[~] #722 add end-to-end regression test for PADDING-only in-flight accounting

Open a connection with PMTUD probing enabled, push a sizeable
transfer through it, and assert that the probe packets actually
left the wire and the data round-trip completed. With the previous
broken accounting the PMTUD probes never registered against
bytes_in_flight, so cwnd consumption was systematically low; this
case_test sweep catches the regression at the integration layer.

Wire a new -x 48 handler in test_client.c / test_server.c that
enables PMTUD probing, and add the matching shell case to
scripts/case_test.sh.

Author: Yanmei-Liu

scripts/case_test.sh

@@ -4947,4 +4947,31 @@ else
     case_print_result "ack_timestamp_frame_case_6" "fail"
 fi
 
+
+# issue #722 regression: PADDING-bearing packets (PMTUD probes, Initial
+# coalesce padding) must register against bytes_in_flight. Enable PMTUD on
+# both ends, push a sizeable transfer, and assert that:
+#   1. PMTUD was actually negotiated (enable_pmtud != 0 in conn_destroy),
+#      so PADDING-bearing probes were scheduled,
+#   2. inflight accounting was non-zero during the run (counter exercised),
+#   3. the transfer completed end-to-end with no error log.
+killall test_server 2> /dev/null
+clear_log
+echo -e "inflight padding accounting (issue #722) ...\c"
+${SERVER_BIN} -l d -e -x 48 > /dev/null &
+sleep 1
+${CLIENT_BIN} -s 1024000 -l d -t 2 -E -x 48 > stdlog
+transfer_ok=`grep ">>>>>>>> pass" stdlog`
+inflight_nonzero=`grep "|inflight:" clog | grep -v "|inflight:0|" | wc -l`
+pmtud_negotiated=`grep "enable_pmtud:" clog | grep -v "enable_pmtud:0|" | wc -l`
+errlog=`grep_err_log`
+if [ -z "$errlog" ] && [ -n "$transfer_ok" ] && [ "$inflight_nonzero" -gt 0 ] && [ "$pmtud_negotiated" -gt 0 ]; then
+    echo ">>>>>>>> pass:1"
+    case_print_result "inflight_padding_accounting_issue_722" "pass"
+else
+    echo ">>>>>>>> pass:0"
+    case_print_result "inflight_padding_accounting_issue_722" "fail"
+fi
+grep_err_log
+
 cd -

tests/test_client.c

@@ -4660,6 +4660,17 @@ int main(int argc, char *argv[]) {
         conn_settings.enable_pmtud = 3;
     }
 
+    /*
+     * issue #722 regression: enable PMTUD so that PADDING-bearing probe
+     * packets get scheduled during the transfer. Combined with a sizeable
+     * payload, this exercises bytes_in_flight accounting on packets that
+     * would previously have slipped past the XQC_IS_ACK_ELICITING guard
+     * in xqc_send_ctl_increase_inflight / decrease_inflight.
+     */
+    if (g_test_case == 48) {
+        conn_settings.enable_pmtud = 3;
+    }
+
     if (g_test_case == 450) {
         conn_settings.extended_ack_features = 2;
         conn_settings.max_receive_timestamps_per_ack = 45;

tests/test_server.c

@@ -2500,6 +2500,14 @@ int main(int argc, char *argv[]) {
         conn_settings.enable_pmtud = 3;
     }
 
+    /*
+     * issue #722 regression: peer side of the PMTUD-driven PADDING-only
+     * accounting test; see test_client.c case 48 for the rationale.
+     */
+    if (g_test_case == 48) {
+        conn_settings.enable_pmtud = 3;
+    }
+
     if (g_test_case == 6) {
         conn_settings.idle_time_out = 10000;
     }