// DeviceChan binds the udev_monitor socket to the event source and spawns a
// goroutine. The goroutine efficiently waits on the monitor socket using epoll.
// Data is received from the udev monitor socket and a new Device is created
// with the data received. Pointers to the device are sent on the returned channel.
// The function takes a done signalling channel as a parameter, which when
// triggered will stop the goroutine and close the device channel.
// Only socket connections with uid=0 are accepted.
func (m *Monitor) DeviceChan(done <-chan struct{}) (<-chan *Device, error) {
var event unix.EpollEvent
var events [maxEpollEvents]unix.EpollEvent
// Lock the context
m.lock()
defer m.unlock()
// Enable receiving
if C.udev_monitor_enable_receiving(m.ptr) != 0 {
return nil, errors.New("udev: udev_monitor_enable_receiving failed")
}
// Set the fd to non-blocking
fd := C.udev_monitor_get_fd(m.ptr)
if e := unix.SetNonblock(int(fd), true); e != nil {
return nil, errors.New("udev: unix.SetNonblock failed")
}
// Create an epoll fd
epfd, e := unix.EpollCreate1(0)
if e != nil {
return nil, errors.New("udev: unix.EpollCreate1 failed")
}
// Add the fd to the epoll fd
event.Events = unix.EPOLLIN | unix.EPOLLET
event.Fd = int32(fd)
if e = unix.EpollCtl(epfd, unix.EPOLL_CTL_ADD, int(fd), &event); e != nil {
return nil, errors.New("udev: unix.EpollCtl failed")
}
// Create the channel
ch := make(chan *Device)
// Create goroutine to epoll the fd
go func(done <-chan struct{}, fd int32) {
// Close the epoll fd when goroutine exits
defer unix.Close(epfd)
// Close the channel when goroutine exits
defer close(ch)
// Loop forever
for {
// Poll the file descriptor
nevents, e := unix.EpollWait(epfd, events[:], epollTimeout)
if e != nil {
return
}
// Process events
for ev := 0; ev < nevents; ev++ {
if events[ev].Fd == fd {
if (events[ev].Events & unix.EPOLLIN) != 0 {
for d := m.receiveDevice(); d != nil; d = m.receiveDevice() {
ch <- d
}
}
}
}
// Check for done signal
select {
case <-done:
return
default:
}
}
}(done, int32(fd))
return ch, nil
}
// Wait using epoll.
// Returns true if something is ready to be read,
// false if there is not.
func (poller *fdPoller) wait() (bool, error) {
// 3 possible events per fd, and 2 fds, makes a maximum of 6 events.
// I don't know whether epoll_wait returns the number of events returned,
// or the total number of events ready.
// I decided to catch both by making the buffer one larger than the maximum.
events := make([]unix.EpollEvent, 7)
for {
n, errno := unix.EpollWait(poller.epfd, events, -1)
if n == -1 {
if errno == unix.EINTR {
continue
}
return false, errno
}
if n == 0 {
// If there are no events, try again.
continue
}
if n > 6 {
// This should never happen. More events were returned than should be possible.
return false, errors.New("epoll_wait returned more events than I know what to do with")
}
ready := events[:n]
epollhup := false
epollerr := false
epollin := false
for _, event := range ready {
if event.Fd == int32(poller.fd) {
if event.Events&unix.EPOLLHUP != 0 {
// This should not happen, but if it does, treat it as a wakeup.
epollhup = true
}
if event.Events&unix.EPOLLERR != 0 {
// If an error is waiting on the file descriptor, we should pretend
// something is ready to read, and let unix.Read pick up the error.
epollerr = true
}
if event.Events&unix.EPOLLIN != 0 {
// There is data to read.
epollin = true
}
}
if event.Fd == int32(poller.pipe[0]) {
if event.Events&unix.EPOLLHUP != 0 {
// Write pipe descriptor was closed, by us. This means we're closing down the
// watcher, and we should wake up.
}
if event.Events&unix.EPOLLERR != 0 {
// If an error is waiting on the pipe file descriptor.
// This is an absolute mystery, and should never ever happen.
return false, errors.New("Error on the pipe descriptor.")
}
if event.Events&unix.EPOLLIN != 0 {
// This is a regular wakeup, so we have to clear the buffer.
err := poller.clearWake()
if err != nil {
return false, err
}
}
}
}
if epollhup || epollerr || epollin {
return true, nil
}
return false, nil
}
}