LCOV - code coverage report
Current view: top level - glib/gio - giounix-private.c (source / functions) Hit Total Coverage
Test: unnamed Lines: 8 13 61.5 %
Date: 2024-03-26 05:16:46 Functions: 1 2 50.0 %
Branches: 1 8 12.5 %

           Branch data     Line data    Source code
       1                 :            : /*
       2                 :            :  * Copyright © 2021 Ole André Vadla Ravnås
       3                 :            :  *
       4                 :            :  * SPDX-License-Identifier: LGPL-2.1-or-later
       5                 :            :  *
       6                 :            :  * This library is free software; you can redistribute it and/or
       7                 :            :  * modify it under the terms of the GNU Lesser General Public
       8                 :            :  * License as published by the Free Software Foundation; either
       9                 :            :  * version 2.1 of the License, or (at your option) any later version.
      10                 :            :  *
      11                 :            :  * This library is distributed in the hope that it will be useful,
      12                 :            :  * but WITHOUT ANY WARRANTY; without even the implied warranty of
      13                 :            :  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
      14                 :            :  * Lesser General Public License for more details.
      15                 :            :  *
      16                 :            :  * You should have received a copy of the GNU Lesser General Public
      17                 :            :  * License along with this library; if not, see <http://www.gnu.org/licenses/>.
      18                 :            :  */
      19                 :            : 
      20                 :            : #include "config.h"
      21                 :            : 
      22                 :            : #include <errno.h>
      23                 :            : #include <unistd.h>
      24                 :            : #include <sys/stat.h>
      25                 :            : #include <sys/types.h>
      26                 :            : #if defined (HAVE_EPOLL_CREATE)
      27                 :            : #include <sys/epoll.h>
      28                 :            : #elif defined (HAVE_KQUEUE)
      29                 :            : #include <sys/event.h>
      30                 :            : #include <sys/time.h>
      31                 :            : #endif
      32                 :            : 
      33                 :            : #include "giounix-private.h"
      34                 :            : 
      35                 :            : #define G_TEMP_FAILURE_RETRY(expression)      \
      36                 :            :   ({                                          \
      37                 :            :     gssize __result;                          \
      38                 :            :                                               \
      39                 :            :     do                                        \
      40                 :            :       __result = (gssize) (expression);       \
      41                 :            :     while (__result == -1 && errno == EINTR); \
      42                 :            :                                               \
      43                 :            :     __result;                                 \
      44                 :            :   })
      45                 :            : 
      46                 :            : static gboolean g_fd_is_regular_file (int fd) G_GNUC_UNUSED;
      47                 :            : 
      48                 :            : gboolean
      49                 :       1005 : _g_fd_is_pollable (int fd)
      50                 :            : {
      51                 :            :   /*
      52                 :            :    * Determining whether a file-descriptor (FD) is pollable turns out to be
      53                 :            :    * quite hard.
      54                 :            :    *
      55                 :            :    * We used to detect this by attempting to lseek() and check if it failed with
      56                 :            :    * ESPIPE, and if so we'd consider the FD pollable. But this turned out to not
      57                 :            :    * work on e.g. PTYs and other devices that are pollable.
      58                 :            :    *
      59                 :            :    * Another approach that was considered was to call fstat() and if it failed
      60                 :            :    * we'd assume that the FD is pollable, and if it succeeded we'd consider it
      61                 :            :    * pollable as long as it's not a regular file. This seemed to work alright
      62                 :            :    * except for FDs backed by simple devices, such as /dev/null.
      63                 :            :    *
      64                 :            :    * There are however OS-specific methods that allow us to figure this out with
      65                 :            :    * absolute certainty:
      66                 :            :    */
      67                 :            : 
      68                 :            : #if defined (HAVE_EPOLL_CREATE)
      69                 :            :   /*
      70                 :            :    * Linux
      71                 :            :    *
      72                 :            :    * The answer we seek is provided by the kernel's file_can_poll():
      73                 :            :    * https://github.com/torvalds/linux/blob/2ab38c17aac10bf55ab3efde4c4db3893d8691d2/include/linux/poll.h#L81-L84
      74                 :            :    * But we cannot probe that by using poll() as the returned events for
      75                 :            :    * non-pollable FDs are always IN | OUT.
      76                 :            :    *
      77                 :            :    * The best option then seems to be using epoll, as it will refuse to add FDs
      78                 :            :    * where file_can_poll() returns FALSE.
      79                 :            :    */
      80                 :            : 
      81                 :            :   int efd;
      82                 :       1005 :   struct epoll_event ev = { 0, };
      83                 :            :   gboolean add_succeeded;
      84                 :            : 
      85                 :       1005 :   efd = epoll_create1 (EPOLL_CLOEXEC);
      86         [ -  + ]:       1005 :   if (efd == -1)
      87                 :          0 :     g_error ("epoll_create1 () failed: %s", g_strerror (errno));
      88                 :            : 
      89                 :       1005 :   ev.events = EPOLLIN;
      90                 :            : 
      91                 :       1005 :   add_succeeded = epoll_ctl (efd, EPOLL_CTL_ADD, fd, &ev) == 0;
      92                 :            : 
      93                 :       1005 :   close (efd);
      94                 :            : 
      95                 :       1005 :   return add_succeeded;
      96                 :            : #elif defined (HAVE_KQUEUE)
      97                 :            :   /*
      98                 :            :    * Apple OSes and BSDs
      99                 :            :    *
     100                 :            :    * Like on Linux, we cannot use poll() to do the probing, but kqueue does
     101                 :            :    * the trick as it will refuse to add non-pollable FDs. (Except for regular
     102                 :            :    * files, which we need to special-case. Even though kqueue does support them,
     103                 :            :    * poll() does not.)
     104                 :            :    */
     105                 :            : 
     106                 :            :   int kfd;
     107                 :            :   struct kevent ev;
     108                 :            :   gboolean add_succeeded;
     109                 :            : 
     110                 :            :   if (g_fd_is_regular_file (fd))
     111                 :            :     return FALSE;
     112                 :            : 
     113                 :            :   kfd = kqueue ();
     114                 :            :   if (kfd == -1)
     115                 :            :     g_error ("kqueue () failed: %s", g_strerror (errno));
     116                 :            : 
     117                 :            :   EV_SET (&ev, fd, EVFILT_READ, EV_ADD, 0, 0, NULL);
     118                 :            : 
     119                 :            :   add_succeeded =
     120                 :            :       G_TEMP_FAILURE_RETRY (kevent (kfd, &ev, 1, NULL, 0, NULL)) == 0;
     121                 :            : 
     122                 :            :   close (kfd);
     123                 :            : 
     124                 :            :   return add_succeeded;
     125                 :            : #else
     126                 :            :   /*
     127                 :            :    * Other UNIXes (AIX, QNX, Solaris, etc.)
     128                 :            :    *
     129                 :            :    * We can rule out regular files, but devices such as /dev/null will be
     130                 :            :    * reported as pollable even though they're not. This is hopefully good
     131                 :            :    * enough for most use-cases, but easy to expand on later if needed.
     132                 :            :    */
     133                 :            : 
     134                 :            :   return !g_fd_is_regular_file (fd);
     135                 :            : #endif
     136                 :            : }
     137                 :            : 
     138                 :            : static gboolean
     139                 :          0 : g_fd_is_regular_file (int fd)
     140                 :            : {
     141                 :            :   struct stat st;
     142                 :            : 
     143   [ #  #  #  #  :          0 :   if (G_TEMP_FAILURE_RETRY (fstat (fd, &st)) == -1)
                   #  # ]
     144                 :          0 :     return FALSE;
     145                 :            : 
     146                 :          0 :   return S_ISREG (st.st_mode);
     147                 :            : }

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