gh-145342: Add _guest_mode flag and guest-mode signal-handler guards

In guest mode the host event loop owns signal handling:
add_signal_handler() and remove_signal_handler() raise RuntimeError so
that asyncio never touches the process-global signal wakeup fd.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: congzhangzh

Lib/asyncio/base_events.py

@@ -432,6 +432,10 @@ def __init__(self):
         # Identifier of the thread running the event loop, or None if the
         # event loop is not running
         self._thread_id = None
+        # True while the loop is driven as a guest inside a host event
+        # loop (see asyncio.start_guest_run); signal handling is then
+        # delegated to the host.
+        self._guest_mode = False
         self._clock_resolution = time.get_clock_info('monotonic').resolution
         self._exception_handler = None
         self.set_debug(coroutines._is_debug_mode())

Lib/asyncio/unix_events.py

@@ -100,6 +100,11 @@ def add_signal_handler(self, sig, callback, *args):
                             "with add_signal_handler()")
         self._check_signal(sig)
         self._check_closed()
+        if self._guest_mode:
+            raise RuntimeError(
+                "add_signal_handler() is not supported in asyncio guest "
+                "mode; the host event loop owns signal handling -- "
+                "forward signals to the loop with call_soon_threadsafe()")
         try:
             # set_wakeup_fd() raises ValueError if this is not the
             # main thread.  By calling it early we ensure that an
@@ -149,6 +154,10 @@ def remove_signal_handler(self, sig):
         Return True if a signal handler was removed, False if not.
         """
         self._check_signal(sig)
+        if self._guest_mode:
+            raise RuntimeError(
+                "remove_signal_handler() is not supported in asyncio "
+                "guest mode; the host event loop owns signal handling")
         try:
             del self._signal_handlers[sig]
         except KeyError: