X-Forwarded-Host to X_FORWARDED_HOST in mod_wsgi..

Ever wondered how the parameters in Django's request.META, such as X-Forwarded-Host or X-Forwarded-For get set in the os.environ objects? The headers often get transformed to entire upper-case and the dashes get replaced by underscores (i.e. X-Forwarded-Host becomes X_FORWARDED_HOST). Most of the work gets done inside mod_wsgi.c using the wsgi_http2env() function.

static PyObject *Auth_environ(AuthObject *self, const char *group)
{
            PyDict_SetItemString(vars, wsgi_http2env(r->pool, hdrs[i].key),
                                 object);

The wsgi_http2env() function basically performs this conversion to HTTP environment variables:

static char *wsgi_http2env(apr_pool_t *a, const char *w)
{
    char *res = (char *)apr_palloc(a, sizeof("HTTP_") + strlen(w));
    char *cp = res;
    char c;

    *cp++ = 'H';
    *cp++ = 'T';
    *cp++ = 'T';
    *cp++ = 'P';
    *cp++ = '_';

    while ((c = *w++) != 0) {
        if (!apr_isalnum(c)) {
            *cp++ = '_';
        }
        else {
            *cp++ = apr_toupper(c);
        }
    }
    *cp = 0;

    return res;
}