httpx randonly hangs on start_tls when using proxy

[alex-oleshkevich]

Checklist

• The bug is reproducible against the latest release and/or master.
• There are no similar issues or pull requests to fix it yet.

Describe the bug

I run a script that periodically parses a 3rd-party website. These requests are proxies.
The problem is that some times httpx may hang up completely.

To reproduce

There is no 100% way to reproduce it but here is a code that I run:

import httpx, asyncio

async for i in range(1, 120):
    async with httpx.AsyncClient(proxy={'all': 'http://someproxy.com') as client:
        await client.get('https://somesite.com')
    asyncio.sleep(2)

Note, all proxies a HTTP, not HTTPS.

Logs

DEBUG [2021-03-13 19:02:27] - [av.by] create HTTP client with this proxy: {'all': 'http://Qga8Jdsj:SGJuxHrH@45.136.68.4:46314'}.
TRACE [2021-03-13 19:02:27] httpx._config - load_ssl_context verify=True cert=None trust_env=True http2=False
TRACE [2021-03-13 19:02:27] - load_ssl_context verify=True cert=None trust_env=True http2=False
TRACE [2021-03-13 19:02:27] httpx._config - load_verify_locations cafile=/home/alex/projects/autobot-parsers/.venv/lib/python3.9/site-packages/certifi/cacert.pem
TRACE [2021-03-13 19:02:27] - load_verify_locations cafile=/home/alex/projects/autobot-parsers/.venv/lib/python3.9/site-packages/certifi/cacert.pem
TRACE [2021-03-13 19:02:27] httpcore._async.http_proxy - tunnel_request proxy_origin=(b'http', b'45.136.68.4', 46314) proxy_headers=[(b'Proxy-Authorization', b'Basic UWdhOEpkc2o6U000000=')] method=b'POST' url=(b'https', b'autobot.by', None, b'/offer-types/cars/filters/main/apply')
TRACE [2021-03-13 19:02:27] - tunnel_request proxy_origin=(b'http', b'45.136.68.4', 46314) proxy_headers=[(b'Proxy-Authorization', b'Basic UWdhOEpkc2o6U000000=')] method=b'POST' url=(b'https', b'autobot.by', None, b'/offer-types/cars/filters/main/apply')
TRACE [2021-03-13 19:02:27] httpcore._async.connection - open_socket origin=(b'http', b'45.136.68.4', 46314) timeout={'connect': 10, 'read': 10, 'write': 10, 'pool': 10}
TRACE [2021-03-13 19:02:27] - open_socket origin=(b'http', b'45.136.68.4', 46314) timeout={'connect': 10, 'read': 10, 'write': 10, 'pool': 10}
TRACE [2021-03-13 19:02:34] httpcore._async.connection - create_connection socket=<httpcore._backends.asyncio.SocketStream object at 0x7f1438cf1370> http_version='HTTP/1.1'
TRACE [2021-03-13 19:02:34] - create_connection socket=<httpcore._backends.asyncio.SocketStream object at 0x7f1438cf1370> http_version='HTTP/1.1'
TRACE [2021-03-13 19:02:34] httpcore._async.connection - connection.arequest method=b'CONNECT' url=(b'http', b'45.136.68.4', 46314, b'autobot.by:443') headers=[(b'Host', b'autobot.by:443'), (b'Accept', b'*/*'), (b'Proxy-Authorization', b'Basic UWdhOEpkc2o6U000000=')]
TRACE [2021-03-13 19:02:34] - connection.arequest method=b'CONNECT' url=(b'http', b'45.136.68.4', 46314, b'autobot.by:443') headers=[(b'Host', b'autobot.by:443'), (b'Accept', b'*/*'), (b'Proxy-Authorization', b'Basic UWdhOEpkc2o6U000000=')]
TRACE [2021-03-13 19:02:34] httpcore._async.http11 - send_request method=b'CONNECT' url=(b'http', b'45.136.68.4', 46314, b'autobot.by:443') headers=[(b'Host', b'autobot.by:443'), (b'Accept', b'*/*'), (b'Proxy-Authorization', b'Basic UWdhOEpkc2o6U000000=')]
TRACE [2021-03-13 19:02:34] - send_request method=b'CONNECT' url=(b'http', b'45.136.68.4', 46314, b'autobot.by:443') headers=[(b'Host', b'autobot.by:443'), (b'Accept', b'*/*'), (b'Proxy-Authorization', b'Basic UWdhOEpkc2o6U000000=')]
TRACE [2021-03-13 19:02:34] httpcore._async.http11 - send_data=Data(<0 bytes>)
TRACE [2021-03-13 19:02:34] - send_data=Data(<0 bytes>)
TRACE [2021-03-13 19:02:35] httpcore._async.http_proxy - tunnel_response proxy_status_code=200 proxy_reason='OK' 
TRACE [2021-03-13 19:02:35] - tunnel_response proxy_status_code=200 proxy_reason='OK' 
TRACE [2021-03-13 19:02:35] httpcore._async.http11 - receive_event=PAUSED
TRACE [2021-03-13 19:02:35] - receive_event=PAUSED
TRACE [2021-03-13 19:02:35] httpcore._async.connection - start_tls hostname=b'autobot.by' timeout={'connect': 10, 'read': 10, 'write': 10, 'pool': 10}
TRACE [2021-03-13 19:02:35] - start_tls hostname=b'autobot.by' timeout={'connect': 10, 'read': 10, 'write': 10, 'pool': 10}

There is nothing logged after this line.

Environment

• OS: Arch Linux
• Python version: 3.9
• HTTPX version: 0.17
• Async environment: asyncio
• HTTP proxy: yes
• Custom certificates: no

Additional context