Make firmware sweep cancellable to prevent shutdown deadlock

send_with_timeout now uses a 2s per-byte timeout with a deadline
loop instead of one long blocking read, checking a cancel event
between reads. SerialBridge.disconnect() sets the cancel event
before acquiring the lock, so a blocked firmware sweep aborts
within ~2s and releases the lock for clean port shutdown.

src/birdcage/protocol.py

@@ -10,6 +10,7 @@ from __future__ import annotations
 
 import logging
 import re
+import threading
 import time
 from abc import ABC, abstractmethod
 from collections.abc import Callable
@@ -420,20 +421,55 @@ class CarryoutG2Protocol(FirmwareProtocol):
 
         raise ValueError(f"Could not parse RSSI from: {response!r}")
 
-    def send_with_timeout(self, cmd: str, timeout: float = 90) -> str:
-        """Send a command with a custom serial timeout.
+    def send_with_timeout(
+        self,
+        cmd: str,
+        timeout: float = 90,
+        cancel: threading.Event | None = None,
+    ) -> str:
+        """Send a command with a deadline-based timeout and optional cancellation.
 
-        Used for long-running firmware commands (azscanwxp, azscan) that stream
-        output over tens of seconds before the final prompt.
+        Uses a short per-byte read timeout (2s) so the cancel event can be
+        checked between reads. The overall deadline limits total wall-clock
+        time. Used for long-running firmware commands (azscanwxp, azscan)
+        that stream output before the final prompt.
+
+        Args:
+            cmd: Command string to send.
+            timeout: Maximum wall-clock seconds to wait for the prompt.
+            cancel: Optional event; if set, the read aborts immediately.
+
+        Raises:
+            InterruptedError: If *cancel* is set during the read.
+            TimeoutError: If the prompt is not received within *timeout*.
         """
         if not self._serial:
             raise RuntimeError("Not connected")
-        original = self._serial.timeout
-        self._serial.timeout = timeout
+
+        original_timeout = self._serial.timeout
+        self._serial.timeout = 2.0  # short per-byte timeout for cancel checks
         try:
-            return self._send(cmd)
+            self._serial.write(f"{cmd}\r".encode("ascii"))
+
+            resp_data = bytearray()
+            deadline = time.monotonic() + timeout
+            while time.monotonic() < deadline:
+                if cancel and cancel.is_set():
+                    raise InterruptedError(f"Cancelled during: {cmd!r}")
+
+                byte = self._serial.read(1)
+                if len(byte) == 0:
+                    continue  # short timeout expired, loop and check again
+                resp_data.append(byte[0])
+                if byte[0] == self.PROMPT_CHAR:
+                    break
+            else:
+                raise TimeoutError(f"No prompt after {timeout}s for: {cmd!r}")
+
+            time.sleep(0.001)
+            return resp_data.decode("utf-8", errors="ignore")
         finally:
-            self._serial.timeout = original
+            self._serial.timeout = original_timeout
 
     def send_raw(self, cmd: str) -> str:
         """Send arbitrary command, return raw prompt-terminated response."""

tui/src/birdcage_tui/bridge.py

@@ -63,6 +63,7 @@ class SerialBridge:
     def __init__(self, protocol: CarryoutG2Protocol) -> None:
         self._proto = protocol
         self._lock = threading.Lock()
+        self._cancel = threading.Event()
         self._menu = Menu.UNKNOWN
         self._connected = False
 
@@ -158,13 +159,20 @@ class SerialBridge:
             self._menu = self._detect_menu()
 
     def disconnect(self) -> None:
-        """Close the serial connection."""
+        """Close the serial connection.
+
+        Signals cancellation first to unblock any long-running serial
+        reads (e.g. firmware sweep), then acquires the lock to close
+        the port cleanly.
+        """
+        self._cancel.set()
         with self._lock:
             with contextlib.suppress(Exception):
                 self._go_to_root()
             self._proto.disconnect()
             self._connected = False
             self._menu = Menu.UNKNOWN
+            self._cancel.clear()  # reset for potential reconnect
 
     @property
     def is_connected(self) -> bool:
@@ -424,9 +432,11 @@ class SerialBridge:
             # Move to start position and wait for prompt.
             self._send(f"a 0 {start_az}")
             # Execute firmware sweep with extended timeout.
+            # Pass cancel event so disconnect() can interrupt the read.
             response = self._proto.send_with_timeout(
                 f"azscanwxp 0 {span} {step_cdeg} {num_xponders}",
                 timeout=timeout,
+                cancel=self._cancel,
             )
 
         # Parse streaming output lines.