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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 : 1003 : _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 : : struct epoll_event ev = { 0, };
83 : : gboolean add_succeeded;
84 : :
85 : : efd = epoll_create1 (EPOLL_CLOEXEC);
86 : : if (efd == -1)
87 : : g_error ("epoll_create1 () failed: %s", g_strerror (errno));
88 : :
89 : : ev.events = EPOLLIN;
90 : :
91 : : add_succeeded = epoll_ctl (efd, EPOLL_CTL_ADD, fd, &ev) == 0;
92 : :
93 : : close (efd);
94 : :
95 : : 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 : 1003 : return !g_fd_is_regular_file (fd);
135 : : #endif
136 : : }
137 : :
138 : : static gboolean
139 : 1003 : g_fd_is_regular_file (int fd)
140 : : {
141 : : struct stat st;
142 : :
143 : 1003 : if (G_TEMP_FAILURE_RETRY (fstat (fd, &st)) == -1)
144 : 0 : return FALSE;
145 : :
146 : 1003 : return S_ISREG (st.st_mode);
147 : : }
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