(I'm not sure if this is a newbie question or a bug report or something in
between. I apologize in advance if its off-topic. Let me know if I should
post this somewhere else.)
If a task is cancelled while SSL is being negotiated, then an SSLError is
raised, but there's 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)
Is this a bug that I should file, or is there some reason that it's
intended to work this way? I can work around it with asyncio.shield(), but
I think I would prefer for the asyncio/sslproto.py to catch the SSLError
and ignore it. Maybe I'm being short sighted.
Thanks,
Mark
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