`qemu -serial stdio` leaves stdout in non-blocking mode

Host environment

• Operating system: Arch Linux
• OS/kernel version: Linux xxx 6.9.5-arch1-1 #1 SMP PREEMPT_DYNAMIC Sun, 16 Jun 2024 19:06:37 +0000 x86_64 GNU/Linux
• Architecture: x86_64
• QEMU flavor: qemu-system-x86_64
• QEMU version: 9.0.1
• QEMU command line:

  qemu-system-x86_64 -nodefaults -display none -no-reboot -serial stdio

Emulated/Virtualized environment

• None / doesn't matter

Description of problem

When -serial stdio is used, qemu exits leaving stdout in non-blocking mode. Although it attempts to restore stdin to blocking mode, it misses that stdout also gets O_NONBLOCK by qemu_chr_open_fd (here). It causes the next applications in the script misbehave because they get unexpected EAGAIN on write to stdout.

Steps to reproduce

Run the following script:

#!/usr/bin/env bash

qemu-system-x86_64 -nodefaults -display none -no-reboot -serial stdio &
PID="$!"
sleep 5
kill "$PID"
wait "$PID"
echo "EXITING $?"

sleep 5
seq 1 400000

The seq command will be interrupted prematurely:

...
5143
5144
5145⏎                                                                                                                                                                                                                wResource temporarily unavailable
write: Resource temporarily unavailable
write: Resource temporarily unavailable

When run from fish shell, it will also start misbehaving when running next commands (fish bug report: https://github.com/fish-shell/fish-shell/issues/10600).

Additional information

Expect a patch from me soon.