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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 : : }