nbd/server: fix sparse read

In case of io error in nbd_co_send_sparse_read we should not
"goto reply:", as it was a fatal error and the common behavior
is to disconnect in this case. We should not try to send the
client an additional error reply, since we already hit a
channel-io error on our previous attempt to send one.

Fix this by handling block-status error in nbd_co_send_sparse_read,
so nbd_co_send_sparse_read fails only on io error. Then just skip
common "reply:" code path in nbd_trip.
Signed-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20180308184636.178534-3-vsementsov@virtuozzo.com>
[eblake: grammar tweaks]
Signed-off-by: NEric Blake <eblake@redhat.com>

nbd/server.c

@@ -1366,6 +1366,10 @@ static int coroutine_fn nbd_co_send_structured_error(NBDClient *client,
     return nbd_co_send_iov(client, iov, 1 + !!iov[1].iov_len, errp);
 }
 
+/* Do a sparse read and send the structured reply to the client.
+ * Returns -errno if sending fails. bdrv_block_status_above() failure is
+ * reported to the client, at which point this function succeeds.
+ */
 static int coroutine_fn nbd_co_send_sparse_read(NBDClient *client,
                                                 uint64_t handle,
                                                 uint64_t offset,
@@ -1386,8 +1390,13 @@ static int coroutine_fn nbd_co_send_sparse_read(NBDClient *client,
         bool final;
 
         if (status < 0) {
-            error_setg_errno(errp, -status, "unable to check for holes");
-            return status;
+            char *msg = g_strdup_printf("unable to check for holes: %s",
+                                        strerror(-status));
+
+            ret = nbd_co_send_structured_error(client, handle, -status, msg,
+                                               errp);
+            g_free(msg);
+            return ret;
         }
         assert(pnum && pnum <= size - progress);
         final = progress + pnum == size;
@@ -1568,7 +1577,7 @@ static coroutine_fn void nbd_trip(void *opaque)
                                           request.from, req->data, request.len,
                                           &local_err);
             if (ret < 0) {
-                goto reply;
+                goto replied;
             }
             goto done;
         }
@@ -1665,6 +1674,8 @@ reply:
                                        req->data, reply_data_len, &local_err);
     }
     g_free(msg);
+
+replied:
     if (ret < 0) {
         error_prepend(&local_err, "Failed to send reply: ");
         goto disconnect;