Hi,

Would it not be a good idea to add tests at conf-read time to see if
headers/filters need to be added into the source code to avoid the
unnecessary overhead of checking whether they need to be used or not at
runtime?

For example, the SSI module. The filter is automatically added to the
list in:

static ngx_int_t
ngx_http_ssi_filter_init(ngx_conf_t *cf)
{
ngx_http_next_header_filter = ngx_http_top_header_filter;
ngx_http_top_header_filter = ngx_http_ssi_header_filter;

ngx_http_next_body_filter = ngx_http_top_body_filter;
ngx_http_top_body_filter = ngx_http_ssi_body_filter;

return NGX_OK;
}

I believe a better implementation would include a variable (it might as
well be global) to test whether the filter needs to be installed, e.g.

ngx_int_t http_ssi_filter_use = 0;
...
[NEW FUNCTION - replacing the use of ngx_conf_set_flag_slot for
directive 'ssi']
static char*
ngx_http_ssi (ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
{
// test for on/off (like ngx_conf_set_flag_slot)
http_ssi_filter_use = 1;
return NGX_CONF_OK;
}

static ngx_int_t
ngx_http_ssi_filter_init (ngx_conf_t *cf)
{
if (http_ssi_filter_use) {
ngx_http_next_header_filter = ngx_http_top_header_filter;
ngx_http_top_header_filter = ngx_http_ssi_header_filter;

ngx_http_next_body_filter = ngx_http_top_body_filter;
ngx_http_top_body_filter = ngx_http_ssi_body_filter;
}

return NGX_OK;
}


Doing this for all the modules, including headers and footers (if it's
not already done) I think would make a difference in speed. Some
modules wouldn't need creating a new function, just adding the
http_[module]_use = 1 to an existing conf function, and the tests for
adding handlers / filters.

Most configurations probably do not use that many handlers / filters,
and by eliminating all the additional function pointer calls, checks and
returns could add a little performance increase to the server as a
whole, and would not affect the functionality of the server in any way.

Cheers,

Marcus.