Hang in tcdrain(3) after write(3)

[Jeffrey Walton]

Hi Everyone,

I'm testing some software on DragonFly. There's not much to it. It
talks to the modem, and sends an ATZ and then reads the response.
Linux, FreeBSD, NetBSD, OpenBSD and OS X are OK.

My test rig is DragonFly 5.6-RELEASE x86_64 (fully patched) with a
USR5637 modem, https://www.amazon.com/gp/product/B0013FDLM0. The modem
is located at /dev/cuaU0.

DragonFly hangs on the call to tcdrain(3). Looking at the man page I
don't see any special handling. Cf.,
http://man.dragonflybsd.org/?command=tcdrain.

Attached is the reproducer. The trace is:

% ./test.exe
Setting TIOCEXCL
Getting tty
Setting tty options
Flushing tty
Setting tty
Writing ATZ
Waiting for write  <<-- call to tcdrain(fd)

Thanks.
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