source: branches/samba-3.0/source/lib/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 2 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, write to the Free Software
   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/

#include "includes.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().
*/

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)
{
        if (!initialised) return;

        if (pipe_written > pipe_read+256) return;

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

/*******************************************************************
 Like select() but avoids the signal race using a pipe
 it also guuarantees that fds on return only ever contains bits set
 for file descriptors that were readable.
********************************************************************/

int sys_select(int maxfd, fd_set *readfds, fd_set *writefds, fd_set *errorfds, struct timeval *tval)
{
        int ret, saved_errno;
        fd_set *readfds2, readfds_buf;

        if (initialised != sys_getpid()) {
#ifndef __OS2__
                pipe(select_pipe);
#else
                socketpair(AF_UNIX, SOCK_STREAM,0, select_pipe);
#endif

                /*
                 * 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.\n");
                if(set_blocking(select_pipe[1],0)==-1)
                        smb_panic("select_pipe[1]: O_NONBLOCK failed.\n");

                initialised = sys_getpid();
        }

        maxfd = MAX(select_pipe[0]+1, maxfd);

        /* If readfds is NULL we need to provide our own set. */
        if (readfds) {
                readfds2 = readfds;
        } else {
                readfds2 = &readfds_buf;
                FD_ZERO(readfds2);
        }
        FD_SET(select_pipe[0], readfds2);

        errno = 0;
        ret = select(maxfd,readfds2,writefds,errorfds,tval);

        if (ret <= 0) {
                FD_ZERO(readfds2);
                if (writefds)
                        FD_ZERO(writefds);
                if (errorfds)
                        FD_ZERO(errorfds);
        } else if (FD_ISSET(select_pipe[0], readfds2)) {
                char c;
                saved_errno = errno;
                if (read(select_pipe[0], &c, 1) == 1) {
                        pipe_read++;
                        /* 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 {
                        FD_CLR(select_pipe[0], readfds2);
                        ret--;
                        errno = saved_errno;
                }
        }

        return ret;
}

/*******************************************************************
 Similar to sys_select() but catch EINTR and continue.
 This is what sys_select() used to do in Samba.
********************************************************************/

int sys_select_intr(int maxfd, fd_set *readfds, fd_set *writefds, fd_set *errorfds, struct timeval *tval)
{
        int ret;
        fd_set *readfds2, readfds_buf, *writefds2, writefds_buf, *errorfds2, errorfds_buf;
        struct timeval tval2, *ptval, end_time;

        readfds2 = (readfds ? &readfds_buf : NULL);
        writefds2 = (writefds ? &writefds_buf : NULL);
        errorfds2 = (errorfds ? &errorfds_buf : NULL);
        if (tval) {
                GetTimeOfDay(&end_time);
                end_time.tv_sec += tval->tv_sec;
                end_time.tv_usec += tval->tv_usec;
                end_time.tv_sec += end_time.tv_usec / 1000000;
                end_time.tv_usec %= 1000000;
                errno = 0;
                tval2 = *tval;
                ptval = &tval2;
        } else {
                ptval = NULL;
        }

        do {
                if (readfds)
                        readfds_buf = *readfds;
                if (writefds)
                        writefds_buf = *writefds;
                if (errorfds)
                        errorfds_buf = *errorfds;
                if (ptval && (errno == EINTR)) {
                        struct timeval now_time;
                        SMB_BIG_INT tdif;

                        GetTimeOfDay(&now_time);
                        tdif = usec_time_diff(&end_time, &now_time);
                        if (tdif <= 0) {
                                ret = 0; /* time expired. */
                                break;
                        }
                        ptval->tv_sec = tdif / 1000000;
                        ptval->tv_usec = tdif % 1000000;
                }

                /* We must use select and not sys_select here. If we use
                   sys_select we'd lose the fact a signal occurred when sys_select
                   read a byte from the pipe. Fix from Mark Weaver
                   <mark-clist@npsl.co.uk>
                */
                ret = select(maxfd, readfds2, writefds2, errorfds2, ptval);
//printf("select2 called, ret = %d\n");
        } while (ret == -1 && errno == EINTR);

        if (readfds)
                *readfds = readfds_buf;
        if (writefds)
                *writefds = writefds_buf;
        if (errorfds)
                *errorfds = errorfds_buf;

        return ret;
}