From b56b23007664525e13f7f7ec50a9a6adff0bdfd9 Mon Sep 17 00:00:00 2001
From: Johannes Altmanninger <aclopte@gmail.com>
Date: Sun, 25 Apr 2021 12:56:34 +0200
Subject: [PATCH] Avoid excessive polling of universal variable file

Reproducible at least on Linux, where the "named pipe" universal
variable notifier is used:

	rm -rf build/test/xdg_config
	XDG_CONFIG_HOME=build/test/xdg_config ./build/fish -c "xterm -e ./build/fish"

The child fish reacts to keyboard input with a noticeable initial
delay.  This is because the universal variable file is polled over
a million times, even when I immediately press Control-D. This polling
prevents readb() from handling keyboard input.
Before commit 939aba02d ("Refactor input_common.cpp:readb"), readb()
reacted to keyboard input even when there were universal variable
notifications.  Restore this behavior, but make sure to call the
universal variable notifier after the new "prepare_to_select" logic.
Maybe the problem is in the notifier but the old behavior was sane.

Fixes the problems described in
https://github.com/fish-shell/fish-shell/commit/7a556ec6f21f16b528be3fb94f5160e2209c3cbf#commitcomment-49773677

Adding "-d uvars-file" to the reproducesr shows that we are checking
the uvar file repeatedly:

	uvar-file: universal log sync
	uvar-file: universal log sync elided based on fast stat()
	uvar-file: universal log no modifications
---
 src/input_common.cpp            | 9 +++++----
 tests/checks/tmux-complete.fish | 3 ---
 2 files changed, 5 insertions(+), 7 deletions(-)

diff --git a/src/input_common.cpp b/src/input_common.cpp
index f473720d2..6a025d7a9 100644
--- a/src/input_common.cpp
+++ b/src/input_common.cpp
@@ -57,7 +57,7 @@ enum {
 };
 using readb_result_t = int;
 
-static readb_result_t readb(int in_fd) {
+static readb_result_t readb(int in_fd, bool queue_is_empty) {
     assert(in_fd >= 0 && "Invalid in fd");
     universal_notifier_t& notifier = universal_notifier_t::default_notifier();
     select_wrapper_t fdset;
@@ -98,7 +98,9 @@ static readb_result_t readb(int in_fd) {
         // This may come about through readability, or through a call to poll().
         if (notifier.poll() ||
             (fdset.test(notifier_fd) && notifier.notification_fd_became_readable(notifier_fd))) {
-            return readb_uvar_notified;
+            if (env_universal_barrier() && !queue_is_empty) {
+                return readb_uvar_notified;
+            }
         }
 
         // Check stdin.
@@ -167,7 +169,7 @@ char_event_t input_event_queue_t::readch() {
             return mevt.acquire();
         }
 
-        readb_result_t rr = readb(in_);
+        readb_result_t rr = readb(in_, queue_.empty());
         switch (rr) {
             case readb_eof:
                 return char_event_type_t::eof;
@@ -178,7 +180,6 @@ char_event_t input_event_queue_t::readch() {
                 break;
 
             case readb_uvar_notified:
-                env_universal_barrier();
                 break;
 
             case readb_ioport_notified:
diff --git a/tests/checks/tmux-complete.fish b/tests/checks/tmux-complete.fish
index 4dd86aed2..28cd8154b 100644
--- a/tests/checks/tmux-complete.fish
+++ b/tests/checks/tmux-complete.fish
@@ -3,9 +3,6 @@
 # Don't run this on GitHub Actions since it's flaky.
 #REQUIRES: test "$CI" != true
 
-# HACK: THIS IS DISABLED for now, since it doesn't work at the moment.
-#REQUIRES: false
-
 # Isolated tmux.
 set -g tmpdir (mktemp -d)
 set -g tmux tmux -S $tmpdir/.tmux-socket -f /dev/null