Add a regression test for putting a socket on kqueue, and then doing

listen(2) on it (see r313043).  Based on Hartmut's code.

tools/regression/sockets/listen_kqueue/Makefile

@@ -0,0 +1,7 @@
+# $FreeBSD$
+
+PROG=	listen_kqueue
+MAN=
+WARNS?=	6
+
+.include <bsd.prog.mk>

tools/regression/sockets/listen_kqueue/listen_kqueue.c

@@ -0,0 +1,114 @@
+/*-
+ * Copyright (c) 2017 Hartmut Brandt <harti@FreeBSD.org>
+ * Copyright (c) 2017 Gleb Smirnoff <glebius@FreeBSD.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ * $FreeBSD$
+ */
+
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <sys/event.h>
+#include <sys/filio.h>
+#include <sys/ioctl.h>
+#include <netinet/in.h>
+#include <err.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+
+/*
+ * This regression test is against a scenario when a socket is first added
+ * to a kqueue, and only then is put into listening state.
+ * This weird scenario was made a valid one in r313043, and shouldn't be
+ * broken later.
+ */
+
+int
+main()
+{
+	struct sockaddr_in addr;
+	struct kevent ev[2];
+	socklen_t socklen;
+	int kq, sock, opt, pid, nev, fd;
+
+	if ((kq = kqueue()) == -1)
+		err(1, "kqueue");
+
+	if ((sock = socket(PF_INET, SOCK_STREAM, 0)) == -1)
+		err(1, "socket");
+
+	EV_SET(&ev[0], sock, EVFILT_READ, EV_ADD | EV_CLEAR, 0, 0, NULL);
+	EV_SET(&ev[1], sock, EVFILT_WRITE, EV_ADD | EV_CLEAR, 0, 0, NULL);
+
+	opt = 1;
+	if (setsockopt(sock, SOL_SOCKET, SO_NOSIGPIPE, &opt, sizeof(opt)) == -1)
+		err(1, "setsockopt");
+
+	if (kevent(kq, ev, 2, NULL, 0, NULL) == -1)
+	    err(1, "kevent");
+
+	if (setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt)) == -1)
+		err(1, "setsockopt");
+
+	memset(&addr, 0, sizeof(addr));
+	addr.sin_family = AF_INET;
+	addr.sin_port = htons(10000);
+
+	if (bind(sock, (struct sockaddr *)&addr, sizeof(addr)) == -1)
+		err(1, "bind");
+	if (listen(sock, 0x80) == -1)
+		err(1, "listen");
+
+	if (ioctl(sock, FIONBIO, &opt) == -1)
+		err(1, "ioctl(FIONBIO)");
+
+	if (kevent(kq, ev, 2, NULL, 0, NULL) == -1)
+		err(1, "kevent");
+
+	pid = fork();
+	if (pid == -1)
+		err(1, "fork");
+	if (pid == 0) {
+		if (close(sock) == -1)
+			err(1, "close");
+		if ((sock = socket(PF_INET, SOCK_STREAM, 0)) == -1)
+			err(1, "socket");
+		if (connect(sock, (struct sockaddr *)&addr, sizeof(addr)) == -1)
+			err(1, "connect");
+	} else {
+		nev = kevent(kq, NULL, 0, ev, 2, NULL);
+		if (nev < 1)
+			err(1, "kevent");
+		for (int i = 0; i < nev; ++i) {
+			if (ev[i].ident == (uintptr_t )sock) {
+				fd = accept(ev[i].ident,
+				    (struct sockaddr *)&addr, &socklen);
+				if (fd == -1)
+					err(1, "accept");
+				printf("OK\n");
+			}
+		}
+	}
+}