lib/util/select.c

   1 /*
   2    Unix SMB/Netbios implementation.
   3    Version 3.0
   4    Samba select/poll implementation
   5    Copyright (C) Andrew Tridgell 1992-1998
   6
   7    This program is free software; you can redistribute it and/or modify
   8    it under the terms of the GNU General Public License as published by
   9    the Free Software Foundation; either version 3 of the License, or
  10    (at your option) any later version.
  11
  12    This program is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15    GNU General Public License for more details.
  16
  17    You should have received a copy of the GNU General Public License
  18    along with this program.  If not, see <http://www.gnu.org/licenses/>.
  19 */
  20
  21 #include "includes.h"
  22 #include "system/filesys.h"
  23 #include "system/select.h"
  24 #include "lib/util/select.h"
  25
  26 /* This is here because it allows us to avoid a nasty race in signal handling.
  27    We need to guarantee that when we get a signal we get out of a select immediately
  28    but doing that involves a race condition. We can avoid the race by getting the
  29    signal handler to write to a pipe that is in the select/poll list
  30
  31    This means all Samba signal handlers should call sys_select_signal().
  32 */
  33
  34 static pid_t initialised;
  35 static int select_pipe[2];
  36 static volatile unsigned pipe_written, pipe_read;
  37
  38 /*******************************************************************
  39  Call this from all Samba signal handlers if you want to avoid a
  40  nasty signal race condition.
  41 ********************************************************************/
  42
  43 void sys_select_signal(char c)
  44 {
  45         int saved_errno = errno;
  46
  47         if (!initialised) return;
  48
  49         if (pipe_written > pipe_read+256) return;
  50
  51         if (write(select_pipe[1], &c, 1) == 1) pipe_written++;
  52
  53         errno = saved_errno;
  54 }
  55
  56 /*
  57  * sys_poll expects pollfd's to be a talloc'ed array.
  58  *
  59  * It expects the talloc_array_length(fds) >= num_fds+1 to give space
  60  * to the signal pipe.
  61  */
  62
  63 int sys_poll(struct pollfd *fds, int num_fds, int timeout)
  64 {
  65         int ret;
  66
  67         if (talloc_array_length(fds) < num_fds+1) {
  68                 errno = ENOSPC;
  69                 return -1;
  70         }
  71
  72         if (initialised != sys_getpid()) {
  73                 if (pipe(select_pipe) == -1)
  74                 {
  75                         int saved_errno = errno;
  76                         DEBUG(0, ("sys_poll: pipe failed (%s)\n",
  77                                 strerror(errno)));
  78                         errno = saved_errno;
  79                         return -1;
  80                 }
  81
  82                 /*
  83                  * These next two lines seem to fix a bug with the Linux
  84                  * 2.0.x kernel (and probably other UNIXes as well) where
  85                  * the one byte read below can block even though the
  86                  * select returned that there is data in the pipe and
  87                  * the pipe_written variable was incremented. Thanks to
  88                  * HP for finding this one. JRA.
  89                  */
  90
  91                 if(set_blocking(select_pipe[0],0)==-1)
  92                         smb_panic("select_pipe[0]: O_NONBLOCK failed");
  93                 if(set_blocking(select_pipe[1],0)==-1)
  94                         smb_panic("select_pipe[1]: O_NONBLOCK failed");
  95
  96                 initialised = sys_getpid();
  97         }
  98
  99         ZERO_STRUCT(fds[num_fds]);
 100         fds[num_fds].fd = select_pipe[0];
 101         fds[num_fds].events = POLLIN|POLLHUP;
 102
 103         errno = 0;
 104         ret = poll(fds, num_fds+1, timeout);
 105
 106         if ((ret >= 0) && (fds[num_fds].revents & (POLLIN|POLLHUP|POLLERR))) {
 107                 char c;
 108                 int saved_errno = errno;
 109
 110                 if (read(select_pipe[0], &c, 1) == 1) {
 111                         pipe_read += 1;
 112
 113                         /* Mark Weaver <mark-clist@npsl.co.uk> pointed out a critical
 114                            fix to ensure we don't lose signals. We must always
 115                            return -1 when the select pipe is set, otherwise if another
 116                            fd is also ready (so ret == 2) then we used to eat the
 117                            byte in the pipe and lose the signal. JRA.
 118                         */
 119                         ret = -1;
 120 #if 0
 121                         /* JRA - we can use this to debug the signal messaging... */
 122                         DEBUG(0,("select got %u signal\n", (unsigned int)c));
 123 #endif
 124                         errno = EINTR;
 125                 } else {
 126                         ret -= 1;
 127                         errno = saved_errno;
 128                 }
 129         }
 130
 131         return ret;
 132 }
 133
 134 int sys_poll_intr(struct pollfd *fds, int num_fds, int timeout)
 135 {
 136         int orig_timeout = timeout;
 137         struct timespec start;
 138         int ret;
 139
 140         clock_gettime_mono(&start);
 141
 142         while (true) {
 143                 struct timespec now;
 144                 int64_t elapsed;
 145
 146                 ret = poll(fds, num_fds, timeout);
 147                 if (ret != -1) {
 148                         break;
 149                 }
 150                 if (errno != EINTR) {
 151                         break;
 152                 }
 153                 clock_gettime_mono(&now);
 154                 elapsed = nsec_time_diff(&now, &start);
 155                 timeout = (orig_timeout - elapsed) / 1000000;
 156         };
 157         return ret;
 158 }