src/iopoll.c

   1 /* iopoll.c -- broken pipe detection / non blocking output handling
   2    Copyright (C) 2022-2024 Free Software Foundation, Inc.
   3
   4    This program is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU General Public License as published by
   6    the Free Software Foundation, either version 3 of the License, or
   7    (at your option) any later version.
   8
   9    This program is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program.  If not, see <https://www.gnu.org/licenses/>.
  16
  17    Written by Carl Edquist in collaboration with Arsen Arsenović.  */
  18
  19 #include <config.h>
  20
  21 /* poll(2) is needed on AIX (where 'select' gives a readable
  22    event immediately) and Solaris (where 'select' never gave
  23    a readable event).  Also use poll(2) on systems we know work
  24    and/or are already using poll (linux).  */
  25
  26 #if defined _AIX || defined __sun || defined __APPLE__ || \
  27     defined __linux__ || defined __ANDROID__
  28 # define IOPOLL_USES_POLL 1
  29   /* Check we've not enabled gnulib's poll module
  30      as that will emulate poll() in a way not
  31      currently compatible with our usage.  */
  32 # if defined HAVE_POLL
  33 #  error "gnulib's poll() replacement is currently incompatible"
  34 # endif
  35 #endif
  36
  37 #if IOPOLL_USES_POLL
  38 # include <poll.h>
  39 #else
  40 # include <sys/select.h>
  41 #endif
  42
  43 #include "system.h"
  44 #include "assure.h"
  45 #include "iopoll.h"
  46 #include "isapipe.h"
  47
  48
  49 /* BROKEN_OUTPUT selects the mode of operation of this function.
  50    If BROKEN_OUTPUT, wait for FDIN to become ready for reading
  51    or FDOUT to become a broken pipe.
  52    If !BROKEN_OUTPUT, wait for FDIN or FDOUT to become ready for writing.
  53    If either of those are -1, then they're not checked.  Set BLOCK to true
  54    to wait for an event, otherwise return the status immediately.
  55    Return 0 if not BLOCKing and there is no event on the requested descriptors.
  56    Return 0 if FDIN can be read() without blocking, or IOPOLL_BROKEN_OUTPUT if
  57    FDOUT becomes a broken pipe. If !BROKEN_OUTPUT return 0 if FDOUT writable.
  58    Otherwise return IOPOLL_ERROR if there is a poll() or select() error.  */
  59
  60 static int
  61 iopoll_internal (int fdin, int fdout, bool block, bool broken_output)
  62 {
  63   affirm (fdin != -1 || fdout != -1);
  64
  65 #if IOPOLL_USES_POLL
  66   struct pollfd pfds[2] = {  /* POLLRDBAND needed for illumos, macOS.  */
  67     { .fd = fdin,  .events = POLLIN | POLLRDBAND, .revents = 0 },
  68     { .fd = fdout, .events = POLLRDBAND, .revents = 0 },
  69   };
  70   int check_out_events = POLLERR | POLLHUP | POLLNVAL;
  71   int ret = 0;
  72
  73   if (! broken_output)
  74     {
  75       pfds[0].events = pfds[1].events = POLLOUT;
  76       check_out_events = POLLOUT;
  77     }
  78
  79   while (0 <= ret || errno == EINTR)
  80     {
  81       ret = poll (pfds, 2, block ? -1 : 0);
  82
  83       if (ret < 0)
  84         continue;
  85       if (ret == 0 && ! block)
  86         return 0;
  87       affirm (0 < ret);
  88       if (pfds[0].revents) /* input available or pipe closed indicating EOF; */
  89         return 0;          /* should now be able to read() without blocking  */
  90       if (pfds[1].revents & check_out_events)
  91         return broken_output ? IOPOLL_BROKEN_OUTPUT : 0;
  92     }
  93
  94 #else  /* fall back to select()-based implementation */
  95
  96   int nfds = (fdin > fdout ? fdin : fdout) + 1;
  97   int ret = 0;
  98
  99   if (FD_SETSIZE < nfds)
 100     {
 101       errno = EINVAL;
 102       ret = -1;
 103     }
 104
 105   /* If fdout has an error condition (like a broken pipe) it will be seen
 106      as ready for reading.  Assumes fdout is not actually readable.  */
 107   while (0 <= ret || errno == EINTR)
 108     {
 109       fd_set fds;
 110       FD_ZERO (&fds);
 111       if (0 <= fdin)
 112         FD_SET (fdin, &fds);
 113       if (0 <= fdout)
 114         FD_SET (fdout, &fds);
 115
 116       struct timeval delay = {0};
 117       ret = select (nfds,
 118                     broken_output ? &fds : nullptr,
 119                     broken_output ? nullptr : &fds,
 120                     nullptr, block ? nullptr : &delay);
 121
 122       if (ret < 0)
 123         continue;
 124       if (ret == 0 && ! block)
 125         return 0;
 126       affirm (0 < ret);
 127       if (0 <= fdin && FD_ISSET (fdin, &fds))    /* input available or EOF; */
 128         return 0;          /* should now be able to read() without blocking */
 129       if (0 <= fdout && FD_ISSET (fdout, &fds))  /* equiv to POLLERR        */
 130         return broken_output ? IOPOLL_BROKEN_OUTPUT : 0;
 131     }
 132
 133 #endif
 134   return IOPOLL_ERROR;
 135 }
 136
 137 extern int
 138 iopoll (int fdin, int fdout, bool block)
 139 {
 140   return iopoll_internal (fdin, fdout, block, true);
 141 }
 142
 143
 144
 145 /* Return true if fdin is relevant for iopoll().
 146    An fd is not relevant for iopoll() if it is always ready for reading,
 147    which is the case for a regular file or block device.  */
 148
 149 extern bool
 150 iopoll_input_ok (int fdin)
 151 {
 152   struct stat st;
 153   bool always_ready = fstat (fdin, &st) == 0
 154                       && (S_ISREG (st.st_mode)
 155                           || S_ISBLK (st.st_mode));
 156   return ! always_ready;
 157 }
 158
 159 /* Return true if fdout is suitable for iopoll().
 160    Namely, fdout refers to a pipe.  */
 161
 162 extern bool
 163 iopoll_output_ok (int fdout)
 164 {
 165   return isapipe (fdout) > 0;
 166 }
 167
 168 #ifdef EWOULDBLOCK
 169 # define IS_EAGAIN(errcode) ((errcode) == EAGAIN || (errcode) == EWOULDBLOCK)
 170 #else
 171 # define IS_EAGAIN(errcode) ((errcode) == EAGAIN)
 172 #endif
 173
 174 /* Inspect the errno of the previous syscall.
 175    On EAGAIN, wait for the underlying file descriptor to become writable.
 176    Return true, if EAGAIN has been successfully handled. */
 177
 178 static bool
 179 fwait_for_nonblocking_write (FILE *f)
 180 {
 181   if (! IS_EAGAIN (errno))
 182     /* non-recoverable write error */
 183     return false;
 184
 185   int fd = fileno (f);
 186   if (fd == -1)
 187     goto fail;
 188
 189   /* wait for the file descriptor to become writable */
 190   if (iopoll_internal (-1, fd, true, false) != 0)
 191     goto fail;
 192
 193   /* successfully waited for the descriptor to become writable */
 194   clearerr (f);
 195   return true;
 196
 197 fail:
 198   errno = EAGAIN;
 199   return false;
 200 }
 201
 202
 203 /* wrapper for fclose() that also waits for F if non blocking.  */
 204
 205 extern bool
 206 fclose_wait (FILE *f)
 207 {
 208   for (;;)
 209     {
 210       if (fflush (f) == 0)
 211         break;
 212
 213       if (! fwait_for_nonblocking_write (f))
 214         break;
 215     }
 216
 217   return fclose (f) == 0;
 218 }
 219
 220
 221 /* wrapper for fwrite() that also waits for F if non blocking.  */
 222
 223 extern bool
 224 fwrite_wait (char const *buf, ssize_t size, FILE *f)
 225 {
 226   for (;;)
 227     {
 228       const size_t written = fwrite (buf, 1, size, f);
 229       size -= written;
 230       affirm (size >= 0);
 231       if (size <= 0)  /* everything written */
 232         return true;
 233
 234       if (! fwait_for_nonblocking_write (f))
 235         return false;
 236
 237       buf += written;
 238     }
 239 }