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BUGS.md

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libkqueue

This documents the current behavioural differences or defficiencies of libkqueue vs the native BSD kqueue implementations.

Common

  • Some functions should crash instead of silently printing a debug message.. for example, note_release().

  • There are a number of stub functions that silently fail or succeed. These need to be cleaned up; at a minimum, they should emit very loud debugging output saying "FIXME -- UNIMPLEMENTED".

    $ grep STUB src/*/*.c
src/linux/read.c:    return (-1); /* STUB */
src/linux/timer.c:    return (0); /* STUB */
src/linux/vnode.c:    return (-1); /* FIXME - STUB */
src/linux/write.c:    return (-1); /* STUB */
src/posix/timer.c:    return (-1); /* STUB */
src/solaris/socket.c:    return (-1); /* STUB */
src/solaris/timer.c:    return (-1); /* STUB */
src/windows/read.c:    return (-1); /* STUB */
src/windows/timer.c:    return (0); /* STUB */

POSIX

  • EVFILT_PROC - The POSIX implmentation requires that SIGCHLD be delivered to its global waiter thread so that the waiter can discover a when child process exits. To prevent SIGCHLD being delivered to another thread sigprocmask(2) is used to mask SIGCHLD at a process level.
    If the application unmasks SIGCHLD or installs a handler for it, the POSIX EVFILT_PROC code will not function.

  • EVFILT_PROC - If using the POSIX EVFILT_PROC the number of monitored processes should be kept low (< 100). Because the Linux kernel coalesces SIGCHLD (and other signals), the only way to reliably determine if a monitored process has exited, is to loop through all PIDs registered by any kqueue when we receive a SIGCHLD. This involves many calls to waitid(2) and may have a negative performance impact.

  • EVFILT_PROC - The notification list of the global waiter thread and the ready lists of individual kqueues share the same mutex. This may cause performance issues where large numbers of processes are monitored.

Linux

  • If a file descriptor outside of kqueue is closed, the internal kqueue state is not cleaned up. On Linux this is pretty much impossible to fix as there's no mechanism to retrieve file descriptor reference counts and no way to be notified of when a file descriptor is closed. Applications should ensure that file descriptors are removed from the kqueue before they are closed.

  • EVFILT_PROC - Only NOTE_EXIT is currently supported. Other functionality is possible, but it requires integrating with netlink to receive notifications. If building against Kernels < 5.3 (where pidfd_open() is not available) the POSIX EVFILT_PROC code is used. See the POSIX section above for limitations of the POSIX EVFILT_PROC code.

  • EVFILT_PROC - Native kqueue provides notifications for any process that is visible to the application process. On Linux/POSIX platforms only direct children of the application process can be monitored for exit.

Solaris

  • Solaris unit test failure.

    LD_LIBRARY_PATH="..:/usr/sfw/lib/64" ./kqtest
1: test_peer_close_detection()
2: test_kqueue()
3: test_kevent_socket_add()
4: test_kevent_socket_del()
5: test_kevent_socket_add_without_ev_add()
6: test_kevent_socket_get()

[read.c:84]: Unexpected event:_test_no_kevents(): [ident=7, filter=-1, flags = 1 (EV_ADD), fflags = 0, data=0, udata=fffffd7fff08c6b4]: Error 0

  • If a file descriptor outside of kqueue is closed, the internal kqueue state is not cleaned up. Applications should ensure that file descriptors are removed from the kqueue before they are closed.

  • We need to uninitialize library after fork() using pthread_atfork(). BSD kqueue file descriptors are not inherited by the fork copy and will be closed automatically in the fork. With libkqueue, because we don't unitialize the library, kqueue file descriptors will persist in the fork.

Windows

  • If a file descriptor outside of kqueue is closed, the internal kqueue state is not cleaned up. Applications should ensure that file descriptors are removed from the kqueue before they are closed.