Repository: zoofIO/flexx (python)

Issue:
Generic way to avoid object deletion for properties that are a list of Models
Properties on `class Both` that are a list of Model instances, need some extra care to avoid deletion of the model objects. The reason is that "jitter" of the property when it is set from both Python and JS (e.g. during initialization) causes the property to fluctuate, making the objects go out of scope, being cleaned up by the GC, and subsequently deleted in JS as well.

One example of such a case is the `children` and `parent` property of `ui.Widget`.

The current solution is to explicitly keep the objects alive for a set amount of time when the property is set, using a simple "liveKeeper" object. However, there are other cases where such an approach would be needed, and having to implement such a thing feels hacky.

Ideally, Flexx would be able to handle this in an automatic way. If that is unfeasible, perhaps there is a more streamlined approach to help circumvent this problem.

Pull request description:
Better deletion prevention of Model objects
Fixes #141 (good enough)

* Adds a `keep_alive()` method to ~~`Model`~~ `Session`, which keeps an object alive for a period of time that is expressed in ping-roundtrips. This means that the object will always be around long enough to survive synchronisation jitter, even if Python or JS are busy for several seconds (or hours, really).
* The `keep_alive()` method is called upon Model creation, which means users won't have to think about it in a great deal of cases.
* This also improves the ping-pong mechanism to determine connection timeouts. You should now not get these "closed due to pong" errors if you do heavy lifting in Python.
* This adds a secondary ping-pong mechanism to make `keep_alive()` work. One that is slightly higher level, so that it stalls when JS is doing work (which is what we want for the keep-alive use-case).



Patch:
diff --git a/flexx/app/clientcore.py b/flexx/app/clientcore.py
index 1d05225..025d835 100644
--- a/flexx/app/clientcore.py
+++ b/flexx/app/clientcore.py
@@ -185,11 +185,13 @@ class FlexxJS:
     def command(self, msg):
         """ Execute a command received from the server.
         """
-        if msg.startswith('PRINT '):
+        if msg.startswith('PING '):
+            self.ws.send('PONG ' + msg[5:])
+        elif msg.startswith('PRINT '):
             window.console.ori_log(msg[6:])
         elif msg.startswith('EVAL '):
             window._ = eval(msg[5:])
-            window.flexx.ws.send('RET ' + window._)  # send back result
+            self.ws.send('RET ' + window._)  # send back result
         elif msg.startswith('EXEC '):
             eval(msg[5:])  # like eval, but do not return result
         elif msg.startswith('DEFINE-JS '):
diff --git a/flexx/app/model.py b/flexx/app/model.py
index fa41f6f..92806c9 100644
--- a/flexx/app/model.py
+++ b/flexx/app/model.py
@@ -401,6 +401,7 @@ class Model(with_metaclass(ModelMeta, event.HasEvents)):
         # so that they can connect to newly created sub Models.
         self._init_handlers()
         self._session._exec('flexx.instances.%s._init_handlers();' % self._id)
+        self._session.keep_alive(self)
     
     def __repr__(self):
         clsname = self.__class__.__name__
@@ -433,7 +434,6 @@ class Model(with_metaclass(ModelMeta, event.HasEvents)):
         """
         pass
     
-    
     def __check_not_active(self):
         active_models = _get_active_models()
         if self in active_models:
diff --git a/flexx/app/session.py b/flexx/app/session.py
index 0b7482f..98651c9 100644
--- a/flexx/app/session.py
+++ b/flexx/app/session.py
@@ -248,6 +248,9 @@ class Session(SessionAssets):
         # commands, which are send to the client as soon as it connects
         self._pending_commands = []
         
+        # Objects that are guarded from deletion: id: (ping_count, instance)
+        self._instances_guarded = {}
+        
         self._creation_time = time.time()
     
     def __repr__(self):
@@ -379,6 +382,28 @@ class Session(SessionAssets):
         else:
             logger.warn('Unknown command received from JS:\n%s' % command)
     
+    def keep_alive(self, ob, iters=4):
+        """ Keep an object alive for a certain amount of time, expressed
+        in Python-JS ping roundtrips. This is intended for making Model
+        objects survive jitter due to synchronisation, though any type
+        of object can be given.
+        """
+        obid = id(ob)
+        counter = 0 if self._ws is None else self._ws.ping_counter
+        lifetime = counter + int(iters)
+        if lifetime > self._instances_guarded.get(obid, (0, ))[0]:
+            self._instances_guarded[obid] = lifetime, ob
+    
+    def _receive_pong(self, count):
+        """ Called by ws when it gets a pong. Thus gets called about
+        every sec. Clear the guarded Model instances for which the
+        "timeout counter" has expired.
+        """
+        objects_to_clear = [ob for c, ob in
+                           self._instances_guarded.values() if c <= count]
+        for ob in objects_to_clear:
+            self._instances_guarded.pop(id(ob))
+    
     def _exec(self, code):
         """ Like eval, but without returning the result value.
         """
diff --git a/flexx/app/tornadoserver.py b/flexx/app/tornadoserver.py
index 82888db..0b54f6d 100644
--- a/flexx/app/tornadoserver.py
+++ b/flexx/app/tornadoserver.py
@@ -446,7 +446,8 @@ class WSHandler(tornado.websocket.WebSocketHandler):
         
         logger.debug('New websocket connection %s' % path)
         if manager.has_app_name(self.app_name):
-            IOLoop.current().spawn_callback(self.pinger)
+            IOLoop.current().spawn_callback(self.pinger1)
+            IOLoop.current().spawn_callback(self.pinger2)
         else:
             self.close(1003, "Could not associate socket with an app.")
     
@@ -472,6 +473,8 @@ class WSHandler(tornado.websocket.WebSocketHandler):
                     self.close(1003, "Could not launch app: %r" % err)
                     raise
                 self.write_message("PRINT Flexx server says hi", binary=BINARY)
+        elif message.startswith('PONG '):
+            self.on_pong2(message[5:])
         else:
             try:
                 self._session._receive_command(message)
@@ -491,24 +494,79 @@ class WSHandler(tornado.websocket.WebSocketHandler):
             self._session = None  # Allow cleaning up
     
     @gen.coroutine
-    def pinger(self):
+    def pinger1(self):
         """ Check for timeouts. This helps remove lingering false connections.
+        
+        This uses the websocket's native ping-ping mechanism. On the
+        browser side, pongs work even if JS is busy. On the Python side
+        we perform a check whether we were really waiting or whether Python 
+        was too busy to detect the pong.
         """
         self._pongtime = time.time()
+        self._pingtime = pingtime = 0
+        
         while self.close_code is None:
-            self.ping(b'x')
-            yield gen.sleep(2)
-            if time.time() - self._pongtime > config.ws_timeout:
+            dt = config.ws_timeout
+            
+            # Ping, but don't spam
+            if pingtime <= self._pongtime:
+                self.ping(b'x')
+                pingtime = self._pingtime = time.time()
+                iters_since_ping = 0
+            
+            yield gen.sleep(dt / 5)
+            
+            # Check pong status
+            iters_since_ping += 1
+            if iters_since_ping < 5:
+                pass  # we might have missed the pong
+            elif time.time() - self._pongtime > dt:
+                # Delay is so big that connection probably dropped.
+                # Note that a browser sends a pong even if JS is busy
                 logger.warn('Closing connection due to lack of pong')
                 self.close(1000, 'Conection timed out (no pong).')
                 return
     
     def on_pong(self, data):
-        """ Called when our ping is returned.
+        """ Implement the ws's on_pong() method. Called when our ping
+        is returned by the browser.
         """
-        #logger.debug('pong')
         self._pongtime = time.time()
     
+    @property
+    def ping_counter(self):
+        """ Counter indicating the number of pings so far. This measure is
+        used by ``Session.keep_alive()``.
+        """
+        return self._ping_counter
+    
+    @gen.coroutine
+    def pinger2(self):
+        """ Ticker so we have a signal of sorts to indicate round-trips.
+        
+        This is used to implement session.call_on_next_pong(), which
+        is sort of like call_later(), but waits for both Py and JS to "flush"
+        their current events.
+        
+        This uses a ping-pong mechanism implemented *atop* the websocket.
+        When JS is working, it is not able to send a pong (which is what we
+        want in this case).
+        """
+        self._ping_counter = 0
+        self._pong_counter = 0
+        while self.close_code is None:
+            if self._pong_counter >= self._ping_counter:
+                self._ping_counter += 1
+                self.command('PING %i' % self._ping_counter)
+            yield gen.sleep(1.0)
+    
+    def on_pong2(self, data):
+        """ Called when our ping is returned by Flexx.
+        """
+        self._pong_counter = int(data)
+        if self._session:
+            self._session._receive_pong(self._pong_counter)
+    
     # --- methods
     
     def command(self, cmd):
diff --git a/flexx/ui/_widget.py b/flexx/ui/_widget.py
index 36754c7..63c1a52 100644
--- a/flexx/ui/_widget.py
+++ b/flexx/ui/_widget.py
@@ -10,29 +10,19 @@
 """
 
 from .. import event
-from ..app import Model, call_later, get_active_model
+from ..app import Model, get_active_model
 from ..pyscript import undefined, window
+from . import logger
 
-
+# todo: remove this (left for now for backward compat)
 class LiveKeeper:
-    """ This little utility keeps objects alive for a set period of
-    time. This is used to prevent Widget objects from being cleaned up
-    by the garbadge collector when they are only referenced by the
-    "children" property of their parent. Due to synchronization, the
-    children property can "jitter", causing references of objects to
-    be lost.
-    """
-
-    def __init__(self):
-        self._objects = {}
-
-    def keep(self, ob, timeout=5.0):
-        i = id(ob)
-        self._objects[i] = ob
-        call_later(timeout, self.clear, i)
-
-    def clear(self, i):
-        self._objects.pop(i, None)
+    _warned = False
+    
+    def keep(self, ob):
+        if not self._warned:
+            self._warned = True
+            logger.warn('liveKeeper is deprecated, use Session.keep_alive() instead.')
+        ob.session.keep_alive(ob)
 
 liveKeeper = LiveKeeper()
 
@@ -169,7 +159,7 @@ class Widget(Model):
     def __keep_alive(self, *events):
         # When the parent changes, we prevent the widget from being deleted
         # for a few seconds, to it will survive parent-children "jitter".
-        liveKeeper.keep(self)
+        self._session.keep_alive(self)
     
     parent = event.prop(parent)
     
diff --git a/flexx/ui/examples/stayin_alive.py b/flexx/ui/examples/stayin_alive.py
new file mode 100644
index 0000000..d3475c2
--- /dev/null
+++ b/flexx/ui/examples/stayin_alive.py
@@ -0,0 +1,75 @@
+"""
+Example that demonstrates/tests how objects survive synchronisation
+jitter even if Python and JS are busy. We tried hard to make this
+as painless as possible in Flexx, which is why this example may look
+a bit dull. But the fact that this works is not trivial :)
+
+What happens is that the ``the_thing`` property is set in the
+``init()``. This will sync to JS, and then back again to Py (syncing
+always ends at the Python side, for eventual synchronicity). However,
+in the mean time, Python has set the property again, so by the time
+that the prop gets synced back to Python, the first Thing is not there
+anymore, and would be deleted (if we had not taken measures to prevent
+that), which would cause problems.
+"""
+
+import time
+from flexx import app, event, ui
+
+
+class Thing(app.Model):
+    
+    @event.prop
+    def value(self, v):
+        return v
+
+
+class Example(ui.Widget):
+    
+    def init(self):
+        self.the_thing = Thing(value=2)
+    
+    @event.prop
+    def foo(self, v=0):
+        print('in foo setter')
+        return v
+
+    @event.connect('the_thing')
+    def on_the_thing(self, *events):
+        for ev in events:
+            print('the thing became %s with value %s' % 
+                  (ev.new_value.id, ev.new_value.value))
+
+    @event.connect('foo')
+    def on_foo(self, *events):
+        print('sleep in Py')
+        time.sleep(10)
+        print('Done sleeping in Py')
+            
+    class Both:
+        
+        @event.prop
+        def the_thing(self, v):
+            assert isinstance(v, Thing)
+            return v
+   
+    class JS:
+        
+        def init(self):
+            print('sleep in JS')
+            self.sleep(10)
+            print('Done sleeping in JS')
+        
+        def sleep(self, t):
+            import time
+            etime = time.time() + t
+            while time.time() < etime:
+                pass
+
+
+m = app.launch(Example)
+with m:
+    m.the_thing = Thing(value=3)
+
+print('starting event loop')
+app.run()


Test patch:
diff --git a/flexx/app/tests/test_model.py b/flexx/app/tests/test_model.py
index 6188e36..3f3d929 100644
--- a/flexx/app/tests/test_model.py
+++ b/flexx/app/tests/test_model.py
@@ -3,6 +3,8 @@
 
 from flexx.util.testing import run_tests_if_main, raises
 
+import weakref
+import gc
 import logging
 import tornado
 
@@ -261,4 +263,68 @@ def test_can_emit_in_init():
     assert m.res1 == [2]
     assert m.res2 == [1]
 
+
+def test_keep_alive():
+    
+    session = app.manager.get_default_session()
+    if session is None:
+        app.manager.create_default_session()
+    
+    class Foo:
+        pass
+    
+    foo1, foo2, foo3 = Foo(), Foo(), Foo()
+    foo1_ref = weakref.ref(foo1)
+    foo2_ref = weakref.ref(foo2)
+    foo3_ref = weakref.ref(foo3)
+    
+    session.keep_alive(foo1, 10)
+    session.keep_alive(foo1, 5)  # should do nothing, longest time counts
+    session.keep_alive(foo2, 5)
+    session.keep_alive(foo2, 11)  # longest timeout counts
+    session.keep_alive(foo3, 15)
+    
+    # Delete objects, session keeps them alive
+    del foo1, foo2, foo3
+    gc.collect()
+    assert foo1_ref() is not None
+    assert foo2_ref() is not None
+    assert foo3_ref() is not None
+    
+    # Pong 4, too soon for the session to release the objects
+    session._receive_pong(4)
+    gc.collect()
+    assert foo1_ref() is not None
+    assert foo2_ref() is not None
+    assert foo3_ref() is not None
+    
+    # Pong 7, still too soon
+    session._receive_pong(7)
+    gc.collect()
+    assert foo1_ref() is not None
+    assert foo2_ref() is not None
+    assert foo3_ref() is not None
+    
+    # Pong 10, should remove foo1
+    session._receive_pong(10)
+    gc.collect()
+    assert foo1_ref() is None
+    assert foo2_ref() is not None
+    assert foo3_ref() is not None
+    
+    # Pong 11, should remove foo2
+    session._receive_pong(11)
+    gc.collect()
+    assert foo1_ref() is None
+    assert foo2_ref() is None
+    assert foo3_ref() is not None
+    
+    # Pong 20, should remove foo3
+    session._receive_pong(20)
+    gc.collect()
+    assert foo1_ref() is None
+    assert foo2_ref() is None
+    assert foo3_ref() is None
+
+
 run_tests_if_main()