os: avoid deadlock in packet stream (fix #1186)

Respectively resend a packet-stream-not-empty signal when a new signal handler
gets registered.

repos/base/include/base/signal.h

@@ -132,6 +132,13 @@ namespace Genode {
 			 */
 			void context(Signal_context_capability context);
 
+
+			/**
+			 * Return signal context
+			 */
+			Signal_context_capability context();
+
+
 			/**
 			 * Trigger signal submission to context
 			 *

repos/base/src/base/signal/common.cc

@@ -78,6 +78,8 @@ Signal_transmitter::Signal_transmitter(Signal_context_capability context)
 void Signal_transmitter::context(Signal_context_capability context) {
 	_context = context; }
 
+Signal_context_capability Signal_transmitter::context() { return _context; }
+
 
 /*********************
  ** Signal_receiver **

repos/os/include/os/packet_stream.h

@@ -260,6 +260,14 @@ class Packet_descriptor_transmitter
 		void register_rx_ready_cap(Genode::Signal_context_capability cap)
 		{
 			_rx_ready.context(cap);
+
+			/*
+			 * if a packet was already put into the queue
+			 * before a signal handler was registered,
+			 * a signal has to be send again
+			 */
+			if (!_tx_queue->empty())
+				_rx_ready.submit();
 		}
 
 		bool ready_for_tx()
@@ -341,6 +349,14 @@ class Packet_descriptor_receiver
 		void register_tx_ready_cap(Genode::Signal_context_capability cap)
 		{
 			_tx_ready.context(cap);
+
+			/*
+			 * if a packet was already put into the queue
+			 * before a signal handler was registered,
+			 * a signal has to be send again
+			 */
+			if (!_rx_queue->empty())
+				_tx_ready.submit();
 		}
 
 		bool ready_for_rx()