Add a regresion test to determine whether or not a file descriptor is

allocated in a fork(2)-inheritable way at the beginning or end of an
accept(2) system call.  This test creates a test thread and blocks it
in accept(2), then forks a child process which tests to see if the
next available file descriptor is defined or not (EBADF vs EINVAL for
ftruncate(2)).

This detects a regression introduced during the network stack locking
work, in which a very narrow race during which fork(2) from one
thread during accept(2) in a second thread lead to an extra inherited
file descriptor turned into a very wide race ensuring that a
descriptor was leaked into the child even though it hadn't been
returned.

PR:		kern/130348

tools/regression/file/newfileops_on_fork/Makefile

@@ -0,0 +1,8 @@
+# $FreeBSD$
+
+PROG=	newfileops_on_fork
+NO_MAN=
+WARNS?=	6
+LDFLAGS=	-lpthread
+
+.include <bsd.prog.mk>

tools/regression/file/newfileops_on_fork/newfileops_on_fork.c

@@ -0,0 +1,121 @@
+/*-
+ * Copyright (c) 2009 Robert N. M. Watson
+ * All rights reserved.
+ *
+ * This software was developed at the University of Cambridge Computer
+ * Laboratory with support from a grant from Google, Inc. 
+ *
+ * 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.
+ *
+ * $FreeBSD$
+ */
+
+/*
+ * When a multi-threaded application calls fork(2) from one thread while
+ * another thread is blocked in accept(2), we prefer that the file descriptor
+ * to be returned by accept(2) not appear in the child process.  Test this by
+ * creating a thread blocked in accept(2), then forking a child and seeing if
+ * the fd it would have returned is defined in the child or not.
+ */
+
+#include <sys/socket.h>
+#include <sys/wait.h>
+
+#include <netinet/in.h>
+
+#include <err.h>
+#include <errno.h>
+#include <pthread.h>
+#include <signal.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#define	PORT	9000
+
+static int listen_fd;
+
+static void *
+do_accept(__unused void *arg)
+{
+	int accept_fd;
+
+	accept_fd = accept(listen_fd, NULL, NULL);
+	if (accept_fd < 0)
+		err(-1, "accept");
+
+	return (NULL);
+}
+
+static void
+do_fork(void)
+{
+	int pid;
+
+	pid = fork();
+	if (pid < 0)
+		err(-1, "fork");
+	if (pid > 0) {
+		waitpid(pid, NULL, 0);
+		exit(0);
+	}
+
+	/*
+	 * We will call ftruncate(2) on the next available file descriptor,
+	 * listen_fd+1, and get back EBADF if it's not a valid descriptor,
+	 * and EINVAL if it is.  This (currently) works fine in practice.
+	 */
+	if (ftruncate(listen_fd + 1, 0 < 0)) {
+		if (errno == EBADF)
+			exit(0);
+		else if (errno == EINVAL)
+			errx(-1, "file descriptor still open in child");
+		else
+			err(-1, "unexpected error");
+	} else
+		errx(-1, "ftruncate succeeded");
+}
+
+int
+main(__unused int argc, __unused char *argv[])
+{
+	struct sockaddr_in sin;
+	pthread_t accept_thread;
+
+	listen_fd = socket(PF_INET, SOCK_STREAM, 0);
+	if (listen_fd < 0)
+		err(-1, "socket");
+	bzero(&sin, sizeof(sin));
+	sin.sin_family = AF_INET;
+	sin.sin_len = sizeof(sin);
+	sin.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
+	sin.sin_port = htons(PORT);
+	if (bind(listen_fd, (struct sockaddr *)&sin, sizeof(sin)) < 0)
+		err(-1, "bind");
+	if (listen(listen_fd, -1) <0)
+		err(-1, "listen");
+	if (pthread_create(&accept_thread, NULL, do_accept, NULL) < 0)
+		err(-1, "pthread_create");
+	sleep(1);	/* Easier than using a CV. */;
+	do_fork();
+	exit(0);
+}