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 b6f5d3b.

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 d2febed 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
b6f5d3b 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: [email protected]
Signed-off-by: Eric Blake <[email protected]>
Message-Id: <[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>

nbd/client.c

@@ -996,15 +996,9 @@ int nbd_receive_reply(QIOChannel *ioc, NBDReply *reply, Error **errp)
         if (ret < 0) {
             break;
         }
-
         trace_nbd_receive_simple_reply(reply->simple.error,
                                        nbd_err_lookup(reply->simple.error),
                                        reply->handle);
-        if (reply->simple.error == NBD_ESHUTDOWN) {
-            /* This works even on mingw which lacks a native ESHUTDOWN */
-            error_setg(errp, "server shutting down");
-            return -EINVAL;
-        }
         break;
     case NBD_STRUCTURED_REPLY_MAGIC:
         ret = nbd_receive_structured_reply_chunk(ioc, &reply->structured, errp);