Re: [Qemu-devel] [PULL 3/4] nbd/client: Don't hard-disconnect on ESHUTDOWN from server

[Michael Roth]

Quoting Eric Blake (2017-11-17 09:05:55)
> 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: address@hidden

Since 2.10.x lacks d2febedb, the patch would look something like:

diff --git a/nbd/client.c b/nbd/client.c
index 4caff77119..f04e95542f 100644
--- a/nbd/client.c
+++ b/nbd/client.c
@@ -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) {

If that seems reasonable I can apply it as such.

> Signed-off-by: Eric Blake <address@hidden>
> Message-Id: <address@hidden>
> Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> ---
>  nbd/client.c | 6 ------
>  1 file changed, 6 deletions(-)
> 
> diff --git a/nbd/client.c b/nbd/client.c
> index 4e15fc484d..eea236ca06 100644
> --- a/nbd/client.c
> +++ b/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);
> -- 
> 2.13.6
> 
>