[alert] epoll_ctl(1, 575) failed (17: File exists)

[Valentin V. Bartenev]

On Wednesday 11 October 2017 15:58:28 Richard Stanway via nginx wrote:
> Hello,
> I have a location that proxies to a websocket server. Clients connect over
> HTTPS (HTTP2, wss://). Sometimes clients generate the following alerts in
> the error log when hitting the websocket location:
> 
> 2017/10/11 21:03:23 [alert] 34381#34381: *1020125 epoll_ctl(1, 603) failed
> (17: File exists) while proxying upgraded connection, client: x.158,
> server: www.example.com, request: "GET /websocketpath HTTP/2.0", upstream:
> "http://<websocketserver>/<url>", host: "www.example.com"
> 
> 2017/10/11 21:44:15 [alert] 34374#34374: *1274194 epoll_ctl(1, 1131) failed
> (17: File exists) while proxying upgraded connection, client: x.42, server:
> www.example.com, request: "GET /websocketpath HTTP/2.0", upstream:
> "http://<websocketserver>/<url>",
> host: "www.example.com"
> 
[..]
> 
> This seems to have started after upgrading to Debian 9 (which upgraded the
> OpenSSL library, allowing ALPN and thus HTTP2 to be usable). Previously the
> connections were mostly HTTP/1.1 and I didn't notice any such messages.
> 
> Despite the alerts, the access log shows the clients with a 101 status code.
> 
> Any idea if this is something on my end I should start looking at, or is
> this a possible issue with http2 and websockets?
> 
[..]

Websockets cannot work over HTTP/2.

  wbr, Valentin V. Bartenev