nbd: make nbd_read* return -EIO on error

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>
Message-Id: <20210129073859.683063-4-rvkagan@yandex-team.ru>
Signed-off-by: Eric Blake <eblake@redhat.com>

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;                                                           \