Avoid nested MQTT preference copies on the Companion loop stack

docs/nimble_companion_trial.md

@@ -93,6 +93,7 @@ python3 -B test/test_heltec_v4_wifi_setup_page.py
 python3 -B test/test_firmware_ram.py
 python3 -B test/test_esp32_dram.py
 python3 -B test/test_shared_mota_queue.py
+python3 -B test/test_companion_mqtt_stack.py
 pio test -e native -f test_companion_node_prefs \
   -f test_ble_tx_stall_watchdog -f test_companion_frame_queue \
   -f test_display_driver
@@ -104,6 +105,9 @@ checks PIN authentication and stealth; and exercises 176-byte frames, MTU and
 notification subscription gating, retry and duplicate prevention. Embedded
 builds compile against the pinned real library. RAM reports retain the existing
 wireless budget; linked headroom is not a measurement of live free heap.
+The MQTT save callback also has a compiler-enforced stack-frame regression
+check: nested preference copies previously overflowed the V4 loop task when
+saving MQTT settings through WebConfig, despite sufficient free heap.
 
 Hardware qualification must additionally verify pairing and reconnection,
 advertised addresses for every MAC policy, contact synchronization, WiFi/BLE

examples/companion_radio/MyMesh.cpp

@@ -3163,9 +3163,10 @@ void MyMesh::onConfigBatchEnd() {
     CompanionMqttSetupPortal::saveStoredConfig(_mqtt_prefs);
     if (_mqtt_started && _mqtt_bridge) _mqtt_bridge->end();
     _mqtt_started = false;
-    MQTTPrefs verified;
-    _mqtt_configured = CompanionMqttSetupPortal::loadStoredConfig(verified);
-    if (_mqtt_configured) _mqtt_prefs = verified;
+    // loadStoredConfig only replaces its destination after validation. Pass
+    // the live preferences directly: another 2.8 KB copy here nests with the
+    // loader's scratch copy and NVS calls, overflowing the ESP32 loop stack.
+    _mqtt_configured = CompanionMqttSetupPortal::loadStoredConfig(_mqtt_prefs);
     // The standalone Companion setting remains canonical even if MQTT config
     // verification reloaded an older copy of this field.
     _mqtt_prefs.wifi_power_save = getCompanionWiFiPowerSave();

test/test_companion_mqtt_stack.py

@@ -0,0 +1,71 @@
+#!/usr/bin/env python3
+"""Bound the real Companion MQTT save callback's additional stack use."""
+from pathlib import Path
+import subprocess
+import tempfile
+import unittest
+
+ROOT = Path(__file__).resolve().parents[1]
+CALLBACK = "void MyMesh::onConfigBatchEnd()"
+PREAMBLE = r"""
+#include <helpers/MQTTPrefsStorage.h>
+void reloadCompanionWiFiPowerSave();
+uint8_t getCompanionWiFiPowerSave();
+struct MQTTBridge { void end(); };
+struct CompanionMqttSetupPortal {
+  static bool saveStoredConfig(const MQTTPrefs&);
+  static bool loadStoredConfig(MQTTPrefs&);
+};
+struct MyMesh {
+  void onConfigBatchEnd();
+  void syncWiFiPowerSaving();
+  bool _wc_mqtt_dirty, _mqtt_started, _mqtt_configured;
+  MQTTBridge* _mqtt_bridge;
+  MQTTPrefs _mqtt_prefs;
+};
+"""
+
+
+def callback_source():
+    source = (ROOT / "examples/companion_radio/MyMesh.cpp").read_text()
+    start = source.index(CALLBACK)
+    return source[start:source.index("void MyMesh::execCommand", start)]
+
+
+class CompanionMqttStackTests(unittest.TestCase):
+    def compile_callback(self, source):
+        with tempfile.TemporaryDirectory(prefix="meshcore-mqtt-stack-") as temp:
+            path = Path(temp) / "callback.cpp"
+            path.write_text(PREAMBLE + source)
+            return subprocess.run([
+                "c++", "-std=c++17", "-Os", "-fno-inline",
+                "-Werror=frame-larger-than=512",
+                "-DESP32=1", "-DWIFI_SSID=1", "-DWITH_MQTT_BRIDGE=1",
+                "-I", str(ROOT / "src"), "-c", str(path),
+                "-o", str(Path(temp) / "callback.o"),
+            ], text=True, capture_output=True)
+
+    def test_save_callback_leaves_stack_for_nvs_and_validation(self):
+        # The loader already needs a full preference scratch copy. The save
+        # callback must not add another one to the ESP32's 8 KB loop stack.
+        result = self.compile_callback(callback_source())
+        self.assertEqual(result.returncode, 0, result.stderr)
+
+    def test_guard_rejects_the_hardware_stack_overflow_regression(self):
+        source = callback_source()
+        call = "CompanionMqttSetupPortal::loadStoredConfig(_mqtt_prefs)"
+        self.assertIn(call, source)
+        source = source.replace("{", "{\n  MQTTPrefs verified;", 1)
+        source = source.replace(call, "CompanionMqttSetupPortal::loadStoredConfig(verified)")
+        source = source.replace(
+            "// The standalone Companion setting",
+            "if (_mqtt_configured) _mqtt_prefs = verified;\n"
+            "    // The standalone Companion setting",
+        )
+        result = self.compile_callback(source)
+        self.assertNotEqual(result.returncode, 0)
+        self.assertIn("frame size", result.stderr)
+
+
+if __name__ == "__main__":
+    unittest.main()