I have been getting an intermittent errors when using asyncio with SSL. The error always occurs in the _process_write_backlog method in asyncio's sslproto.py file. I have looked at lots of possibilities as to what the cause is and found that for some reason when in _process_write_backlog's loop the deque seems to be empty, I added some quick terrible hacky code to confirm it fixed the issue and checking at each point it is used wether it is empty fixes the issue, I am unusure as to what causes it to become empty but still run through the loop. The most frequent time it happens is after we have a successful message the client sends a request to join a data stream this request mostly causes the error but sometimes it happens while the client is receiving data. I am currently using python 3.7.1 but have also tested my code on 3.7.3 with the same result. 

NOTE: I am currently working on a minimal sample to show the issue easier.

Fatal error on SSL transport
protocol: <asyncio.sslproto.SSLProtocol object at 0x7f267462e780>
transport: <_SelectorSocketTransport fd=38 read=polling write=<idle, bufsize=0>>
Traceback (most recent call last):
    File "/usr/local/lib/python3.7/asyncio/sslproto.py", line 689, in _process_write_backlog
    del self._write_backlog[0]
IndexError: deque index out of range

Fatal error on SSL transport
protocol: <asyncio.sslproto.SSLProtocol object at 0x7f45f802ec88>
transport: <_SelectorSocketTransport fd=29 read=polling write=<idle, bufsize=0>>
Traceback (most recent call last):
    File "/usr/local/lib/python3.7/asyncio/sslproto.py", line 664, in _process_write_backlog
    data, offset = self._write_backlog[0]
IndexError: deque index out of range

This looks more like an asyncio problem than an SSL problem. Andrew, Yury,  please take a look.

Just to clarify: what OpenSSL version is used?

The OpenSSL version is OpenSSL 1.1.0j  20 Nov 2018

That's what I afraid.
asyncio/sslproto.py fails on new OpenSSL at several places :(

That's a shame, which version should it work on I don't mind downgrading for now to fix the issue as a workaround.

Is 1.1.0 also a problem? I have only seen issue with 1.1.1 and TLS 1.3.

I am using the version I mentioned above 1.1.0 and TLS 1.2, I am sorry I can't be of more help.

I tested an older version of OpenSSL 1.0.2g and I get the same error on that

I have created a minimal example if that helps to show the issue https://gist.github.com/bobthemac/031213b8e37960ee805f2ae1e6990b60

Thank you!
Yuri, you might be interested too

Hi is there any update on this issue, were you able to replicate the error with the minimal example I provided or is there any additional information I can provide.

Any updates on this issue? We're also encountering it as part of our websocket clients.

Maayan Keshet do you have a minimal example it would be interesting to compare against our code and see if there is something we are both doing that could help narrow down the issue.

Hey Ben! Unfortunately our use case is too involved to distill into a neat example. We use a websocket library (autobahn) to connect to a server of a 3rd party which is outside our control (which is not written in python, but rather node.js). We receive the errors on our client.

With some more research it looks like the issue is flow related, I experimented with the StreamWriter in a simple server and using that plus drain() appears to have worked an I no longer get the error. I have added my new server to the gist I posted above.

I can fully confirm the issue is due to flow control or lack of in my code the system runs out of resources and this is when it errors, I have implemented flow control in my protocol and it now works without errors. One thing I did find is that the documentation on flow control was lacking but luckily I found this post which makes implementation clearer https://medium.com/@pgjones/an-asyncio-socket-tutorial-5e6f3308b8b0

I am not sure if you would still classify this as a bug the error could be clearer but it could be expected behaviour when you run out of resources, I think this can now be closed I am just unsure of the etiquette as to who closes it and what resolution should be selected.