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From: Stefan Eissing <stefan.eissing@greenbytes.de>
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Subject: variables
Message-Id: <1A4A9DE8-ACF3-4FEB-BE4E-756E850ACF39@greenbytes.de>
Date: Wed, 16 Dec 2015 12:40:25 +0100
To: dev@httpd.apache.org
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I would like to expose some HTTP/2 properties, for example =
"HTTP2=3Don|off" similar to what mod_ssl does with HTTPS and other SSL =
variables. Current implementation does not look like something to =
repeat, at first glance, but maybe it's the only way...

The optional function ssl_is_https is used:
- from many modules
- server/util_expr_eval.c

The optional function ssl_var_lookup is used:
- from even more modules
- *not* in server

Subprocesses get their environment via r->subprocess_env, filled via the =
"fixups" hook. This is the usual way how we involve modules in request =
processing. But, as I read it, subprocess_env gets filled with all =
variables for every request, even though they maybe never read? Hmm, =
probably a little late to do anything about that without code breakage.

So, the least work and most 2.4.x compatible way for mod_http2 is to =
mimmic the mod_ssl way. http2_is_http2() and http2_var_lookup(), =
optional. Add fixups hook for subprocess_env, add $HTTP2 handling to =
server/util_expr_eval.c for checks in config/htaccess?

//Stefan

PS. For trunk, it might be a nice TODO to add a variable registry that =
allows lazy evaluation. Like with
  struct ap_var_handler_t {
    const char *name;
    const char *lookup(struct ap_var_handler_t *h, server_rec *s, =
conn_rec *c, request_rec *r);
    void *ctx;
    int server_specific :1;
    int conn_specific :1;
    int cacheable :1;
    int scope_config :1;
    int scope_subprocess :1;
  }