SSLError when trying to reply disconnected streaming client

[lukjak]

python 3.12.8, werkzeug 3.1.3 on flask 3.1.0
(no issue with the previous setup: python 3.8, werkzeug 2.0.1 on flask 2.0.1 and the same flask app)

Conditions:

• werkzeug runs with SSL certificates set up (ssl_context provided to make_server, also threaded=True)
• there exists some specific network setup between werkzeug and connecting client; I haven't been able to reproduce the problem running the app locally and unfortunately I have no means to debug in the actual environment where the problem happens - I may try to check some specific things if needed
• a flask route is set up to produce SSE-compatible streaming response (return Response(stream_with_context(my_data_generator(), mimetype="text/event-stream")), 200)

Error scenario:

• a client successfully subscribes to the SSE stream (with GET /streaming-endpoint)
• after some the client is shut down (it may have received some messages already or not)
• my_data_generator() produces a new message
• werkzeug always fails when attempting to send it to the now-dead client with the below:

[ERROR] (_internal.py/_log:97) Error on request:
Traceback (most recent call last):
  File "D:\CondaEnvs\default\Lib\site-packages\werkzeug\serving.py", line 370, in run_wsgi
    execute(self.server.app)
  File "D:\CondaEnvs\default\Lib\site-packages\werkzeug\serving.py", line 334, in execute
    write(data)
  File "D:\CondaEnvs\default\Lib\site-packages\werkzeug\serving.py", line 306, in write
    self.wfile.write(hex(len(data))[2:].encode())
  File "D:\CondaEnvs\default\Lib\socketserver.py", line 840, in write
    self._sock.sendall(b)
  File "D:\CondaEnvs\default\Lib\ssl.py", line 1210, in sendall
    v = self.send(byte_view[count:])
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "D:\CondaEnvs\default\Lib\ssl.py", line 1179, in send
    return self._sslobj.write(data)
           ^^^^^^^^^^^^^^^^^^^^^^^^
ssl.SSLError: [SYS] unknown error (_ssl.c:2417)

Expected:

• the error is ignored