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Re: [Qemu-devel] [Nbd] NBD_CMD_DISC

From: Daniel P. Berrange
Subject: Re: [Qemu-devel] [Nbd] NBD_CMD_DISC
Date: Tue, 12 Apr 2016 11:34:06 +0100
User-agent: Mutt/1.5.24 (2015-08-30)

On Tue, Apr 12, 2016 at 10:48:20AM +0100, Daniel P. Berrange wrote:
> On Sun, Apr 10, 2016 at 10:49:00AM +0100, Alex Bligh wrote:
> > (Daniel: if you want to replicate the issue, just run qemu-img info
> > against my gonbdserver with TLS. Every fifth NBD_CMD_DISC doesn't
> > get through before the TCP session closes).
> Hmm, I'll have a look at this - I'm not sure its caused by
> the lack of gnutls_bye, as opposed to incorrect handling
> of EAGAIN when the block layer sends CMD_DISC

I have tried to reproduce with your server yet, but I did look at the
code and identified one potential flaw that might cause the behaviour
you mention. Can you try testing with the following change applied
to QEMU:

diff --git a/nbd/common.c b/nbd/common.c
index 8ddb2dd..47757b6 100644
--- a/nbd/common.c
+++ b/nbd/common.c
@@ -50,7 +50,7 @@ ssize_t nbd_wr_syncv(QIOChannel *ioc,
                  * qio_channel_yield() that works with AIO contexts
                  * and consider using that in this branch */
-            } else if (done) {
+            } else if (done || !do_read) {
                 /* XXX this is needed by nbd_reply_ready.  */
                                  do_read ? G_IO_IN : G_IO_OUT);

The current code will cause it to silently not send CMD_DISC if the socket
returns EAGAIN on send(). This change will cause it to do a poll to wait
and retry the send(). That said I'd be pretty surprised if we do actually
get EAGAIN in this scenario, as I'd expect the outgoing TCP buffers to be
empty at the point in which we close the client connection, but this fix
is worth a try.

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