On 2021-01-19 19:15, Nick Holland wrote:
On 1/19/21 4:35 PM, Adam Thompson wrote:
I ran into this exact problem last year. It'll be in the list
archives.
According to Theo (if I understood him correctly) it's partly due to
the
way BSD serial ports have always worked, i.e. in a rather
under-specified manner.
Apparently the core tty(4) code around this particular symptom hasn't
really changed at all since OpenBSD forked.
I'm curious what you think "this exact problem" is.
Based on later messages in the thread, it's clear I did *not* have the
exact same problem, after all.
I was experiencing ttys that remained busy after the process that opened
them had closed.
So, for example, if I used "screen /dev/cua0 9600", after exiting
screen(1), confirming with ps(1) that all screen processes were gone,
and confirming with lsof/fstat that nothing still had either cua00 or
tty00 open, if I re-ran "screen /dev/cua00 9600" - or ANY other
application, including cu(1) - the new process would simply block.
ktrace(8) confirmed that the open(2) call was simply blocking...
forever. I could reproduce it with cu(1) as well as screen. Don't
think I tried any other apps.
I recall that someone suggested at the time that it might be signal line
related, but it didn't matter whether I had full RS-232 CTS/RTS and/or
DCD/DSR signalling, basic 3-wire signalling only, or even if I unplugged
the cable entirely, the blocking behaviour stuck around until I
rebooted. Granted, rebooting on a single-port terminal server isn't the
end of the world, but it was a PITA that I got tired of. (Oh, if I used
a USB serial adapter, unplugging and replugging the USB dongle also
cleared the problem.)
I'm not going to say serial support in OpenBSD is perfect, but it works
Darned Well,
Mostly, I agree. I still don't know what was different about this one
use case. Pretty sure it wasn't hardware - I reproduced it on three
vastly different amd64 systems, and with both traditional UARTs and USB
dongles. Pretty sure it wasn't (Exclusively) a PEBCAK issue. But still
not sure what it was.
I'll try again at some point, and I'll post if/when it happens to me
again.
-Adam
