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nbd/client: Don't hard-disconnect on ESHUTDOWN from server 

The NBD spec says that a server may fail any transmission request
with ESHUTDOWN when it is apparent that no further request from
the client can be successfully honored.  The client is supposed
to then initiate a soft shutdown (wait for all remaining in-flight
requests to be answered, then send NBD_CMD_DISC).  However, since
qemu's server never uses ESHUTDOWN errors, this code was mostly
untested since its introduction in commit b6f5d3b5.

More recently, I learned that nbdkit as the NBD server is able to
send ESHUTDOWN errors, so I finally tested this code, and noticed
that our client was special-casing ESHUTDOWN to cause a hard
shutdown (immediate disconnect, with no NBD_CMD_DISC), but only
if the server sends this error as a simple reply.  Further
investigation found that commit d2febedb introduced a regression
where structured replies behave differently than simple replies -
but that the structured reply behavior is more in line with the
spec (even if we still lack code in nbd-client.c to properly quit
sending further requests).  So this patch reverts the portion of
b6f5d3b5 that introduced an improper hard-disconnect special-case
at the lower level, and leaves the future enhancement of a nicer
soft-disconnect at the higher level for another day.

CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20171113194857.13933-1-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
(cherry picked from commit 01b05c66a3)
 Conflicts:
	nbd/client.c
*drop dep on d2febedb
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
This commit is contained in:
Eric Blake 2017-11-13 13:48:57 -06:00 committed by Michael Roth
parent 0fd80ef569
commit b81833fe7d
1 changed files with 0 additions and 6 deletions
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6
nbd/client.c
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@ -945,12 +945,6 @@ ssize_t nbd_receive_reply(QIOChannel *ioc, NBDReply *reply, Error **errp)
reply->handle = ldq_be_p(buf + 8);
reply->error = nbd_errno_to_system_errno(reply->error);
if (reply->error == ESHUTDOWN) {
/* This works even on mingw which lacks a native ESHUTDOWN */
error_setg(errp, "server shutting down");
return -EINVAL;
}
trace_nbd_receive_reply(magic, reply->error, reply->handle);
if (magic != NBD_REPLY_MAGIC) {