[Qemu-devel] Question about nonblocking stderr and lost logs

[Sam Bobroff <sam.bobroff@au1.ibm.com>]

Hi QEMU programmers,

While doing some experimental work on QEMU that has involved adding a lot of
new log messages (using qemu_log_mask()), I've discovered that under some
conditions a lot of my log messages go missing.

I've tracked the issue down to qemu_logfile being left at the default (stderr)
(so when not using -D) and according to strace what is happening is that the
debug messages are being passed to write() but write() is returning EWOULDBLOCK
and the messags are dropped.

This seems to be happening because stderr is being set non-blocking (which is a
bit odd to me), and it seems like NONBLOCK is being set as qmp_chardev_add() adds a
device for stdout (yes stdout, not stderr; perhaps file descriptors have been
dup'd by that point?).

Is this by design to prevent a slow console from blocking QEMU? If not, should
I delve further and try to prevent non-blocking being set on stderr?

(Unfortunately I don't have a replication for this using an unmodified QEMU but
I suspect I could find one if necessary.)

Cheers,
Sam.