reconnect: Add Python implementation of received_attempt(), and test.

This follows up on commit 4241d652e4 ("jsonrpc: Avoid disconnecting
prematurely due to long poll intervals."), which implemented the same
thing in C.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Requested-by: Ilya Maximets <i.maximets@ovn.org>
Acked-by: Ilya Maximets <i.maximets@ovn.org>

python/ovs/jsonrpc.py

@@ -570,13 +570,16 @@ class Session(object):
         if self.rpc is not None:
             received_bytes = self.rpc.get_received_bytes()
             error, msg = self.rpc.recv()
+
+            now = ovs.timeval.msec()
+            self.reconnect.receive_attempted(now)
             if received_bytes != self.rpc.get_received_bytes():
                 # Data was successfully received.
                 #
                 # Previously we only counted receiving a full message as
                 # activity, but with large messages or a slow connection that
                 # policy could time out the session mid-message.
-                self.reconnect.activity(ovs.timeval.msec())
+                self.reconnect.activity(now)
 
             if not error:
                 if msg.type == Message.T_REQUEST and msg.method == "echo":

python/ovs/reconnect.py

@@ -95,7 +95,10 @@ class Reconnect(object):
         def deadline(fsm):
             if fsm.probe_interval:
                 base = max(fsm.last_activity, fsm.state_entered)
-                return base + fsm.probe_interval
+                expiration = base + fsm.probe_interval
+                if (fsm.last_receive_attempt is None or
+                    fsm.last_receive_attempt >= expiration):
+                    return expiration
             return None
 
         @staticmethod
@@ -113,7 +116,10 @@ class Reconnect(object):
         @staticmethod
         def deadline(fsm):
             if fsm.probe_interval:
-                return fsm.state_entered + fsm.probe_interval
+                expiration = fsm.state_entered + fsm.probe_interval
+                if (fsm.last_receive_attempt is None or
+                    fsm.last_receive_attempt >= expiration):
+                    return expiration
             return None
 
         @staticmethod
@@ -153,6 +159,7 @@ class Reconnect(object):
         self.last_activity = now
         self.last_connected = None
         self.last_disconnected = None
+        self.last_receive_attempt = now
         self.max_tries = None
         self.backoff_free_tries = 0
 
@@ -472,6 +479,16 @@ class Reconnect(object):
             self._transition(now, Reconnect.Active)
         self.last_activity = now
 
+    def receive_attempted(self, now):
+        """Tell 'fsm' that some attempt to receive data on the connection was
+        made at 'now'.  The FSM only allows probe interval timer to expire when
+        some attempt to receive data on the connection was received after the
+        time when it should have expired.  This helps in the case where there's
+        a long delay in the poll loop and then reconnect_run() executes before
+        the code to try to receive anything from the remote runs.  (To disable
+        this feature, pass None for 'now'.)"""
+        self.last_receive_attempt = now
+
     def _transition(self, now, state):
         if self.state == Reconnect.ConnectInProgress:
             self.n_attempted_connections += 1