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
7a556ec6f2 (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
b56b230076