issue: 4082058 Fixing return from rx_wait_helper without data

In case poll_and_process_element_rx polled packets without draining the CQ but m_n_rx_pkt_ready_list_count==0,
meaning no data bytes were received, we need to return positive return code, to make the caller call us again.
Returning EAGAIN without draining the CQ will make the caller return EGAIAN as well, and so up to the application,
although packets are still available in the CQ.
This can happen, for example, if the polling received only ACK packets. Returning EAGAIN in this case is incorrect.
Callers may decide falsly to go to sleep.

Signed-off-by: Alexander Grissik <[email protected]>

Author: AlexanderGrissik

src/core/sock/sockinfo_tcp.cpp

@@ -5096,8 +5096,9 @@ int sockinfo_tcp::rx_wait_helper(int &poll_count, bool blocking)
     // It can be too expansive for the application to get nothing just because of lock contention.
     // In this case it will be better to have a lock() version of poll_and_process_element_rx.
     // And then we should continue polling untill we have ready packets or we drained the CQ.
+    bool all_drained = true;
     if (likely(m_p_rx_ring)) {
-        m_p_rx_ring->poll_and_process_element_rx(&poll_sn);
+        all_drained = m_p_rx_ring->poll_and_process_element_rx(&poll_sn);
     } else { // There's more than one CQ, go over each one
         for (rx_ring_iter = m_rx_ring_map.begin(); rx_ring_iter != m_rx_ring_map.end();
              rx_ring_iter++) {
@@ -5106,13 +5107,13 @@ int sockinfo_tcp::rx_wait_helper(int &poll_count, bool blocking)
                 continue;
             }
 
-            rx_ring_iter->first->poll_and_process_element_rx(&poll_sn);
+            all_drained &= rx_ring_iter->first->poll_and_process_element_rx(&poll_sn);
         }
     }
     m_rx_ring_map_lock.unlock();
     lock_tcp_con(); // We must take a lock before checking m_n_rx_pkt_ready_list_count
 
-    if (likely(m_n_rx_pkt_ready_list_count)) { // got completions from CQ
+    if (likely(m_n_rx_pkt_ready_list_count || !all_drained)) { // Got completions from CQ
         __log_entry_funcall("Ready %d packets. sn=%llu", m_n_rx_pkt_ready_list_count,
                             (unsigned long long)poll_sn);
         IF_STATS(m_p_socket_stats->counters.n_rx_poll_hit++);