Safe accessors for the module configuration

[bavshin-f5]

There's a certain unsafe pattern that is repeated in most of the examples:

let co = unsafe { request.get_module_loc_conf::<ModuleConfig>(&*addr_of!(ngx_http_curl_module)) };
let co = co.expect("module config is none");

The problems here are:

• ngx_http_curl_module is a global mutable static, requiring unsafe for any access.
• Nothing guarantees that the type requested matches an actual type allocated for the module config.

Something we can optimize here is to tie config type to a context type and to a module ptr, and hide most of the unsafe details from end user.

E.g.

impl HttpLocationConf for ngx_http_core_module_t {
    type ConfType = ngx_http_core_loc_conf_t;

    fn module() -> &'static ngx_module_t { unsafe { &*addr_of!(ngx_http_core_module) }  }
}

fn get_loc_conf<T: HttpLocationConf>(req) -> <T as HttpLocationConf>::ConfType;

req.get_loc_conf::<ngx_http_core_module_t>().ok_or(...)?;

or with an opposite direction of the mapping

impl HttpModuleConf for ngx_http_core_loc_conf_t {
    const CONTEXT: &HttpModuleConfType = HttpModuleConfType::Location;

    fn module() -> &'static ngx_module_t { unsafe { &*addr_of!(ngx_http_core_module) }  }
}

fn get_conf<T: HttpModuleConf>(req) -> &T;

req.get_conf::<ngx_http_core_loc_conf_t>().ok_or(...)?;

Extending the ngx::http::module::HTTPModule trait is also a viable option.

An important thing to consider is that the module configs can be obtained from ngx_cycle_t, ngx_conf_t, ngx_http_request_t or ngx_http_upstream_t objects and all of those should provide safe accessors.