[Bug 234258] SIGIO not delivered on socket when incoming connection arrives

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=234258

            Bug ID: 234258
           Summary: SIGIO not delivered on socket when incoming connection
                    arrives
           Product: Base System
           Version: 12.0-RELEASE
          Hardware: i386
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: adelman at adelman.com

If a socket is set:

fcntl(sock, F_SETOWN, getpid()) and 
fcntl(sock, F_SETFL,O_ASYNC)

when a connection arrives a SIGIO signal should be delivered to the appropriate
process. Under FreeBSD 11.2 and prior releases, this works. Under FreeBSD
12.0-RELEASE, no signal is delivered.  This is the case with at least TCP and
UNIX domain sockets.

Duplicate by compiling and running the attached source code. "telnet 127.0.0.1
2666" to tickle the code with an incoming connection. Expected behavior is for
the signal handler to fire and print "signalfired(): SIGIO fired" to stderr.
Under FreeBSD 11.2 and prior (back to at least FreeBSD 2 the code that relied
on this worked), this works. Under FreeBSD 12.0-RELEASE the signal is not
delivered.

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>

#include <stdio.h>
#include <signal.h>
#include <errno.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>


static void signalfired()
{
        fprintf(stderr,"signalfired(): SIGIO fired\n");
}

int main(void)
{
        int sock;
        struct sockaddr_in sin = { AF_INET };  /* socket address */

        sin.sin_addr.s_addr = INADDR_ANY;
        sin.sin_port = htons(2666);

        sock = socket(AF_INET, SOCK_STREAM, 0);
        if (sock < 0) {
            perror("socket");
            exit(1);
        }

        if (bind(sock, (struct sockaddr *) &sin, sizeof(sin)) < 0){
            perror("bind");
            exit(1);
        }

        if (signal(SIGIO,(void *) signalfired)) {
            perror("trapping signal");
            exit(1);
        }
        if (fcntl(sock, F_SETOWN, getpid()) == -1) {
            perror("fcntl(F_SETOWN)");
            exit(1);
        }
        if (fcntl(sock, F_SETFL, O_ASYNC) == -1) {
            perror("fcntl(F_SETFL)");
            exit(1);
        }

        if (listen(sock, 10) < 0) {
            perror("listen");
            exit(1);
        }
        fprintf(stderr,"sleeping, test by \"telnet localhost %d\"\n",2666);
        sleep(10000);
}

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