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);
}