Major refactoring of the rpc locking mechanisms.

Add a send_call() method that computes a new msgid and hands the
message off to the smac layer.  Uses __msgid_lock()

call() still uses __call_lock, but callAsync() does not.  callAsync()
does not use any lock beyond what send_call() does.

src/ZEO/zrpc/connection.py

@@ -137,8 +137,12 @@ class Connection(smac.SizedMessageAsyncConnection):
         self.trigger = None
         self._prepare_async()
         self._map = {self._fileno: self}
+        # __msgid_lock guards access to msgid
+        self.__msgid_lock = threading.Lock()
+        # __call_lock prevents more than one synchronous call from
+        # being issued at one time.
         self.__call_lock = threading.Lock()
-        # The reply lock is used to block when a synchronous call is
+        # __reply_lock is used to block when a synchronous call is
         # waiting for a response
         self.__reply_lock = threading.Lock()
         self.__reply_lock.acquire()
@@ -281,6 +285,20 @@ class Connection(smac.SizedMessageAsyncConnection):
     # The next two public methods (call and callAsync) are used by
     # clients to invoke methods on remote objects
 
+    def send_call(self, method, args, flags):
+        # send a message and return its msgid
+        self.__msgid_lock.acquire()
+        try:
+            msgid = self.msgid
+            self.msgid = self.msgid + 1
+        finally:
+            self.__msgid_lock.release()
+        if __debug__:
+            log("send msg: %d, %d, %s, ..." % (msgid, flags, method))
+        buf = self.marshal.encode(msgid, flags, method, args)
+        self.message_output(buf)
+        return msgid
+
     def call(self, method, *args):
         self.__call_lock.acquire()
         try:
@@ -291,14 +309,7 @@ class Connection(smac.SizedMessageAsyncConnection):
     def _call(self, method, args):
         if self.closed:
             raise DisconnectedError()
-        msgid = self.msgid
-        self.msgid = self.msgid + 1
-        if __debug__:
-            log("send msg: %d, 0, %s, ..." % (msgid, method))
-        self.message_output(self.marshal.encode(msgid, 0, method, args))
-
-        # XXX implementation of promises would start here
-
+        msgid = self.send_call(method, args, 0)
         self.__reply = None
         self.wait() # will release reply lock before returning
         r_msgid, r_flags, r_args = self.__reply
@@ -314,20 +325,9 @@ class Connection(smac.SizedMessageAsyncConnection):
             return r_args
 
     def callAsync(self, method, *args):
-        self.__call_lock.acquire()
-        try:
-            self._callAsync(method, args)
-        finally:
-            self.__call_lock.release()
-
-    def _callAsync(self, method, args):
         if self.closed:
             raise DisconnectedError()
-        msgid = self.msgid
-        self.msgid += 1
-        if __debug__:
-            log("send msg: %d, %d, %s, ..." % (msgid, ASYNC, method))
-        self.message_output(self.marshal.encode(msgid, ASYNC, method, args))
+        self.send_call(method, args, ASYNC)
         self.poll()
 
     # handle IO, possibly in async mode