pgrep: Better warning if pidfd_open not implemented

If procps is built on a Linux 5.3+ system then pidwait
is built with pidfd_open(). If that program is run on
a system < 5.3 then it gives an odd generic error.

If we get a ENOSYS from one pid, we will get it for all
the others so its better to explain what happens and terminate.

The man page is updated to note this issue.

This came up due to killall in psmisc using pidfd_send_signal

References:
 https://bugs.debian.org/1015228

Signed-off-by: Craig Small <csmall@dropbear.xyz>

NEWS

@@ -2,6 +2,7 @@ procps-ng-NEXT
   * library
     Re-add elogind support                                 merge #151
     Used memory is Total - Available
+  * pidwait: Better warning if pidfd_open not implemented
   * pmap: Dont reuse stdin filehandle                      issue #231
   * ps: threads again display when -L is used with -q      issue #234
   * ps: proper aix format string behavior was restored

pgrep.1

@@ -7,7 +7,7 @@
 .\" the Free Software Foundation; either version 2 of the License, or
 .\" (at your option) any later version.
 .\"
-.TH PGREP "1" "2021-12-25" "procps-ng" "User Commands"
+.TH PGREP "1" "2022-07-18" "procps-ng" "User Commands"
 .SH NAME
 pgrep, pkill, pidwait \- look up, signal, or wait for processes based on name and other attributes
 .SH SYNOPSIS
@@ -281,7 +281,11 @@ can not be combined.  Let
 me know if you need to do this.
 .PP
 Defunct processes are reported.
-
+.PP
+.B pidwait
+requires the
+.BR pidfd_open (2)
+system call which first appeared in Linux 5.3.
 .SH "SEE ALSO"
 .BR ps (1),
 .BR regex (7),

pgrep.c

@@ -1064,6 +1064,8 @@ int main (int argc, char **argv)
                 printf(_("waiting for %s (pid %lu)\n"), procs[i].str, procs[i].num);
             int pidfd = pidfd_open(procs[i].num, 0);
             if (pidfd == -1) {
+		if (errno == ENOSYS)
+		    xerrx(EXIT_FAILURE, _("pidfd_open() not implemented in Linux < 5.3"));
                 /* ignore ESRCH, same as pkill */
                 if (errno != ESRCH)
                     xwarn(_("opening pid %ld failed"), procs[i].num);