Strange status 500 with empty response

[Carlos Eduardo Ferreira Rodrigues]

Hi,

Since upgrading to nginx 1.8.0, we started seeing some requests being logged with status 500 and a response of 0 bytes ($bytes_sent). Not many, only one for about 36000 requests.
I've been trying to reliably reproduce this and/or figure out the cause without success, at least to know if this is an nginx bug or some module's. Any ideas or suggestions of how I can go about doing this?

These requests use proxy_pass (with cache) and header_filter_by_lua, so I've been able to figure out some things for these failed requests:

   * They are always a cache hit;
   * Headers created by the Lua code are correctly logged with $sent_http_*;
   * Errors in Lua code result in status 500 with a non-empty response (our custom error page);

Also, the error log shows nothing along with these errors.

Thanks in advance,

-- 
Carlos Rodrigues