Set opentracing context for a request in an earlier phase

[ElvinEfendi]

I'm working on kubernetes/ingress-nginx#3783 to enable tracing of each Nginx phase we run Lua code. I've noticed that ngx.var.opentracing_binary_context is not set at rewrite phase and when I try to access it in Lua I get:

2019/02/20 14:40:54 [error] 42#42: *374 failed to expand opentracing_context_ for request 0000564FD8E6D060: no OpenTracingContext attached to request, client: 172.17.0.1, server: example.com, request: "GET /echo HTTP/1.1", host: "example.com"

Looking at the source code, I noticed that opentracing runs after rewrite phase:

nginx-opentracing/opentracing/src/ngx_http_opentracing_module.cpp

Line 172 in 9cde691

&core_main_config->phases[NGX_HTTP_PREACCESS_PHASE].handlers));

Can we inject it at NGX_HTTP_POST_READ_PHASE or NGX_HTTP_SERVER_REWRITE_PHASE? According to http://nginx.org/en/docs/dev/development_guide.html#http_phases NGX_HTTP_POST_READ_PHASE is the earliest phase.

[rnburn]

I think that would be ok. Can you try making the change and seeing if it breaks anything?

[ElvinEfendi]

I'll give it a try, but not sure when.

[terrynsun]

I moved it to the SERVER_REWRITE phase, because when I moved it earlier than the SERVER_REWRITE phase, the module doesn't get loaded for subrequests. This seems to work fine for me. [Edit: except apparently I'm not passing the tests. I'll take a look.]

[ElvinEfendi]

@terrynsun SERVER_REWRITE is probably good enough, maybe open a PR so that the maintainer can help with tests.

[rnburn]

Done in #98