nbd: do not hang nbd_wr_syncv if outside a coroutine and no available data

Until commit 1c778ef7 ("nbd: convert to using I/O channels for actual
socket I/O", 2016-02-16), nbd_wr_sync returned -EAGAIN this scenario.
nbd_reply_ready required these semantics because it has two conflicting
requirements:

1) if a reply can be received on the socket, nbd_reply_ready needs
to read the header outside coroutine context to identify _which_
coroutine to enter to process the rest of the reply

2) on the other hand, nbd_reply_ready can find a false positive if
another thread (e.g. a VCPU thread running aio_poll) sneaks in and
calls nbd_reply_ready too.  In this case nbd_reply_ready does nothing
and expects nbd_wr_syncv to return -EAGAIN.

Currently, the solution to the first requirement is to wait in the very
rare case of a read() that doesn't retrieve the reply header in its
entirety; this is what nbd_wr_syncv does by calling qio_channel_wait().
However, the unconditional call to qio_channel_wait() breaks the second
requirement.  To fix this, the patch makes nbd_wr_syncv return -EAGAIN
if done is zero, similar to the code before commit 1c778ef7.

This is okay because NBD client-side negotiation is the only other case
that calls nbd_wr_syncv outside a coroutine, and it places the socket
in blocking mode.  On the other hand, it is a bit unpleasant to put
this in nbd_wr_syncv(), because the function is used by both client
and server.

The full fix would be to add a counter to NbdClientSession for how
many bytes have been filled in s->reply.  Then a reply can be filled
by multiple separate invocations of nbd_reply_ready and the
qio_channel_wait() call can be removed completely.  Something to
consider for 2.7...
Reported-by: NChanglong Xie <xiecl.fnst@cn.fujitsu.com>
Reviewed-by: NDaniel P. Berrange <berrange@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

nbd/common.c

@@ -50,9 +50,12 @@ ssize_t nbd_wr_syncv(QIOChannel *ioc,
                  * qio_channel_yield() that works with AIO contexts
                  * and consider using that in this branch */
                 qemu_coroutine_yield();
-            } else {
+            } else if (done) {
+                /* XXX this is needed by nbd_reply_ready.  */
                 qio_channel_wait(ioc,
                                  do_read ? G_IO_IN : G_IO_OUT);
+            } else {
+                return -EAGAIN;
             }
             continue;
         }