From 17a6318ad68092f604223c4077564352c0721ff3 Mon Sep 17 00:00:00 2001
From: Norman Feske <norman.feske@genode-labs.com>
Date: Tue, 1 Sep 2020 18:19:58 +0200
Subject: [PATCH] vfs: process all acks in each iteration

This patch solves a corner case where one long-active job (e.g.,
read-ready request) stays at the beginning of the '_active_jobs' queue
without an ack. In this case, the '_try_acknowledge_jobs' method would
wrongly stop processing the subsequent acknowledgements. In practice,
this can lead to a delayed sending of acknowledgements until new I/O or
client requests occur. In particular, Vim in Sculpt's inspect window
sometimes did not immediately respond to key presses during tab
completion. Here, the read-ready request of the terminal prevented the
acknowledgement for read of directory entry from being delivered until
the next key was pressed.

Fixes #3873
---
 repos/os/src/server/vfs/main.cc | 56 ++++++++++++++-------------------
 1 file changed, 23 insertions(+), 33 deletions(-)

diff --git a/repos/os/src/server/vfs/main.cc b/repos/os/src/server/vfs/main.cc
index c8a527d58a..723ddbc287 100644
--- a/repos/os/src/server/vfs/main.cc
+++ b/repos/os/src/server/vfs/main.cc
@@ -261,46 +261,36 @@ class Vfs_server::Session_component : private Session_resources,
 
 		bool _try_acknowledge_jobs()
 		{
-			bool overall_progress = false;
+			bool progress = false;
 
-			for (;;) {
-				if (!_stream.ready_to_ack())
-					break;
+			Node_queue requeued_nodes { };
 
-				if (_active_nodes.empty())
-					break;
+			_active_nodes.dequeue_all([&] (Node &node) {
 
-				bool progress_in_iteration = false;
+				if (!_stream.ready_to_ack()) {
+					requeued_nodes.enqueue(node);
+					return;
+				}
 
-				_active_nodes.dequeue([&] (Node &node) {
+				if (node.acknowledgement_pending()) {
+					_stream.acknowledge_packet(node.dequeue_acknowledgement());
+					progress = true;
+				}
 
-					/*
-					 * Deliver only one acknowledgement per iteration to
-					 * re-check the 'ready_to_ack' condition for each
-					 * acknowledgement.
-					 */
-					if (node.acknowledgement_pending()) {
-						_stream.acknowledge_packet(node.dequeue_acknowledgement());
-						progress_in_iteration = true;
-					}
+				/*
+				 * If there is still another acknowledgement pending,
+				 * keep the node enqueud to process it in the next call of
+				 * '_try_acknowledge_jobs'. This can happen if there is a
+				 * READ_READY acknowledgement in addition to the
+				 * acknowledgement of an operation.
+				 */
+				if (node.active())
+					requeued_nodes.enqueue(node);
+			});
 
-					/*
-					 * If there is still another acknowledgement pending, keep
-					 * the node enqueud to process it in the next iteration.
-					 * This can happen if there is a READ_READY acknowledgement
-					 * in addition to the acknowledgement of an operation.
-					 */
-					if (node.active())
-						_active_nodes.enqueue(node);
-				});
+			_active_nodes = requeued_nodes;
 
-				overall_progress |= progress_in_iteration;
-
-				if (!progress_in_iteration)
-					break;
-			}
-
-			return overall_progress;
+			return progress;
 		}
 
 	public: