Fix RTL-SDR device conflicts when running ADS-B and airband simultaneously

Problems fixed:
1. Added start_new_session=True to dump1090 Popen - creates proper process
   group for clean shutdown
2. Use os.killpg() to kill entire process group when stopping ADS-B -
   ensures child processes are terminated and device is released
3. Track active device index in adsb_active_device for debugging
4. Add device info to /adsb/status endpoint
5. Add logging when starting/stopping ADS-B with device info

These changes ensure the RTL-SDR device is properly released when ADS-B
stops, allowing another process (e.g., airband) to use a different device.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

routes/adsb.py

@@ -46,6 +46,7 @@ adsb_messages_received = 0
 adsb_last_message_time = None
 adsb_bytes_received = 0
 adsb_lines_received = 0
+adsb_active_device = None  # Track which device index is being used
 
 # Track ICAOs already looked up in aircraft database (avoid repeated lookups)
 _looked_up_icaos: set[str] = set()
@@ -286,6 +287,7 @@ def adsb_status():
 
     return jsonify({
         'tracking_active': adsb_using_service,
+        'active_device': adsb_active_device,
         'connected_to_sbs': adsb_connected,
         'messages_received': adsb_messages_received,
         'bytes_received': adsb_bytes_received,
@@ -303,7 +305,7 @@ def adsb_status():
 @adsb_bp.route('/start', methods=['POST'])
 def start_adsb():
     """Start ADS-B tracking."""
-    global adsb_using_service
+    global adsb_using_service, adsb_active_device
 
     with app_module.adsb_lock:
         if adsb_using_service:
@@ -364,17 +366,20 @@ def start_adsb():
         if not dump1090_path:
             return jsonify({'status': 'error', 'message': f'readsb or dump1090 not found for {sdr_type.value}. Install readsb with SoapySDR support.'})
 
-    # Kill any stale app-started process
+    # Kill any stale app-started process (use process group to ensure full cleanup)
     if app_module.adsb_process:
         try:
-            app_module.adsb_process.terminate()
+            pgid = os.getpgid(app_module.adsb_process.pid)
+            os.killpg(pgid, 15)  # SIGTERM
             app_module.adsb_process.wait(timeout=PROCESS_TERMINATE_TIMEOUT)
-        except (subprocess.TimeoutExpired, OSError):
+        except (subprocess.TimeoutExpired, ProcessLookupError, OSError):
             try:
-                app_module.adsb_process.kill()
-            except OSError:
+                pgid = os.getpgid(app_module.adsb_process.pid)
+                os.killpg(pgid, 9)  # SIGKILL
+            except (ProcessLookupError, OSError):
                 pass
         app_module.adsb_process = None
+        logger.info("Killed stale ADS-B process")
 
     # Create device object and build command via abstraction layer
     sdr_device = SDRFactory.create_default_device(sdr_type, index=device)
@@ -393,10 +398,12 @@ def start_adsb():
         cmd[0] = dump1090_path
 
     try:
+        logger.info(f"Starting dump1090 with device index {device}: {' '.join(cmd)}")
         app_module.adsb_process = subprocess.Popen(
             cmd,
             stdout=subprocess.DEVNULL,
-            stderr=subprocess.PIPE
+            stderr=subprocess.PIPE,
+            start_new_session=True  # Create new process group for clean shutdown
         )
 
         time.sleep(DUMP1090_START_WAIT)
@@ -421,10 +428,11 @@ def start_adsb():
                 return jsonify({'status': 'error', 'message': error_msg})
 
         adsb_using_service = True
+        adsb_active_device = device  # Track which device is being used
         thread = threading.Thread(target=parse_sbs_stream, args=(f'localhost:{ADSB_SBS_PORT}',), daemon=True)
         thread.start()
 
-        return jsonify({'status': 'started', 'message': 'ADS-B tracking started'})
+        return jsonify({'status': 'started', 'message': 'ADS-B tracking started', 'device': device})
     except Exception as e:
         return jsonify({'status': 'error', 'message': str(e)})
 
@@ -432,17 +440,26 @@ def start_adsb():
 @adsb_bp.route('/stop', methods=['POST'])
 def stop_adsb():
     """Stop ADS-B tracking."""
-    global adsb_using_service
+    global adsb_using_service, adsb_active_device
 
     with app_module.adsb_lock:
         if app_module.adsb_process:
-            app_module.adsb_process.terminate()
             try:
+                # Kill the entire process group to ensure all child processes are terminated
+                pgid = os.getpgid(app_module.adsb_process.pid)
+                os.killpg(pgid, 15)  # SIGTERM
                 app_module.adsb_process.wait(timeout=ADSB_TERMINATE_TIMEOUT)
-            except subprocess.TimeoutExpired:
-                app_module.adsb_process.kill()
+            except (subprocess.TimeoutExpired, ProcessLookupError, OSError):
+                try:
+                    # Force kill if terminate didn't work
+                    pgid = os.getpgid(app_module.adsb_process.pid)
+                    os.killpg(pgid, 9)  # SIGKILL
+                except (ProcessLookupError, OSError):
+                    pass
             app_module.adsb_process = None
+            logger.info("ADS-B process stopped")
         adsb_using_service = False
+        adsb_active_device = None
 
     app_module.adsb_aircraft.clear()
     _looked_up_icaos.clear()