pgrep: Use only --signal option for signal

When pgrep was used to match on signal, it makes sense to use
the same signal parsing code as pkill. Unfortunately the
"find the signal" part is a little too enthusaistic about what a
signal is, meaning

pgrep -u -42

fails because the signal becomes "42" and then there is no UID.

This is a bit sad for pkill but has been that way for a long
time. For pgrep this is new so now only the long form
pgrep --signal <X>
will work.

In addition, when using --signal if pgrep/pkill couldn't work
out what the signal was it just silently ignored it. It now
complains and aborts.

References:
 https://bugs.debian.org/1031765
 commit 866abacf88

NEWS

@@ -1,5 +1,6 @@
 procps-ng-NEXT
 ---------------
+  * pgrep: Use only --signal option for signal             Debian #1031765
   * tests: dont compare floats with ==                     issue #271
 
 procps-ng-4.0.3

man/pgrep.1

@@ -60,7 +60,7 @@ the symbolic signal name can be used. In
 .B pgrep
 or
 .B pidwait
-mode this has no effect unless used in conjunction with
+mode only the long option can be used and has no effect unless used in conjunction with
 \fB\-\-require\-handler\fR to filter to processes with a userspace signal
 handler present for a particular signal.
 

src/pgrep.c

@@ -163,6 +163,7 @@ static int __attribute__ ((__noreturn__)) usage(int opt)
         fputs(_(" -w, --lightweight         list all TID\n"), fp);
         break;
     case PKILL:
+        fputs(_(" -<sig>                    signal to send (either number or name)\n"), fp);
         fputs(_(" -H, --require-handler     match only if signal handler is present\n"), fp);
         fputs(_(" -q, --queue <value>       integer value to be sent with the signal\n"), fp);
         fputs(_(" -e, --echo                display what is killed\n"), fp);
@@ -173,7 +174,6 @@ static int __attribute__ ((__noreturn__)) usage(int opt)
         break;
 #endif
     }
-    fputs(_(" -<sig>, --signal <sig>    signal to send (either number or name)\n"), fp);
     fputs(_(" -c, --count               count of matching processes\n"), fp);
     fputs(_(" -f, --full                use full process name to match\n"), fp);
     fputs(_(" -g, --pgroup <PGID,...>   match listed process group IDs\n"), fp);
@@ -184,6 +184,7 @@ static int __attribute__ ((__noreturn__)) usage(int opt)
     fputs(_(" -O, --older <seconds>     select where older than seconds\n"), fp);
     fputs(_(" -P, --parent <PPID,...>   match only child processes of the given parent\n"), fp);
     fputs(_(" -s, --session <SID,...>   match session IDs\n"), fp);
+    fputs(_("     --signal <sig>        signal to send (either number or name)\n"), fp);
     fputs(_(" -t, --terminal <tty,...>  match by controlling terminal\n"), fp);
     fputs(_(" -u, --euid <ID,...>       match by effective IDs\n"), fp);
     fputs(_(" -U, --uid <ID,...>        match by real IDs\n"), fp);
@@ -823,7 +824,6 @@ static int pidfd_open (pid_t pid, unsigned int flags)
 static void parse_opts (int argc, char **argv)
 {
     char opts[64] = "";
-    int sig;
     int opt;
     int criteria_count = 0;
 
@@ -869,9 +869,6 @@ static void parse_opts (int argc, char **argv)
         {NULL, 0, NULL, 0}
     };
 
-    sig = signal_option(&argc, argv);
-    if (-1 < sig)
-        opt_signal = sig;
 
 #ifdef ENABLE_PIDWAIT
     if (strcmp (program_invocation_short_name, "pidwait") == 0 ||
@@ -882,7 +879,11 @@ static void parse_opts (int argc, char **argv)
 #endif
     if (strcmp (program_invocation_short_name, "pkill") == 0 ||
         strcmp (program_invocation_short_name, "lt-pkill") == 0) {
+        int sig;
         prog_mode = PKILL;
+        sig = signal_option(&argc, argv);
+        if (-1 < sig)
+            opt_signal = sig;
 	strcat (opts, "eq:");
     } else {
         strcat (opts, "lad:vw");
@@ -895,8 +896,14 @@ static void parse_opts (int argc, char **argv)
         switch (opt) {
         case SIGNAL_OPTION:
             opt_signal = signal_name_to_number (optarg);
-            if (opt_signal == -1 && isdigit (optarg[0]))
+            if (opt_signal == -1) {
-                opt_signal = atoi (optarg);
+                if (isdigit (optarg[0]))
+                    opt_signal = atoi (optarg);
+                else {
+                    fprintf(stderr, _("Unknown signal \"%s\"."), optarg);
+                    usage('?');
+                }
+            }
             break;
         case 'e':
             opt_echo = 1;