gh-87079: Warn on unintended signal wakeup fd override in `asyncio` (#96807)

Warn on loop initialization, when setting the wakeup fd disturbs a previously set wakeup fd, and on loop closing, when upon resetting the wakeup fd, we find it has been changed by someone else.

1 files changed, 16 insertions, 3 deletions

diff --git a/Lib/asyncio/unix_events.py b/Lib/asyncio/unix_events.py
index cf7683f..6c9a89d 100644
--- a/Lib/asyncio/unix_events.py
+++ b/Lib/asyncio/unix_events.py
@@ -65,7 +65,9 @@ class _UnixSelectorEventLoop(selector_events.BaseSelectorEventLoop):
         self._signal_handlers = {}
 
     def close(self):
-        super().close()
+        # remove signal handlers first to verify
+        # the loop's signal handling setup has not
+        # been tampered with
         if not sys.is_finalizing():
             for sig in list(self._signal_handlers):
                 self.remove_signal_handler(sig)
@@ -77,6 +79,7 @@ class _UnixSelectorEventLoop(selector_events.BaseSelectorEventLoop):
                               ResourceWarning,
                               source=self)
                 self._signal_handlers.clear()
+        super().close()
 
     def _process_self_data(self, data):
         for signum in data:
@@ -102,7 +105,12 @@ class _UnixSelectorEventLoop(selector_events.BaseSelectorEventLoop):
             # main thread.  By calling it early we ensure that an
             # event loop running in another thread cannot add a signal
             # handler.
-            signal.set_wakeup_fd(self._csock.fileno())
+            oldfd = signal.set_wakeup_fd(self._csock.fileno())
+            if oldfd != -1 and oldfd != self._csock.fileno():
+                warnings.warn(
+                    "Signal wakeup fd was already set",
+                    ResourceWarning,
+                    source=self)
         except (ValueError, OSError) as exc:
             raise RuntimeError(str(exc))
 
@@ -166,7 +174,12 @@ class _UnixSelectorEventLoop(selector_events.BaseSelectorEventLoop):
 
         if not self._signal_handlers:
             try:
-                signal.set_wakeup_fd(-1)
+                oldfd = signal.set_wakeup_fd(-1)
+                if oldfd != -1 and oldfd != self._csock.fileno():
+                    warnings.warn(
+                        "Got unexpected signal wakeup fd",
+                        ResourceWarning,
+                        source=self)
             except (ValueError, OSError) as exc:
                 logger.info('set_wakeup_fd(-1) failed: %s', exc)