source: trunk/server/lib/util/select.c

/*
   Unix SMB/Netbios implementation.
   Version 3.0
   Samba select/poll implementation
   Copyright (C) Andrew Tridgell 1992-1998

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#include "includes.h"
#include "system/filesys.h"
#include "system/select.h"
#include "lib/util/select.h"

/* This is here because it allows us to avoid a nasty race in signal handling.
   We need to guarantee that when we get a signal we get out of a select immediately
   but doing that involves a race condition. We can avoid the race by getting the
   signal handler to write to a pipe that is in the select/poll list

   This means all Samba signal handlers should call sys_select_signal().
*/
#ifdef __OS2__
#define pipe(A) os2_pipe(A)
#endif
static pid_t initialised;
static int select_pipe[2];
static volatile unsigned pipe_written, pipe_read;

/*******************************************************************
 Call this from all Samba signal handlers if you want to avoid a
 nasty signal race condition.
********************************************************************/

void sys_select_signal(char c)
{
        int saved_errno = errno;

        if (!initialised) return;

        if (pipe_written > pipe_read+256) return;

        if (write(select_pipe[1], &c, 1) == 1) pipe_written++;

        errno = saved_errno;
}

/*
 * sys_poll expects pollfd's to be a talloc'ed array.
 *
 * It expects the talloc_array_length(fds) >= num_fds+1 to give space
 * to the signal pipe.
 */

int sys_poll(struct pollfd *fds, int num_fds, int timeout)
{
        int ret;

        if (talloc_array_length(fds) < num_fds+1) {
                errno = ENOSPC;
                return -1;
        }

        if (initialised != sys_getpid()) {
                if (pipe(select_pipe) == -1)
                {
                        int saved_errno = errno;
                        DEBUG(0, ("sys_poll: pipe failed (%s)\n",
                                strerror(errno)));
                        errno = saved_errno;
                        return -1;
                }

                /*
                 * These next two lines seem to fix a bug with the Linux
                 * 2.0.x kernel (and probably other UNIXes as well) where
                 * the one byte read below can block even though the
                 * select returned that there is data in the pipe and
                 * the pipe_written variable was incremented. Thanks to
                 * HP for finding this one. JRA.
                 */

                if(set_blocking(select_pipe[0],0)==-1)
                        smb_panic("select_pipe[0]: O_NONBLOCK failed");
                if(set_blocking(select_pipe[1],0)==-1)
                        smb_panic("select_pipe[1]: O_NONBLOCK failed");

                initialised = sys_getpid();
        }

        ZERO_STRUCT(fds[num_fds]);
        fds[num_fds].fd = select_pipe[0];
        fds[num_fds].events = POLLIN|POLLHUP;

        errno = 0;
        ret = poll(fds, num_fds+1, timeout);

        if ((ret >= 0) && (fds[num_fds].revents & (POLLIN|POLLHUP|POLLERR))) {
                char c;
                int saved_errno = errno;

                if (read(select_pipe[0], &c, 1) == 1) {
                        pipe_read += 1;

                        /* Mark Weaver <mark-clist@npsl.co.uk> pointed out a critical
                           fix to ensure we don't lose signals. We must always
                           return -1 when the select pipe is set, otherwise if another
                           fd is also ready (so ret == 2) then we used to eat the
                           byte in the pipe and lose the signal. JRA.
                        */
                        ret = -1;
#if 0
                        /* JRA - we can use this to debug the signal messaging... */
                        DEBUG(0,("select got %u signal\n", (unsigned int)c));
#endif
                        errno = EINTR;
                } else {
                        ret -= 1;
                        errno = saved_errno;
                }
        }

        return ret;
}

int sys_poll_intr(struct pollfd *fds, int num_fds, int timeout)
{
        int orig_timeout = timeout;
        struct timespec start;
        int ret;

        clock_gettime_mono(&start);

        while (true) {
                struct timespec now;
                int64_t elapsed;

                ret = poll(fds, num_fds, timeout);
                if (ret != -1) {
                        break;
                }
                if (errno != EINTR) {
                        break;
                }
                clock_gettime_mono(&now);
                elapsed = nsec_time_diff(&now, &start);
                timeout = (orig_timeout - elapsed) / 1000000;
        };
        return ret;
}