Bound hardware CAD scans

src/helpers/radiolib/CustomSX1276.h

@@ -72,19 +72,24 @@ class CustomSX1276 : public SX1276 {
         | RH_RF95_MODEM_STATUS_HEADER_INFO_VALID)) != 0;
     }
 
-    int tryScanChannel() {
+    int tryScanChannel(unsigned long timeout_ms = 2000UL) {
       // start CAD
       int16_t state = startChannelScan();
       RADIOLIB_ASSERT(state);
 
-      // wait for channel activity detected or timeout
-      unsigned long timeout = millis() + 16;
-      while(!this->mod->hal->digitalRead(this->mod->getIrq()) && millis() < timeout) {
+      // SX127x getChannelScanResult() reports a free channel before CAD has
+      // completed, so wait on its IRQ pins with the same bounded policy used
+      // by the generic wrapper.
+      const unsigned long started = millis();
+      while(!this->mod->hal->digitalRead(this->mod->getIrq())) {
         this->mod->hal->yield();
         if(this->mod->hal->digitalRead(this->mod->getGpio())) {
           return(RADIOLIB_PREAMBLE_DETECTED);
         }
+        if(millis() - started >= timeout_ms) {
+          return(RADIOLIB_ERR_RX_TIMEOUT);
+        }
       }
-      return 0; // timed out
+      return(RADIOLIB_CHANNEL_FREE);
     }
 };

src/helpers/radiolib/CustomSX1276Wrapper.h

@@ -41,6 +41,10 @@ public:
   uint8_t getSpreadingFactor() const override { return ((CustomSX1276 *)_radio)->spreadingFactor; }
 
 protected:
+  int16_t performChannelScan() override {
+    return ((CustomSX1276 *)_radio)->tryScanChannel();
+  }
+
   bool radioDeepInit() override {
     return ((CustomSX1276 *)_radio)->std_init();
   }

src/helpers/radiolib/RadioLibWrappers.cpp

@@ -614,7 +614,21 @@ void RadioLibWrapper::onSendFinished() {
 }
 
 int16_t RadioLibWrapper::performChannelScan() {
-  return _radio->scanChannel();
+  // RadioLib's blocking scanChannel() waits forever if the radio never raises
+  // its CAD-done IRQ. Keep CAD bounded so a missing IRQ cannot starve the main
+  // loop until the MCU watchdog resets the node.
+  int16_t result = _radio->startChannelScan();
+  if (result != RADIOLIB_ERR_NONE) return result;
+
+  const unsigned long started = millis();
+  while (millis() - started < 2000UL) {
+    result = _radio->getChannelScanResult();
+    if (result != RADIOLIB_ERR_UNKNOWN) return result;
+    yield();
+  }
+
+  MESH_DEBUG_PRINTLN("RadioLibWrapper: CAD scan timed out");
+  return RADIOLIB_ERR_RX_TIMEOUT;
 }
 
 bool RadioLibWrapper::isChannelActive() {