec, getpid(), pthread_self(), __FILE__, __LINE__, *sig_char ); fflush(stdout); }
          break;
        }
 
@@ -5901,6 +5904,9 @@ traffic.  */)
 #ifndef SIGTSTP
   error ("No SIGTSTP support");
 #else
+  { struct timespec debug_ts; char debug_dateStr[20]; { clock_gettime(CLOCK_REALTIME, &debug_ts); struct tm mytm; localtime_r(&debug_ts.tv_sec, &mytm); strftime(debug_dateStr, 20, "%Y-%m-%dT%H:%M:%S", &mytm); }
+       printf( "%s.%09ld|pid:%d|tid:%ld|%s|%d| DEBUG: Send SIGTSTP\n", // TODO: debugging
+                         debug_dateStr, debug_ts.tv_nsec, getpid(), pthread_self(), __FILE__, __LINE__ ); fflush(stdout); }
   process_send_signal (process, SIGTSTP, current_group, 0);
 #endif
   return process;

The output during the ill recipe is:

2014-02-13T14:11:49.438466000|pid:22430|tid:47532927609088|process.c|5909| DEBUG: Send SIGTSTP
2014-02-13T14:11:49.438547000|pid:22430|tid:47532927609088|process.c|5758| DEBUG: sig_char=1a

So the signal is being sent it seems. What might be the obstruction?

I am on RHEL 5, using latest Emacs from trunk and GDB 7.1.