issue: 4669986 Fix integer underflow on EINTR

The udp_send tests were failing sporadically due to integer
underflow when recv() was interrupted by a signal.

In the receive accumulation loops, when recv() returned -1 with
errno=EINTR (interrupted system call):

    while (received < recvsize) {
        rc = recv(fd, buff + received, sizeof(buff) - received, 0);
        if (rc < 0 && errno != EINTR) {
            break;
        }
        received += static_cast<size_t>(rc);  // BUG
    }

The EINTR check allowed the loop to continue, but then proceeded
to cast rc=-1 to size_t, creating a huge positive value
(0xFFFFFFFFFFFFFFFF on 64-bit). This caused 'received' to exceed
'recvsize', terminating the loop immediately with an empty buffer.

The sporadic nature occurred because signal delivery (timer signals,
scheduling signals, etc.) is non-deterministic. Most runs succeeded,
but occasional signal interruption triggered the bug.

The fix adds a check to only accumulate positive receive counts:

    if (rc > 0) {
        received += static_cast<size_t>(rc);
    }

This correctly handles three cases:
- rc > 0: Data received, accumulate and continue
- rc == 0: Zero-length UDP datagram, skip and continue
- rc < 0 with EINTR: Interrupted, retry without accumulating
- rc < 0 with other errno: Error, break from loop

Fixed in two test locations:
- udp_send.null_iov_elements (line 524)
- udp_send.null_iov_elements_single_iov (line 744)

Note: tcp_send.cc already had correct handling with explicit
rc==0 check, preventing this class of bug.

Signed-off-by: Tomer Cabouly <[email protected]>

Author: tomerdbz

tests/gtest/udp/udp_send.cc

@@ -521,8 +521,10 @@ TEST_F(udp_send, null_iov_elements)
                         break;
                     }
 
-                    received += static_cast<size_t>(rc);
-                    log_trace("Received %zd\n", received);
+                    if (rc > 0) {
+                        received += static_cast<size_t>(rc);
+                        log_trace("Received %zd\n", received);
+                    }
                 }
 
                 log_trace("Received Final %zd\n", received);
@@ -739,8 +741,10 @@ TEST_F(udp_send, DISABLED_null_iov_elements_fragmented)
                         break;
                     }
 
-                    received += static_cast<size_t>(rc);
-                    log_trace("Received %d\n", rc);
+                    if (rc > 0) {
+                        received += static_cast<size_t>(rc);
+                        log_trace("Received %d\n", rc);
+                    }
                 }
 
                 log_trace("Received Final %zd\n", received);