Process packets in batches at the VFS server

Process I/O packets in batches. If a batch is processed and there are
still packets pending, send a signal locally to the packet handler and
return to the entrypoint signal dispatcher. This prevents clients from
starving each other, which happens when a client continuously submits
packets at a faster rate than the server can process.

Fix #2900

repos/os/run/fs_packet.run

@@ -1,3 +1,14 @@
+#
+# \brief  Test of packet handling and signal scheduling
+# \author Emery Hemingway
+# \date   2018-07-03
+#
+
+if {[get_cmd_switch --autopilot] && [have_include "power_on/qemu"]} {
+	puts "Running fs_packet test in autopilot on Qemu is not recommended.\n"
+	exit
+}
+
 #
 # Build
 #
@@ -53,7 +64,7 @@ append config {
 	</start>
 	<start name="short-test">
 		<binary name="sequence"/>
-		<resource name="RAM" quantum="2M"/>
+		<resource name="RAM" quantum="4M"/>
 		<config>
 			<start name="a">
 				<binary name="test-fs_packet"/>

repos/os/src/server/vfs/main.cc

@@ -301,6 +301,8 @@ class Vfs_server::Session_component : public File_system::Session_rpc_object,
 		 */
 		void _process_packets()
 		{
+			using namespace Genode;
+
 			/*
 			 * XXX Process client backlog before looking at new requests. This
 			 *     limits the number of simultaneously addressed handles (which
@@ -311,7 +313,19 @@ class Vfs_server::Session_component : public File_system::Session_rpc_object,
 				/* backlog not cleared - block for next condition change */
 				return;
 
+			/**
+			 * Process packets in batches, otherwise a client that
+			 * submits packets as fast as they are processed will
+			 * starve the signal handler.
+			 */
+			int quantum = TX_QUEUE_SIZE;
+
 			while (tx_sink()->packet_avail()) {
+				if (--quantum == 0) {
+					/* come back to this later */
+					Signal_transmitter(_process_packet_handler).submit();
+					break;
+				}
 
 				/*
 				 * Make sure that the '_process_packet' function does not