Re: [PULL 3/5] nbd: fix uninitialized variable warning

[Eric Blake <eblake@redhat.com>]

On 1/8/20 10:02 AM, Laurent Vivier wrote:
> From: Pan Nengyuan <pannengyuan@huawei.com>
> 
> Fixes:
> /mnt/sdb/qemu/nbd/server.c: In function 'nbd_handle_request':
> /mnt/sdb/qemu/nbd/server.c:2313:9: error: 'ret' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>       int ret;
> 
> Reported-by: Euler Robot <euler.robot@huawei.com>
> Signed-off-by: Pan Nengyuan <pannengyuan@huawei.com>
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> Reviewed-by: Laurent Vivier <laurent@vivier.eu>
> Message-Id: <20200108025132.46956-1-pannengyuan@huawei.com>
> Signed-off-by: Laurent Vivier <laurent@vivier.eu>
> ---
>   nbd/server.c | 10 +---------
>   1 file changed, 1 insertion(+), 9 deletions(-)
> 

This patch is broken; I first noticed it when testing libnbd, where the 
symptoms are a hung interop/dirty-bitmap.sh test.

> diff --git a/nbd/server.c b/nbd/server.c
> index 24ebc1a80571..87fcd2e7bfac 100644
> --- a/nbd/server.c
> +++ b/nbd/server.c
> @@ -2384,20 +2384,12 @@ static coroutine_fn int nbd_handle_request(NBDClient *client,
>                                                  !client->export_meta.bitmap,
>                                                  NBD_META_ID_BASE_ALLOCATION,
>                                                  errp);
> -                if (ret < 0) {
> -                    return ret;
> -                }
> -            }
> -
> -            if (client->export_meta.bitmap) {
> +            } else {              /* client->export_meta.bitmap */
>                   ret = nbd_co_send_bitmap(client, request->handle,
>                                            client->exp->export_bitmap,
>                                            request->from, request->len,
>                                            dont_fragment,
>                                            true, NBD_META_ID_DIRTY_BITMAP, errp);
> -                if (ret < 0) {
> -                    return ret;
> -                }
>               }

The NBD spec says that clients can request more than one meta context at 
a time. Qemu does not (and hence we didn't notice the breakage until 
now), but libnbd does, and this change prevents qemu from obeying the 
spec (if the client requested both contexts, then we must return both, 
and not short-circuit after the first).

I'll post a patch to fix this unintended regression soon.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org