asyncio: __del__() keep reference to warnings.warn (GH-11491)

* asyncio: __del__() keep reference to warnings.warn

The __del__() methods of asyncio classes now keep a strong reference
to the warnings.warn() to be able to display the ResourceWarning
warning in more cases. Ensure that the function remains available if
instances are destroyed late during Python shutdown (while module
symbols are cleared).

* Rename warn parameter to _warn

"_warn" name is a hint that it's not the regular warnings.warn()
function.

Lib/asyncio/base_events.py

@@ -622,10 +622,9 @@ class BaseEventLoop(events.AbstractEventLoop):
         """Returns True if the event loop was closed."""
         return self._closed
 
-    def __del__(self):
+    def __del__(self, _warn=warnings.warn):
         if not self.is_closed():
-            warnings.warn(f"unclosed event loop {self!r}", ResourceWarning,
-                          source=self)
+            _warn(f"unclosed event loop {self!r}", ResourceWarning, source=self)
             if not self.is_running():
                 self.close()
 

Lib/asyncio/base_subprocess.py

@@ -120,10 +120,9 @@ class BaseSubprocessTransport(transports.SubprocessTransport):
 
             # Don't clear the _proc reference yet: _post_init() may still run
 
-    def __del__(self):
+    def __del__(self, _warn=warnings.warn):
         if not self._closed:
-            warnings.warn(f"unclosed transport {self!r}", ResourceWarning,
-                          source=self)
+            _warn(f"unclosed transport {self!r}", ResourceWarning, source=self)
             self.close()
 
     def get_pid(self):

Lib/asyncio/proactor_events.py

@@ -89,10 +89,9 @@ class _ProactorBasePipeTransport(transports._FlowControlMixin,
             self._read_fut.cancel()
             self._read_fut = None
 
-    def __del__(self):
+    def __del__(self, _warn=warnings.warn):
         if self._sock is not None:
-            warnings.warn(f"unclosed transport {self!r}", ResourceWarning,
-                          source=self)
+            _warn(f"unclosed transport {self!r}", ResourceWarning, source=self)
             self.close()
 
     def _fatal_error(self, exc, message='Fatal error on pipe transport'):

Lib/asyncio/selector_events.py

@@ -658,10 +658,9 @@ class _SelectorTransport(transports._FlowControlMixin,
             self._loop._remove_writer(self._sock_fd)
             self._loop.call_soon(self._call_connection_lost, None)
 
-    def __del__(self):
+    def __del__(self, _warn=warnings.warn):
         if self._sock is not None:
-            warnings.warn(f"unclosed transport {self!r}", ResourceWarning,
-                          source=self)
+            _warn(f"unclosed transport {self!r}", ResourceWarning, source=self)
             self._sock.close()
 
     def _fatal_error(self, exc, message='Fatal error on transport'):

Lib/asyncio/sslproto.py

@@ -316,10 +316,9 @@ class _SSLProtocolTransport(transports._FlowControlMixin,
         self._closed = True
         self._ssl_protocol._start_shutdown()
 
-    def __del__(self):
+    def __del__(self, _warn=warnings.warn):
         if not self._closed:
-            warnings.warn(f"unclosed transport {self!r}", ResourceWarning,
-                          source=self)
+            _warn(f"unclosed transport {self!r}", ResourceWarning, source=self)
             self.close()
 
     def is_reading(self):

Lib/asyncio/unix_events.py

@@ -511,10 +511,9 @@ class _UnixReadPipeTransport(transports.ReadTransport):
         if not self._closing:
             self._close(None)
 
-    def __del__(self):
+    def __del__(self, _warn=warnings.warn):
         if self._pipe is not None:
-            warnings.warn(f"unclosed transport {self!r}", ResourceWarning,
-                          source=self)
+            _warn(f"unclosed transport {self!r}", ResourceWarning, source=self)
             self._pipe.close()
 
     def _fatal_error(self, exc, message='Fatal error on pipe transport'):
@@ -707,10 +706,9 @@ class _UnixWritePipeTransport(transports._FlowControlMixin,
             # write_eof is all what we needed to close the write pipe
             self.write_eof()
 
-    def __del__(self):
+    def __del__(self, _warn=warnings.warn):
         if self._pipe is not None:
-            warnings.warn(f"unclosed transport {self!r}", ResourceWarning,
-                          source=self)
+            _warn(f"unclosed transport {self!r}", ResourceWarning, source=self)
             self._pipe.close()
 
     def abort(self):

Lib/asyncio/windows_utils.py

@@ -107,10 +107,9 @@ class PipeHandle:
             CloseHandle(self._handle)
             self._handle = None
 
-    def __del__(self):
+    def __del__(self, _warn=warnings.warn):
         if self._handle is not None:
-            warnings.warn(f"unclosed {self!r}", ResourceWarning,
-                          source=self)
+            _warn(f"unclosed {self!r}", ResourceWarning, source=self)
             self.close()
 
     def __enter__(self):