To be honest - I feared that when I saw your messages regarding this. Here is
my original message from july. Attached is also a small test program.
Hi,
I'm trying to use asio (that's boost::asio without boost) to handle listening
sockets asynchronuosly. This appears not to work. There are also some reports
on the net about this problem. I was able to reproduce the problem with a small
C-programm that does the same steps as asio. The relevant sequence of system
calls is:
kqueue() = 3 (0x3)
socket(PF_INET,SOCK_STREAM,6) = 4 (0x4)
setsockopt(0x4,0xffff,0x800,0x7fffffffea2c,0x4) = 0 (0x0)
kevent(3,{ 4,EVFILT_READ,EV_ADD|EV_CLEAR,0x0,0x0,0x0
4,EVFILT_WRITE,EV_ADD|EV_CLEAR,0x0,0x0,0x0 },2,0x0,0,0x0) = 0 (0x0)
setsockopt(0x4,0xffff,0x4,0x7fffffffea2c,0x4) = 0 (0x0)
bind(4,{ AF_INET 0.0.0.0:8080 },16) = 0 (0x0)
listen(0x4,0x80) = 0 (0x0)
ioctl(4,FIONBIO,0xffffea2c) = 0 (0x0)
kevent(3,{ 4,EVFILT_READ,EV_ADD|EV_CLEAR,0x0,0x0,0x0
4,EVFILT_WRITE,EV_ADD|EV_CLEAR,0x0,0x0,0x0 },2,0x0,0,0x0) = 0 (0x0)
kevent(3,0x0,0,0x7fffffffe5a0,32,0x0) ERR#4 'Interrupted system call'
The problem here is that asio registers each file descriptor with EVFILT_READ
and EVFILT_WRITE as soon as it is opened (first kevent call).
After bringing the socket into the listening state and when async_accept() is
called it registers the socket a second time. According to the man page this is
perfectly legal and can be used to modify the registration.
With this sequence of calls kevent() does not return when a connection is
established successfully.
I tracked down the problem and the reason is in soo_kqfilter(). This is called
for the first EVFILT_READ registration and decides based on the SO_ACCEPTCONN
flag which filter operations to use solisten_filtops or soread_filtops. In this
case it chooses soread_filtops.
The second EVFILT_READ registration does not call soo_kqfilter() again, but
just updates the filter from the data and fflags field so the listening socket
ends up with the wrong filter operations.
-----Original Message-----
From: Gleb Smirnoff [mailto:[email protected]]
Sent: Wednesday, February 01, 2017 7:08 PM
To: Hartmut Brandt
Cc: [email protected]; [email protected];
[email protected]
Subject: Re: svn commit: r313043 - head/sys/kern
On Wed, Feb 01, 2017 at 01:12:07PM +0000, Hartmut Brandt wrote:
H> Author: harti
H> Date: Wed Feb 1 13:12:07 2017
H> New Revision: 313043
H> URL: https://svnweb.freebsd.org/changeset/base/313043
H>
H> Log:
H> Merge filt_soread and filt_solisten and decide what to do when checking
H> for EVFILT_READ at the point of the check not when the event is registers.
H> This fixes a problem with asio when accepting a connection.
H>
H> Reviewed by: kib@, Scott Mitchell
This goes into opposite direction with what I am doing:
https://reviews.freebsd.org/D9356
Can you please explain the problem with asio when accepting a connection?
--
Totus tuus, Glebius.
#include <sys/socket.h>
#include <sys/types.h>
#include <sys/event.h>
#include <sys/filio.h>
#include <sys/ioctl.h>
#include <netinet/in.h>
#include <stdio.h>
#include <string.h>
#include <err.h>
static void
wait_loop(int kq, int sock)
{
struct kevent ev[32];
struct sockaddr_in addr;
socklen_t socklen;
for (;;) {
int nev = kevent(kq, NULL, 0, ev, 32, NULL);
if (nev < 1)
err(1, "kevent");
for (int i = 0; i < nev; ++i) {
if (ev[i].ident == sock) {
printf("accept\n");
int fd = accept(ev[i].ident,
(struct sockaddr *)&addr, &socklen);
if (fd == -1)
err(1, "accept");
}
}
}
}
int
main()
{
struct sockaddr_in addr;
/* open a TCP socket */
int kq = kqueue();
int sock = socket(PF_INET, SOCK_STREAM, 0);
struct kevent ev[2];
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);
int opt = 1;
setsockopt(sock, SOL_SOCKET, SO_NOSIGPIPE, &opt, sizeof(opt));
if (kevent(kq, ev, 2, NULL, 0, NULL) == -1)
err(1, "kevent");
setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));
memset(&addr, 0, sizeof(addr));
addr.sin_port = htons(10000);
bind(sock, (struct sockaddr *)&addr, sizeof(addr));
listen(sock, 0x80);
ioctl(sock, FIONBIO, &opt);
if (kevent(kq, ev, 2, NULL, 0, NULL) == -1)
err(1, "kevent");
wait_loop(kq, sock);
}
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