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Re: [Qemu-devel] [libvirt] char: Logging serial pty output when disconne

From: Daniel P. Berrange
Subject: Re: [Qemu-devel] [libvirt] char: Logging serial pty output when disconnected
Date: Tue, 31 Jan 2017 09:39:27 +0000
User-agent: Mutt/1.7.1 (2016-10-04)

On Mon, Jan 30, 2017 at 08:58:25PM -0800, Ed Swierk wrote:
> On Fri, Jan 27, 2017 at 1:40 AM, Daniel P. Berrange <address@hidden> wrote:
> > On Thu, Jan 26, 2017 at 05:07:16PM -0800, Ed Swierk wrote:
> >> Currently qemu_chr_fe_write() calls qemu_chr_fe_write_log() only for
> >> data consumed by the backend chr_write function. With the pty backend,
> >> pty_chr_write() returns 0 indicating that the data was not consumed
> >> when the pty is disconnected. Simply changing it to return len instead
> >> of 0 tricks the caller into logging the data even when the pty is
> >> disconnected. I don't know what problems this might cause, but one
> >> data point is that tcp_chr_write() already happens to work this way.
> >>
> >> Alternatively, qemu_chr_fe_write() could be modified to log everything
> >> passed to it, regardless of how much data chr_write claims to have
> >> consumed. The trouble is that the serial device retries writing
> >> unconsumed data, so when the pty is disconnected you'd see every
> >> character duplicated 4 times in the log file.
> >>
> >> Any opinions on either approach, or other suggestions? If there are no
> >> objections to the first one, I'll prepare a patch.
> >
> > If the pty backend intends to just drop data into a blackhole when
> > no client is connected, then its chr_write() impl should return
> > the length of the data discarded, not zero.
> That's exactly the question: when no client is connected, should the
> pty backend just drop the data into a black hole, returning the length
> of the data discarded? Or should it return 0, letting the frontend
> device decide what to do with it?

It should return len of data discarded.

> I can't discern a consistent pattern across all the char backends. The
> closest analog is the tcp backend, which does discard the data and
> return len. In contrast, several backends call
> io_channel_send{,_full}(), which returns -1 if the write would block
> or fails for any other reason.
> It's not clear there's much the frontend can do to recover from an
> error, but there's no consistent pattern across serial devices either.
> Most just ignore the return value. But the 16550A serial device
> retries 4 times after an error. Changing the pty backend to discard
> the data on the first attempt would bypass this retry mechanism. Is
> that a problem?

I don't think so - retrying in this way is pointless IMHO - it is just
going to get the same result on every retry on 99% of occassions.

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