This command
echo $(/bin/echo -n 1; echo -n 2)
sometimes outputs "21" because we implement this as
let bufferfill = IoBufferfill::create_opts(...);
...
let eval_res = parser.eval_with(...);
let buffer = IoBufferfill::finish(bufferfill);
i.e. /bin/echo and builtin echo both output to the same buffer; the
builtin does inside parser.eval_with(), and the external process may
or may not output before that, depending on when the FD monitor thread
gets scheduled (to run item_callback).
(Unrelated to that we make sure to consume all available input in
"IoBufferfill::finish(bufferfill)" but that doesn't help with
ordering.)
Fix this by reading all available data from stdout after the child
process has exited.
This means we need to pass the BufferFill down to
process_mark_finished_children().
We don't need to do this for builtins like "fg" or "wait",
because commands that buffer output do not get job control, see
2ca66cff53 (Disable job control inside command substitutions,
2021-07-26).
We also don't need to do it when reaping from reader because there
should be no buffering(?).
fish still deviates from other shells in that it doesn't wait for
it's child's stdout to be closed, meaning that this will behave
non-deterministically.
fish -c '
echo -n $(
sh -c " ( for i in \$(seq 10000); do printf .; done ) & "
)
' | wc -c
We should fix that later.
Closes#12018
f1c8e6995d