Merge PR #3214: companion USB connection and flow control

Preserve the newer terminal and mOTA passthrough paths while adding real USB client detection and whole-frame transmit pacing.

src/helpers/ArduinoSerialInterface.cpp

@@ -66,7 +66,8 @@ void ArduinoSerialInterface::disable() {
   resetReceiveState();
 }
 
-bool ArduinoSerialInterface::isConnected() const { 
+bool ArduinoSerialInterface::isConnected() const {
+  if (_conn_check) return _conn_check();
   return true;   // no way of knowing, so assume yes
 }
 
@@ -75,6 +76,12 @@ bool ArduinoSerialInterface::isReadBusy() const {
 }
 
 bool ArduinoSerialInterface::isWriteBusy() const {
+  if (_passthroughMode) return false;
+  if (_flow_ctl && isConnected()) {
+    return const_cast<Stream*>(_serial)->availableForWrite() < (int)(MAX_FRAME_SIZE + 3);
+  }
+  // while nobody drains the port the TX buffer stays full, so never report
+  // busy in that case: it would stall the paced streams on all interfaces
   return false;
 }
 
@@ -84,6 +91,17 @@ size_t ArduinoSerialInterface::writeFrame(const uint8_t src[], size_t len) {
     return 0;
   }
   if (_passthroughMode) return len;
+  if (_flow_ctl) {
+    if (!isConnected()) {
+      return len;   // nobody is listening, drop instead of filling the TX buffer
+    }
+    if (_serial->availableForWrite() < (int)(len + 3)) {
+      // a short write would tear the length prefixed framing, and as there is
+      // neither a checksum nor a resync marker the receiver would stay out of
+      // sync forever - so drop the whole frame instead
+      return 0;
+    }
+  }
 
   uint8_t hdr[3];
   hdr[0] = '>';
@@ -133,6 +151,7 @@ size_t ArduinoSerialInterface::checkRecvFrame(uint8_t dest[]) {
           if (_frame_len > MAX_FRAME_SIZE) _frame_len = MAX_FRAME_SIZE;    // truncate
           memcpy(dest, rx_buf, _frame_len);
           _state = RECV_STATE_IDLE;  // reset state, for next frame
+          _last_frame_ms = millis();   // a real client is talking to us
           return _frame_len;
         }
     }