The fix in gh-91227 is incorrect, the SelectorEventLoop has the correct behaviour calling error_received and continuing to run the Transport #156784

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1. graingert

   changed the title [-]This fix in gh-91227 is incorrect, the SelectorEventLoop has the correct behavior raising the exception and continuing to run the Transport[/-] [+]This fix in gh-91227 is incorrect, the SelectorEventLoop has the correct behavior calling error_recieved and continuing to run the Transport[/+] on Sep 1, 2026
2. graingert commented on Sep 1, 2026

   

   graingert

   on Sep 1, 2026

   Last edited by graingert

   ContributorAuthor

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   """Demonstrates that sending a UDP datagram to a port that isn't
   listening now surfaces a recoverable error consistently on both
   SelectorEventLoop and ProactorEventLoop (see gh-91227, gh-127057):
   
     - DatagramProtocol.error_received() is called, and the transport
       keeps working afterwards instead of the read loop dying silently.
   
   Windows only. Run with: python snippet.py
   """
   import asyncio
   import socket
   
   
   class EchoProtocol(asyncio.DatagramProtocol):
       def connection_made(self, transport):
           self.transport = transport
           self.received = []
           self.errors = []
   
       def datagram_received(self, data, addr):
           self.received.append(data)
   
       def error_received(self, exc):
           self.errors.append(exc)
   
   
   def closed_socket_address():
       """Bind and immediately close a UDP socket to get an address that
       is guaranteed not to be listening."""
       sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
       sock.bind(('127.0.0.1', 0))
       addr = sock.getsockname()
       sock.close()
       return addr
   
   
   async def demo_transport(loop):
       print('--- DatagramProtocol.error_received() ---')
       sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
       sock.setblocking(False)
       sock.bind(('127.0.0.1', 0))
       own_addr = sock.getsockname()
   
       transport, protocol = await loop.create_datagram_endpoint(
           EchoProtocol, sock=sock)
   
       transport.sendto(b'ping', closed_socket_address())
   
       for _ in range(100):
           await asyncio.sleep(0.05)
           if protocol.errors:
               break
   
       if protocol.errors:
           print(f'error_received() fired: {protocol.errors[0]!r}')
       else:
           print('error_received() did NOT fire (timed out waiting)')
   
       # The transport must still work afterwards.
       transport.sendto(b'still alive', own_addr)
       for _ in range(100):
           await asyncio.sleep(0.05)
           if protocol.received:
               break
       print(f'transport still usable afterwards: {protocol.received!r}')
   
       transport.close()
   
   
   async def main(loop):
       await demo_transport(loop)
   
   
   def run(name, loop):
       print(f'\n=== {name} ===')
       try:
           loop.run_until_complete(main(loop))
       finally:
           loop.close()
   
   
   if __name__ == '__main__':
       run('SelectorEventLoop', asyncio.SelectorEventLoop())
       run('ProactorEventLoop', asyncio.ProactorEventLoop())

   on 3.14 it reports:

   PS C:\Users\tagra\projects\cpython> py -3.14 .\snippet.py
   
   === SelectorEventLoop ===
   --- DatagramProtocol.error_received() ---
   error_received() fired: ConnectionResetError(10054, 'An existing connection was forcibly closed by the remote host', None, 10054, None)
   transport still usable afterwards: [b'still alive']
   
   === ProactorEventLoop ===
   --- DatagramProtocol.error_received() ---
   error_received() did NOT fire (timed out waiting)
   transport still usable afterwards: [b'still alive']
   

   on my patched branch I get:

   PS C:\Users\tagra\projects\cpython> ./PCbuild/amd64/python_d.exe snippet.py
   
   === SelectorEventLoop ===
   --- DatagramProtocol.error_received() ---
   error_received() fired: ConnectionResetError(10054, 'An existing connection was forcibly closed by the remote host', None, 10054, None)
   transport still usable afterwards: [b'still alive']
   
   === ProactorEventLoop ===
   --- DatagramProtocol.error_received() ---
   error_received() fired: ConnectionResetError(22, 'No service is operating at the destination network endpoint on the remote system', None, 1234, None)
   transport still usable afterwards: [b'still alive']
   
3. graingert

   changed the title [-]This fix in gh-91227 is incorrect, the SelectorEventLoop has the correct behavior calling error_recieved and continuing to run the Transport[/-] [+]This fix in gh-91227 is incorrect, the SelectorEventLoop has the correct behavior calling error_received and continuing to run the Transport[/+] on Sep 1, 2026
4. graingert

   changed the title [-]This fix in gh-91227 is incorrect, the SelectorEventLoop has the correct behavior calling error_received and continuing to run the Transport[/-] [+]The fix in gh-91227 is incorrect, the SelectorEventLoop has the correct behaviour calling error_received and continuing to run the Transport[/+] on Sep 1, 2026
5. aisk

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   topic-asyncio

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   on Sep 2, 2026
6. github-project-automation

   moved this to Todo in asyncioon Sep 2, 2026
7. github-project-automation

   added this to asyncioon Sep 2, 2026
8. aisk

   added

   OS-windows

   on Sep 3, 2026
9. kumaraditya303

   closed this as completedon Sep 16, 2026
10. github-project-automation

    moved this from Todo to Done in asyncioon Sep 16, 2026
11. graingert

    reopened this on Sep 16, 2026
12. github-project-automation

    moved this from Done to In Progress in asyncioon Sep 16, 2026
13. graingert commented on Sep 16, 2026

    

    graingert

    on Sep 16, 2026

    ContributorAuthor

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    @kumaraditya303 this isn't fixed yet, so I re-opened it