[Bug 245318] procstat_getprocs() noise on stderr

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=245318

            Bug ID: 245318
           Summary: procstat_getprocs() noise on stderr
           Product: Base System
           Version: 12.1-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: jwb at freebsd.org

I think a process count of 0 in procstat_getprocs() is not necessarily an error
and this should not trigger a message to stderr.

The code below demonstrates the issue, assuming 1000 is not part of a process
group.  Tweak the value if necessary.

If procstat_getprocs() does not find any processes in the group, it returns 0
via the *count argument as expected, but also prints a warning to stderr using
warnx():

kinfo_proc structure size mismatch (len = 0)

The warnx() call can be found in /usr/src/lib/libprocstat/libprocstat.c.

It seems to be that the action to be taken when count == 0 should be left to
the caller.

#include <stdio.h>
#include <sysexits.h>
#include <sys/param.h>
#include <sys/queue.h>
#include <sys/socket.h>
#include <sys/user.h>
#include <sys/sysctl.h>
#include <libprocstat.h>

int     main(int argc,char *argv[])

{
    unsigned            pgid = 1000,
                        pid_count;
    struct procstat     *proc_info;
    struct kinfo_proc   *proc_list;
    // FILE                *procstat_err;

    proc_info = procstat_open_sysctl();

    // Silence warnx()
    // procstat_err = fopen("/dev/null", "w+");
    // err_set_file(procstat_err);

    proc_list = procstat_getprocs(proc_info, KERN_PROC_PGRP, pgid,
                                    (unsigned *)&pid_count);
    // fclose(procstat_err);
    procstat_freeprocs(proc_info, proc_list);
    procstat_close(proc_info);

    return EX_OK;
}

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