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Re: [libvirt] Help needed: simple python api to obtain events



On Fri, Oct 09, 2009 at 09:58:14AM +0100, Daniel P. Berrange wrote:
> On Fri, Oct 09, 2009 at 10:19:08AM +0200, Dan Kenigsberg wrote:
> > On Wed, Oct 07, 2009 at 12:59:39PM +0100, Daniel P. Berrange wrote:
> > > On Tue, Oct 06, 2009 at 07:04:29PM +0200, Dan Kenigsberg wrote:
> > > > Would someone help me have a shrink-wrapped solution for obtaining libvirt
> > > > events in python?
> > > 
> > > I decided to re-write the demo program so that is shows a serious
> > > production kwalitee event loop implementation that can be used in
> > > real world applications. I think you'll find this much nicer :-)
> > 
> > It sure look nicer (though I still don't get the hows and whys).
> > However, it seems that you have an issue with python's time.time()
> > measured in seconds, not milliseconds.
> 
> Here's a version with more docs & the time in milliseconds
> 
> #!/usr/bin/python -u
> #
> #
> #
> #################################################################################
> # Start off by implementing a general purpose event loop for anyones use
> #################################################################################
> 
> import sys
> import getopt
> import os
> import libvirt
> import select
> import errno
> import time
> import threading
> 
> #
> # This general purpose event loop will support waiting for file handle
> # I/O and errors events, as well as scheduling repeatable timers with
> # a fixed interval.
> #
> # It is a pure python implementation based around the poll() API
> #
> class virEventLoopPure:
>     # This class contains the data we need to track for a
>     # single file handle
>     class virEventLoopPureHandle:
>         def __init__(self, handle, fd, events, cb, opaque):
>             self.handle = handle
>             self.fd = fd
>             self.events = events
>             self.cb = cb
>             self.opaque = opaque
> 
>         def get_id(self):
>             return self.handle
> 
>         def get_fd(self):
>             return self.fd
> 
>         def get_events(self):
>             return self.events
> 
>         def set_events(self, events):
>             self.events = events
> 
>         def dispatch(self, events):
>             self.cb(self.handle,
>                     self.fd,
>                     events,
>                     self.opaque[0],
>                     self.opaque[1])
> 
>     # This class contains the data we need to track for a
>     # single periodic timer
>     class virEventLoopPureTimer:
>         def __init__(self, timer, interval, cb, opaque):
>             self.timer = timer
>             self.interval = interval
>             self.cb = cb
>             self.opaque = opaque
>             self.lastfired = 0
> 
>         def get_id(self):
>             return self.timer
> 
>         def get_interval(self):
>             return self.interval
> 
>         def set_interval(self, interval):
>             self.interval = interval
> 
>         def get_last_fired(self):
>             return self.lastfired
> 
>         def set_last_fired(self, now):
>             self.lastfired = now
> 
>         def dispatch(self):
>             self.cb(self.timer,
>                     self.opaque[0],
>                     self.opaque[1])
> 
> 
>     def __init__(self, debug=False):
>         self.debugOn = debug
>         self.poll = select.poll()
>         self.pipetrick = os.pipe()
>         self.nextHandleID = 1
>         self.nextTimerID = 1
>         self.handles = []
>         self.timers = []
>         self.quit = False
> 
>         # The event loop can be used from multiple threads at once.
>         # Specifically while the main thread is sleeping in poll()
>         # waiting for events to occur, another thread may come along
>         # and add/update/remove a file handle, or timer. When this
>         # happens we need to interrupt the poll() sleep in the other
>         # thread, so that it'll see the file handle / timer changes.
>         #
>         # Using OS level signals for this is very unreliable and
>         # hard to implement correctly. Thus we use the real classic
>         # "self pipe" trick. A anonymous pipe, with one end registered
>         # with the event loop for input events. When we need to force
>         # the main thread out of a poll() sleep, we simple write a
>         # single byte of data to the other end of the pipe.
>         self.debug("Self pipe watch %d write %d" %(self.pipetrick[0], self.pipetrick[1]))
>         self.poll.register(self.pipetrick[0], select.POLLIN)
> 
>     def debug(self, msg):
>         if self.debugOn:
>             print msg
> 
> 
>     # Calculate when the next timeout is due to occurr, returning
>     # the absolute timestamp for the next timeout, or 0 if there is
>     # no timeout due
>     def next_timeout(self):
>         next = 0
>         for t in self.timers:
>             last = t.get_last_fired()
>             interval = t.get_interval()
>             if interval < 0:
>                 continue
>             if next == 0 or (last + interval) < next:
>                 next = last + interval
> 
>         return next
> 
>     # Lookup a virEventLoopPureHandle object based on file descriptor
>     def get_handle_by_fd(self, fd):
>         for h in self.handles:
>             if h.get_fd() == fd:
>                 return h
>         return None
> 
>     # Lookup a virEventLoopPureHandle object based on its event loop ID
>     def get_handle_by_id(self, handleID):
>         for h in self.handles:
>             if h.get_id() == handleID:
>                 return h
>         return None
> 
> 
>     # This is the heart of the event loop, performing one single
>     # iteration. It asks when the next timeout is due, and then
>     # calcuates the maximum amount of time it is able to sleep
>     # for in poll() pending file handle events.
>     #
>     # It then goes into the poll() sleep.
>     #
>     # When poll() returns, there will zero or more file handle
>     # events which need to be dispatched to registered callbacks
>     # It may also be time to fire some periodic timers.
>     #
>     # Due to the coarse granularity of schedular timeslices, if
>     # we ask for a sleep of 500ms in order to satisfy a timer, we
>     # may return upto 1 schedular timeslice early. So even though
>     # our sleep timeout was reached, the registered timer may not
>     # technically be at its expiry point. This leads to us going
>     # back around the loop with a crazy 5ms sleep. So when checking
>     # if timeouts are due, we allow a margin of 20ms, to avoid
>     # these pointless repeated tiny sleeps.
>     def run_once(self):
>         sleep = -1
>         next = self.next_timeout()
>         self.debug("Next timeout due at %d" % next)
>         if next > 0:
>             now = int(time.time() * 1000)
>             if now >= next:
>                 sleep = 0
>             else:
>                 sleep = next - now
> 
>         self.debug("Poll with a sleep of %d" % sleep)
>         events = self.poll.poll(sleep)

Thanks! but there's still a units problem here (should be sleep/1000. )

     |  poll(...)
     |      poll([timeout=-1[, maxevents=-1]]) -> [(fd, events), (...)]
     |      
     |      Wait for events on the epoll file descriptor for a maximum time of timeout
     |      in seconds (as float). -1 makes poll wait indefinitely.
     |      Up to maxevents are returned to the caller.

> 
>         # Dispatch any file handle events that occurred
>         for (fd, revents) in events:
>             # See if the events was from the self-pipe
>             # telling us to wakup. if so, then discard
>             # the data just continue
>             if fd == self.pipetrick[0]:
>                 data = os.read(fd, 1)
>                 continue
> 
>             h = self.get_handle_by_fd(fd)
>             if h:
>                 self.debug("Dispatch fd %d handle %d events %d" % (fd, h.get_id(), revents))
>                 h.dispatch(self.events_from_poll(revents))
> 
>         now = int(time.time() * 1000)
>         for t in self.timers:
>             interval = t.get_interval()
>             if interval < 0:
>                 continue
> 
>             want = t.get_last_fired() + interval
>             # Deduct 20ms, since schedular timeslice
>             # means we could be ever so slightly early
>             if now >= (want-20):
>                 self.debug("Dispatch timer %d now %s want %s" % (t.get_id(), str(now), str(want)))
>                 t.set_last_fired(now)
>                 t.dispatch()
> 
> 
>     # Actually the event loop forever
>     def run_loop(self):
>         self.quit = False
>         while not self.quit:
>             self.run_once()
> 
>     def interrupt(self):
>         os.write(self.pipetrick[1], 'c')
> 
> 
>     # Registers a new file handle 'fd', monitoring  for 'events' (libvirt
>     # event constants), firing the callback  cb() when an event occurs.
>     # Returns a unique integer identier for this handle, that should be
>     # used to later update/remove it
>     def add_handle(self, fd, events, cb, opaque):
>         handleID = self.nextHandleID + 1
>         self.nextHandleID = self.nextHandleID + 1
> 
>         h = self.virEventLoopPureHandle(handleID, fd, events, cb, opaque)
>         self.handles.append(h)
> 
>         self.poll.register(fd, self.events_to_poll(events))
>         self.interrupt()
> 
>         self.debug("Add handle %d fd %d events %d" % (handleID, fd, events))
> 
>         return handleID
> 
>     # Registers a new timer with periodic expiry at 'interval' ms,
>     # firing cb() each time the timer expires. If 'interval' is -1,
>     # then the timer is registered, but not enabled
>     # Returns a unique integer identier for this handle, that should be
>     # used to later update/remove it
>     def add_timer(self, interval, cb, opaque):
>         timerID = self.nextTimerID + 1
>         self.nextTimerID = self.nextTimerID + 1
> 
>         h = self.virEventLoopPureTimer(timerID, interval, cb, opaque)
>         self.timers.append(h)
>         self.interrupt()
> 
>         self.debug("Add timer %d interval %d" % (timerID, interval))
> 
>         return timerID
> 
>     # Change the set of events to be monitored on the file handle
>     def update_handle(self, handleID, events):
>         h = self.get_handle_by_id(handleID)
>         if h:
>             h.set_events(events)
>             self.poll.unregister(h.get_fd())
>             self.poll.register(h.get_fd(), self.events_to_poll(events))
>             self.interrupt()
> 
>             self.debug("Update handle %d fd %d events %d" % (handleID, h.get_fd(), events))
> 
>     # Change the periodic frequency of the timer
>     def update_timer(self, timerID, interval):
>         for h in self.timers:
>             if h.get_id() == timerID:
>                 h.set_interval(interval);
>                 self.interrupt()
> 
>                 self.debug("Update timer %d interval %d"  % (timerID, interval))
>                 break
> 
>     # Stop monitoring for events on the file handle
>     def remove_handle(self, handleID):
>         handles = []
>         for h in self.handles:
>             if h.get_id() == handleID:
>                 self.poll.unregister(h.get_fd())
>                 self.debug("Remove handle %d fd %d" % (handleID, h.get_fd()))
>             else:
>                 handles.append(h)
>         self.handles = handles
>         self.interrupt()
> 
>     # Stop firing the periodic timer
>     def remove_timer(self, timerID):
>         timers = []
>         for h in self.timers:
>             if h.get_id() != timerID:
>                 timers.append(h)
>                 self.debug("Remove timer %d" % timerID)
>         self.timers = timers
>         self.interrupt()
> 
>     # Convert from libvirt event constants, to poll() events constants
>     def events_to_poll(self, events):
>         ret = 0
>         if events & libvirt.VIR_EVENT_HANDLE_READABLE:
>             ret |= select.POLLIN
>         if events & libvirt.VIR_EVENT_HANDLE_WRITABLE:
>             ret |= select.POLLOUT
>         if events & libvirt.VIR_EVENT_HANDLE_ERROR:
>             ret |= select.POLLERR;
>         if events & libvirt.VIR_EVENT_HANDLE_HANGUP:
>             ret |= select.POLLHUP;
>         return ret
> 
>     # Convert from poll() event constants, to libvirt events constants
>     def events_from_poll(self, events):
>         ret = 0;
>         if events & select.POLLIN:
>             ret |= libvirt.VIR_EVENT_HANDLE_READABLE;
>         if events & select.POLLOUT:
>             ret |= libvirt.VIR_EVENT_HANDLE_WRITABLE;
>         if events & select.POLLNVAL:
>             ret |= libvirt.VIR_EVENT_HANDLE_ERROR;
>         if events & select.POLLERR:
>             ret |= libvirt.VIR_EVENT_HANDLE_ERROR;
>         if events & select.POLLHUP:
>             ret |= libvirt.VIR_EVENT_HANDLE_HANGUP;
>         return ret;
> 
> 
> ###########################################################################
> # Now glue an instance of the general event loop into libvirt's event loop
> ###########################################################################
> 
> # This single global instance of the event loop wil be used for
> # monitoring libvirt events
> eventLoop = virEventLoopPure(debug=False)
> 
> # This keeps track of what thread is running the event loop,
> # (if it is run in a background thread)
> eventLoopThread = None
> 
> 
> # These next set of 6 methods are the glue between the official
> # libvirt events API, and our particular impl of the event loop
> #
> # There is no reason why the 'virEventLoopPure' has to be used.
> # An application could easily may these 6 glue methods hook into
> # another event loop such as GLib's, or something like the python
> # Twisted event framework.
> 
> def virEventAddHandleImpl(fd, events, cb, opaque):
>     global eventLoop
>     return eventLoop.add_handle(fd, events, cb, opaque)
> 
> def virEventUpdateHandleImpl(handleID, events):
>     global eventLoop
>     return eventLoop.update_handle(handleID, events)
> 
> def virEventRemoveHandleImpl(handleID):
>     global eventLoop
>     return eventLoop.remove_handle(handleID)
> 
> def virEventAddTimerImpl(interval, cb, opaque):
>     global eventLoop
>     return eventLoop.add_timer(interval, cb, opaque)
> 
> def virEventUpdateTimerImpl(timerID, interval):
>     global eventLoop
>     return eventLoop.update_timer(timerID, interval)
> 
> def virEventRemoveTimerImpl(timerID):
>     global eventLoop
>     return eventLoop.remove_timer(timerID)
> 
> # This tells libvirt what event loop implementation it
> # should use
> def virEventLoopPureRegister():
>     libvirt.virEventRegisterImpl(virEventAddHandleImpl,
>                                  virEventUpdateHandleImpl,
>                                  virEventRemoveHandleImpl,
>                                  virEventAddTimerImpl,
>                                  virEventUpdateTimerImpl,
>                                  virEventRemoveTimerImpl)
> 
> # Directly run the event loop in the current thread
> def virEventLoopPureRun():
>     global eventLoop
>     eventLoop.run_loop()
> 
> # Spawn a background thread to run the event loop
> def virEventLoopPureStart():
>     global eventLoopThread
>     virEventLoopPureRegister()
>     eventLoopThread = threading.Thread(target=virEventLoopPureRun, name="libvirtEventLoop")
>     eventLoopThread.setDaemon(True)
>     eventLoopThread.start()
> 
> 
> ##########################################################################
> # Everything that now follows is a simple demo of domain lifecycle events
> ##########################################################################
> def eventToString(event):
>     eventStrings = ( "Added",
>                      "Removed",
>                      "Started",
>                      "Suspended",
>                      "Resumed",
>                      "Stopped",
>                      "Saved",
>                      "Restored" );
>     return eventStrings[event];
> 
> def myDomainEventCallback1 (conn, dom, event, detail, opaque):
>     print "myDomainEventCallback1 EVENT: Domain %s(%s) %s %d" % (dom.name(), dom.ID(), eventToString(event), detail)
> 
> def myDomainEventCallback2 (conn, dom, event, detail, opaque):
>     print "myDomainEventCallback2 EVENT: Domain %s(%s) %s %d" % (dom.name(), dom.ID(), eventToString(event), detail)
> 
> def usage():
>         print "usage: "+os.path.basename(sys.argv[0])+" [uri]"
>         print "   uri will default to qemu:///system"
> 
> def main():
>     try:
>         opts, args = getopt.getopt(sys.argv[1:], "h", ["help"] )
>     except getopt.GetoptError, err:
>         # print help information and exit:
>         print str(err) # will print something like "option -a not recognized"
>         usage()
>         sys.exit(2)
>     for o, a in opts:
>         if o in ("-h", "--help"):
>             usage()
>             sys.exit()
> 
>     if len(sys.argv) > 1:
>         uri = sys.argv[1]
>     else:
>         uri = "qemu:///system"
> 
>     print "Using uri:" + uri
> 
>     # Run a background thread with the event loop
>     virEventLoopPureStart()
> 
>     vc = libvirt.open(uri)
> 
>     # Close connection on exit (to test cleanup paths)
>     old_exitfunc = getattr(sys, 'exitfunc', None)
>     def exit():
>         print "Closing " + str(vc)
>         vc.close()
>         if (old_exitfunc): old_exitfunc()
>     sys.exitfunc = exit
> 
>     #Add 2 callbacks to prove this works with more than just one
>     vc.domainEventRegister(myDomainEventCallback1,None)
>     vc.domainEventRegister(myDomainEventCallback2,None)
> 
>     # The rest of your app would go here normally, but for sake
>     # of demo we'll just go to sleep. The other option is to
>     # run the event loop in your main thread if your app is
>     # totally event based.
>     while 1:
>         time.sleep(1)
> 
> 
> if __name__ == "__main__":
>     main()
> 
> -- 
> |: Red Hat, Engineering, London   -o-   http://people.redhat.com/berrange/ :|
> |: http://libvirt.org  -o-  http://virt-manager.org  -o-  http://ovirt.org :|
> |: http://autobuild.org       -o-         http://search.cpan.org/~danberr/ :|
> |: GnuPG: 7D3B9505  -o-  F3C9 553F A1DA 4AC2 5648 23C1 B3DF F742 7D3B 9505 :|


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