DbgMuxConnHandler: proper handling of the FlowControl message

A FlowControl message basically indicates how many data blocks the
remote side is able to receive from us.  Whenever the block limit is
reached, we should queue outgoing data blocks until the remote side
has given us a new quote by sending another FlowControl message.

sedbgmux/ch/base.py

@@ -39,7 +39,9 @@ class DbgMuxConnHandler(abc.ABC):
     def __init__(self):
         self.conn_state: DbgMuxConnState = DbgMuxConnState.NotEstablished
         self._rx_data_queue: queue.Queue = queue.Queue()
+        self._tx_data_queue: queue.Queue = queue.Queue()
         self._tx_queue: Optional[queue.Queue] = None
+        self.DataBlockLimit: int = 0
         self.ConnRef: int = 0xffff
         self.DPRef: int = 0xffff
 
@@ -59,8 +61,13 @@ class DbgMuxConnHandler(abc.ABC):
     def send_data(self, data: bytes) -> None:
         ''' Send connection data to the target '''
         assert self.conn_state == DbgMuxConnState.Established
+        msg_type = DbgMuxFrame.MsgType.ConnData
         msg = dict(ConnRef=self.ConnRef, Data=data)
-        self.send_msg(DbgMuxFrame.MsgType.ConnData, msg)
+        if self.DataBlockLimit > 0:  # Can we send immediately?
+            self.send_msg(msg_type, msg)
+            self.DataBlockLimit -= 1
+        else:  # Postpone transmission until a FlowControl is received
+            self._tx_data_queue.put((msg_type, msg))
 
     def establish(self, DPRef: int, txq: queue.Queue) -> None:
         ''' Establish connection with a DataProvider '''
@@ -81,9 +88,10 @@ class DbgMuxConnHandler(abc.ABC):
     def _handle_established(self, ConnRef: int, DataBlockLimit: int) -> None:
         ''' Called on connection establishment '''
         assert self.conn_state == DbgMuxConnState.Establishing
-        log.info('Connection established: DPRef=0x%04x, ConnRef=0x%04x',
-                 self.DPRef, ConnRef)
+        log.info('Connection established: DPRef=0x%04x, ConnRef=0x%04x, DataBlockLimit=%u',
+                 self.DPRef, ConnRef, DataBlockLimit)
         self.conn_state = DbgMuxConnState.Established
+        self.DataBlockLimit = DataBlockLimit
         self.ConnRef = ConnRef
         self._conn_established()
 
@@ -102,4 +110,13 @@ class DbgMuxConnHandler(abc.ABC):
 
     def _handle_flow_control(self, DataBlockLimit: int):
         ''' Called on reciept of FlowControl message '''
-        # TODO: purpose is not yet understood
+        assert self.conn_state == DbgMuxConnState.Established
+        self.DataBlockLimit = DataBlockLimit
+        while self.DataBlockLimit > 0:
+            try:
+                (msg_type, msg) = self._tx_data_queue.get(block=False)
+                self.send_msg(msg_type, msg)
+                self.DataBlockLimit -= 1
+                self._tx_data_queue.task_done()
+            except queue.Empty:
+                break