NBD reconnect logic considers the error code from the functions that
read NBD messages to tell if reconnect should be attempted or not: it is
attempted on -EIO, otherwise the client transitions to NBD_CLIENT_QUIT
state (see nbd_channel_error).  This error code is propagated from the
primitives like nbd_read.

The problem, however, is that nbd_read itself turns every error into -1
rather than -EIO.  As a result, if the NBD server happens to die while
sending the message, the client in QEMU receives less data than it
expects, considers it as a fatal error, and wouldn't attempt
reestablishing the connection.

Fix it by turning every negative return from qio_channel_read_all into
-EIO returned from nbd_read.  Apparently that was the original behavior,
but got broken later.  Also adjust nbd_readXX to follow.

Fixes: e6798f06a6 ("nbd: generalize usage of nbd_read")
Signed-off-by: Roman Kagan <rvkagan@yandex-team.ru>
---
 include/block/nbd.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/block/nbd.h b/include/block/nbd.h
index 4a52a43ef5..5f34d23bb0 100644
--- a/include/block/nbd.h
+++ b/include/block/nbd.h
@@ -364,7 +364,7 @@ static inline int nbd_read(QIOChannel *ioc, void *buffer, size_t size,
         if (desc) {
             error_prepend(errp, "Failed to read %s: ", desc);
         }
-        return -1;
+        return ret;
     }
 
     return 0;
@@ -375,8 +375,9 @@ static inline int nbd_read##bits(QIOChannel *ioc,                       \
                                  uint##bits##_t *val,                   \
                                  const char *desc, Error **errp)        \
 {                                                                       \
-    if (nbd_read(ioc, val, sizeof(*val), desc, errp) < 0) {             \
-        return -1;                                                      \
+    int ret = nbd_read(ioc, val, sizeof(*val), desc, errp);             \
+    if (ret < 0) {                                                      \
+        return ret;                                                     \
     }                                                                   \
     *val = be##bits##_to_cpu(*val);                                     \
     return 0;                                                           \
-- 
2.29.2

28.01.2021 23:14, Roman Kagan wrote:
> NBD reconnect logic considers the error code from the functions that
> read NBD messages to tell if reconnect should be attempted or not: it is
> attempted on -EIO, otherwise the client transitions to NBD_CLIENT_QUIT
> state (see nbd_channel_error).  This error code is propagated from the
> primitives like nbd_read.
> 
> The problem, however, is that nbd_read itself turns every error into -1
> rather than -EIO.  As a result, if the NBD server happens to die while
> sending the message, the client in QEMU receives less data than it
> expects, considers it as a fatal error, and wouldn't attempt
> reestablishing the connection.
> 
> Fix it by turning every negative return from qio_channel_read_all into
> -EIO returned from nbd_read.  Apparently that was the original behavior,
> but got broken later.  Also adjust nbd_readXX to follow.
> 
> Fixes: e6798f06a6 ("nbd: generalize usage of nbd_read")
> Signed-off-by: Roman Kagan <rvkagan@yandex-team.ru>

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

Really looks like a bug in e6798f06a6: it changes error code from -EIO to -1 without any reasoning.

> ---
>   include/block/nbd.h | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/include/block/nbd.h b/include/block/nbd.h
> index 4a52a43ef5..5f34d23bb0 100644
> --- a/include/block/nbd.h
> +++ b/include/block/nbd.h
> @@ -364,7 +364,7 @@ static inline int nbd_read(QIOChannel *ioc, void *buffer, size_t size,
>           if (desc) {
>               error_prepend(errp, "Failed to read %s: ", desc);
>           }
> -        return -1;
> +        return ret;
>       }
>   
>       return 0;
> @@ -375,8 +375,9 @@ static inline int nbd_read##bits(QIOChannel *ioc,                       \
>                                    uint##bits##_t *val,                   \
>                                    const char *desc, Error **errp)        \
>   {                                                                       \
> -    if (nbd_read(ioc, val, sizeof(*val), desc, errp) < 0) {             \
> -        return -1;                                                      \
> +    int ret = nbd_read(ioc, val, sizeof(*val), desc, errp);             \
> +    if (ret < 0) {                                                      \
> +        return ret;                                                     \
>       }                                                                   \
>       *val = be##bits##_to_cpu(*val);                                     \
>       return 0;                                                           \
> 


-- 
Best regards,
Vladimir