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d7ee70c013
For devices such as PTYs, where not being able to cancel a pending read operation is problematic for many applications. Fixes: #1180
146 lines
4.2 KiB
C
146 lines
4.2 KiB
C
/*
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* Copyright © 2021 Ole André Vadla Ravnås
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*
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* This library is free software; you can redistribute it and/or
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* modify it under the terms of the GNU Lesser General Public
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* License as published by the Free Software Foundation; either
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* version 2.1 of the License, or (at your option) any later version.
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*
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* This library is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* Lesser General Public License for more details.
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*
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* You should have received a copy of the GNU Lesser General Public
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* License along with this library; if not, see <http://www.gnu.org/licenses/>.
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*/
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#include "config.h"
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#include <errno.h>
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#include <unistd.h>
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#include <sys/stat.h>
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#include <sys/types.h>
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#if defined (HAVE_EPOLL_CREATE)
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#include <sys/epoll.h>
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#elif defined (HAVE_KQUEUE)
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#include <sys/event.h>
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#include <sys/time.h>
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#endif
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#include "giounix-private.h"
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#define G_TEMP_FAILURE_RETRY(expression) \
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({ \
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gssize __result; \
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\
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do \
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__result = (gssize) (expression); \
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while (__result == -1 && errno == EINTR); \
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\
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__result; \
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})
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static gboolean g_fd_is_regular_file (int fd) G_GNUC_UNUSED;
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gboolean
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_g_fd_is_pollable (int fd)
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{
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/*
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* Determining whether a file-descriptor (FD) is pollable turns out to be
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* quite hard.
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*
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* We used to detect this by attempting to lseek() and check if it failed with
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* ESPIPE, and if so we'd consider the FD pollable. But this turned out to not
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* work on e.g. PTYs and other devices that are pollable.
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*
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* Another approach that was considered was to call fstat() and if it failed
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* we'd assume that the FD is pollable, and if it succeeded we'd consider it
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* pollable as long as it's not a regular file. This seemed to work alright
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* except for FDs backed by simple devices, such as /dev/null.
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*
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* There are however OS-specific methods that allow us to figure this out with
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* absolute certainty:
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*/
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#if defined (HAVE_EPOLL_CREATE)
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/*
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* Linux
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*
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* The answer we seek is provided by the kernel's file_can_poll():
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* https://github.com/torvalds/linux/blob/2ab38c17aac10bf55ab3efde4c4db3893d8691d2/include/linux/poll.h#L81-L84
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* But we cannot probe that by using poll() as the returned events for
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* non-pollable FDs are always IN | OUT.
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*
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* The best option then seems to be using epoll, as it will refuse to add FDs
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* where file_can_poll() returns FALSE.
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*/
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int efd;
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struct epoll_event ev = { 0, };
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gboolean add_succeeded;
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efd = epoll_create (1);
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if (efd == -1)
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g_error ("epoll_create () failed: %s", g_strerror (errno));
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ev.events = EPOLLIN;
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add_succeeded = epoll_ctl (efd, EPOLL_CTL_ADD, fd, &ev) == 0;
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close (efd);
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return add_succeeded;
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#elif defined (HAVE_KQUEUE)
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/*
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* Apple OSes and BSDs
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*
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* Like on Linux, we cannot use poll() to do the probing, but kqueue does
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* the trick as it will refuse to add non-pollable FDs. (Except for regular
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* files, which we need to special-case. Even though kqueue does support them,
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* poll() does not.)
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*/
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int kfd;
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struct kevent ev;
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gboolean add_succeeded;
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if (g_fd_is_regular_file (fd))
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return FALSE;
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kfd = kqueue ();
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if (kfd == -1)
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g_error ("kqueue () failed: %s", g_strerror (errno));
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EV_SET (&ev, fd, EVFILT_READ, EV_ADD, 0, 0, NULL);
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add_succeeded =
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G_TEMP_FAILURE_RETRY (kevent (kfd, &ev, 1, NULL, 0, NULL)) == 0;
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close (kfd);
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return add_succeeded;
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#else
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/*
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* Other UNIXes (AIX, QNX, Solaris, etc.)
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*
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* We can rule out regular files, but devices such as /dev/null will be
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* reported as pollable even though they're not. This is hopefully good
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* enough for most use-cases, but easy to expand on later if needed.
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*/
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return !g_fd_is_regular_file (fd);
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#endif
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}
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static gboolean
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g_fd_is_regular_file (int fd)
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{
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struct stat st;
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if (G_TEMP_FAILURE_RETRY (fstat (fd, &st)) == -1)
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return FALSE;
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return S_ISREG (st.st_mode);
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}
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