nbd-client: Short-circuit 0-length operations

The NBD spec was recently clarified to state that clients should
not send 0-length requests to the server, as the server behavior
is undefined [1].  We know that qemu-nbd's behavior is a successful
no-op (once it has filtered for read-only exports), but other NBD
implementations might return an error.  To avoid any questionable
server implementations, it is better to just short-circuit such
requests on the client side (we are relying on the block layer to
already filter out requests such as invalid offset, write to a
read-only volume, and so forth); do the short-circuit as late as
possible to still benefit from protections from assertions that
the block layer is not violating our assumptions.

[1] https://github.com/NetworkBlockDevice/nbd/commit/ee926037Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20171108215703.9295-6-eblake@redhat.com>
Reviewed-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

block/nbd-client.c

@@ -674,6 +674,9 @@ int nbd_client_co_preadv(BlockDriverState *bs, uint64_t offset,
     assert(bytes <= NBD_MAX_BUFFER_SIZE);
     assert(!flags);
 
+    if (!bytes) {
+        return 0;
+    }
     ret = nbd_co_send_request(bs, &request, NULL);
     if (ret < 0) {
         return ret;
@@ -705,6 +708,9 @@ int nbd_client_co_pwritev(BlockDriverState *bs, uint64_t offset,
 
     assert(bytes <= NBD_MAX_BUFFER_SIZE);
 
+    if (!bytes) {
+        return 0;
+    }
     return nbd_co_request(bs, &request, qiov);
 }
 
@@ -731,6 +737,9 @@ int nbd_client_co_pwrite_zeroes(BlockDriverState *bs, int64_t offset,
         request.flags |= NBD_CMD_FLAG_NO_HOLE;
     }
 
+    if (!bytes) {
+        return 0;
+    }
     return nbd_co_request(bs, &request, NULL);
 }
 
@@ -759,7 +768,7 @@ int nbd_client_co_pdiscard(BlockDriverState *bs, int64_t offset, int bytes)
     };
 
     assert(!(client->info.flags & NBD_FLAG_READ_ONLY));
-    if (!(client->info.flags & NBD_FLAG_SEND_TRIM)) {
+    if (!(client->info.flags & NBD_FLAG_SEND_TRIM) || !bytes) {
         return 0;
     }