Fix Jython problem for dead-locking issue

Resolves #171

rpyc/core/protocol.py

@@ -153,6 +153,7 @@ def __init__(self, service, channel, config = {}, _lazy = False):
         self._proxy_cache = WeakValueDict()
         self._netref_classes_cache = {}
         self._remote_root = None
+        self._send_queue = []
         self._local_root = service(weakref.proxy(self))
         if not _lazy:
             self._init_service()
@@ -241,15 +242,30 @@ def _send(self, msg, seq, args):
         # if so, a BaseNetref.__del__ might be called
         # BaseNetref.__del__ must call asyncreq,
         # which will cause a deadlock
-        is_gc_enabled = gc.isenabled()
-        gc.disable()
-        self._sendlock.acquire()
-        try:
-            self._channel.send(data)
-        finally:
-            self._sendlock.release()
-            if is_gc_enabled:
-                gc.enable()
+        # Solution:
+        # Add the current request to a queue and let the thread that currently
+        # holds the sendlock send it when it's done with its current job.
+        # NOTE: Atomic list operations should be thread safe,
+        # please call me out if they are not on all implementations!
+        self._send_queue.append(data)
+        # It is crucial to check the queue each time AFTER releasing the lock:
+        while self._send_queue:
+            if not self._sendlock.acquire(False):
+                # Another thread holds the lock. It will send the data after
+                # it's done with its current job. We can safely return.
+                return
+            try:
+                # Can happen if another consumer was scheduled in between
+                # `while` and `acquire`:
+                if not self._send_queue:
+                    # Must `continue` to ensure that `send_queue` is checked
+                    # after releasing the lock! (in case another producer is
+                    # scheduled before `release`)
+                    continue
+                data = self._send_queue.pop(0)
+                self._channel.send(data)
+            finally:
+                self._sendlock.release()
 
     def _send_request(self, seq, handler, args):
         self._send(consts.MSG_REQUEST, seq, (handler, self._box(args)))