Address review feedback for xdebug transform fix

- Update comment to explain 'why' (prevents tight loop that starves
  other transactions) instead of 'what' the code does
- Change verify_no_loop.sh to fail the test when consumed_count != 2
  instead of just warning

Author: bryancall

plugins/xdebug/xdebug_transforms.cc

@@ -297,9 +297,9 @@ body_transform(TSCont contp, TSEvent event, void * /* edata ATS_UNUSED */)
 
     if (TSVIONTodoGet(src_vio) > 0) {
       if (towrite > 0) {
-        // Only reenable and notify upstream if we consumed data.
-        // If no data was available, just return and wait for the
-        // upstream to call us again when more data arrives.
+        // Only reenable when we consumed data. If we reenable and call
+        // WRITE_READY when towrite is 0 (no data available yet), we create
+        // a tight loop that starves other transactions and causes high CPU.
         TSVIOReenable(data->output_vio);
         TSContCall(TSVIOContGet(src_vio), TS_EVENT_VCONN_WRITE_READY, src_vio);
       }

tests/gold_tests/pluginTest/xdebug/x_probe_full_json_slow_origin/verify_no_loop.sh

@@ -56,7 +56,8 @@ consumed_count=$(grep -c "consumed.*bytes" "$LOGFILE" 2>/dev/null || echo "0")
 echo "  'consumed' lines: $consumed_count (expected: 2)"
 
 if [[ $consumed_count -ne 2 ]]; then
-    echo "WARNING: Expected 2 consumed lines but found $consumed_count"
+    echo "FAIL: Expected 2 consumed lines but found $consumed_count"
+    exit 1
 fi
 
 echo "PASS: Transform completed normally"