New submission from Mark Haase:
If a task is cancelled while it waiting for SSL negotiation, then an SSLError
is raised, but there is no way (as far as I can tell) for the caller to catch
it. (The example below is pretty contrived, but in an application I'm working
on, the user can cancel downloads at any time.) Here's an example:
import asyncio, random, ssl
async def download(host):
ssl_context = ssl.create_default_context()
reader, writer = await asyncio.open_connection(host, 443,
ssl=ssl_context)
request = f'HEAD / HTTP/1.1\r\nHost: {host}\r\n\r\n'
writer.write(request.encode('ascii'))
lines = list()
while True:
newdata = await reader.readline()
if newdata == b'\r\n':
break
else:
lines.append(newdata.decode('utf8').rstrip('\r\n'))
return lines[0]
async def main():
while True:
task = asyncio.Task(download('www.python.org'))
await asyncio.sleep(random.uniform(0.0, 0.5))
task.cancel()
try:
response = await task
print(response)
except asyncio.CancelledError:
print('request cancelled!')
except ssl.SSLError:
print('caught SSL error')
await asyncio.sleep(1)
loop = asyncio.get_event_loop()
loop.run_until_complete(main())
loop.close()
Running this script yields the following output:
HTTP/1.1 200 OK
request cancelled!
HTTP/1.1 200 OK
HTTP/1.1 200 OK
<asyncio.sslproto.SSLProtocol object at 0x7fe7c00e5a20>: SSL handshake
failed
Traceback (most recent call last):
File "/usr/lib/python3.6/asyncio/base_events.py", line 803, in
_create_connection_transport
yield from waiter
File "/usr/lib/python3.6/asyncio/tasks.py", line 304, in _wakeup
future.result()
concurrent.futures._base.CancelledError
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/usr/lib/python3.6/asyncio/sslproto.py", line 577, in
_on_handshake_complete
raise handshake_exc
File "/usr/lib/python3.6/asyncio/sslproto.py", line 638, in
_process_write_backlog
ssldata = self._sslpipe.shutdown(self._finalize)
File "/usr/lib/python3.6/asyncio/sslproto.py", line 155, in shutdown
ssldata, appdata = self.feed_ssldata(b'')
File "/usr/lib/python3.6/asyncio/sslproto.py", line 219, in feed_ssldata
self._sslobj.unwrap()
File "/usr/lib/python3.6/ssl.py", line 692, in unwrap
return self._sslobj.shutdown()
ssl.SSLError: [SSL] shutdown while in init (_ssl.c:2299)
I posted this on async-sig, and Nathaniel replied:
> SSLObject.unwrap has the contract that if it finishes successfully, then the
> SSL connection has been cleanly shut down and both sides remain in sync, and
> can continue to use the socket in unencrypted mode. When asyncio calls unwrap
> before the handshake has completed, then this contract is impossible to
> fulfill, and raising an error is the right thing to do. So imo the ssl module
> is correct here, and this is a (minor) bug in asyncio.
The unwrap() call that throws is already wrapped in `try ... except SSLError`
but the exception handler checks for specific SSL error codes and re-throws
this particular SSL error.
except (ssl.SSLError, ssl.CertificateError) as exc:
if getattr(exc, 'errno', None) not in (
ssl.SSL_ERROR_WANT_READ, ssl.SSL_ERROR_WANT_WRITE,
ssl.SSL_ERROR_SYSCALL):
if self._state == _DO_HANDSHAKE and self._handshake_cb:
self._handshake_cb(exc)
raise
self._need_ssldata = (exc.errno == ssl.SSL_ERROR_WANT_READ)
I think this could be fixed checking for SSL_R_SHUTDOWN_WHILE_IN_INIT in this
exception handler, but that constant doesn't exist in _ssl.c.
As an alternative, maybe a new state _ABORT_HANDSHAKE could be introduced (the
existing states are _DO_HANDSHAKE, _WRAPPED, _SHUTDOWN, and _UNWRAP).
I'm happy to try my hand at either one of these approaches if somebody can
point me in the right direction.
----------
components: asyncio
messages: 296720
nosy: mehaase, yselivanov
priority: normal
severity: normal
status: open
title: SSLError when cancelling an SSL connection
versions: Python 3.6
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue30740>
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