mod_filter.c

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        else if (!strcasecmp(arg, "change=1:1")) {
            flags |= AP_FILTER_PROTO_CHANGE;
        }
        else if (!strcasecmp(arg, "byteranges=no")) {
            flags |= AP_FILTER_PROTO_NO_BYTERANGE;
        }
        else if (!strcasecmp(arg, "proxy=no")) {
            flags |= AP_FILTER_PROTO_NO_PROXY;
        }
        else if (!strcasecmp(arg, "proxy=transform")) {
            flags |= AP_FILTER_PROTO_TRANSFORM;
        }
        else if (!strcasecmp(arg, "cache=no")) {
            flags |= AP_FILTER_PROTO_NO_CACHE;
        }
    }

    if (pname) {
        provider->frec->proto_flags = flags;
    }
    else {
        filter->proto_flags = flags;
    }

    return NULL;
}
#endif

static const char *filter_declare(cmd_parms *cmd, void *CFG, const char *fname,
                                  const char *place)
{
    mod_filter_cfg *cfg = (mod_filter_cfg *)CFG;
    ap_filter_rec_t *filter;

    filter = apr_pcalloc(cmd->pool, sizeof(ap_filter_rec_t));
    apr_hash_set(cfg->live_filters, fname, APR_HASH_KEY_STRING, filter);

    filter->name = fname;
    filter->filter_init_func = filter_init;
    filter->filter_func.out_func = filter_harness;
    filter->ftype = AP_FTYPE_RESOURCE;
    filter->next = NULL;

    if (place) {
        if (!strcasecmp(place, "CONTENT_SET")) {
            filter->ftype = AP_FTYPE_CONTENT_SET;
        }
        else if (!strcasecmp(place, "PROTOCOL")) {
            filter->ftype = AP_FTYPE_PROTOCOL;
        }
        else if (!strcasecmp(place, "CONNECTION")) {
            filter->ftype = AP_FTYPE_CONNECTION;
        }
        else if (!strcasecmp(place, "NETWORK")) {
            filter->ftype = AP_FTYPE_NETWORK;
        }
    }

    return NULL;
}

static const char *filter_provider(cmd_parms *cmd, void *CFG, const char *args)
{
    mod_filter_cfg *cfg = CFG;
    int flags;
    ap_filter_provider_t *provider;
    const char *rxend;
    const char *c;
    char *str;
    const char *eq;
    ap_filter_rec_t* frec;
    ap_filter_rec_t* provider_frec;

    /* insist on exactly four arguments */
    const char *fname = ap_getword_conf(cmd->pool, &args) ;
    const char *pname = ap_getword_conf(cmd->pool, &args) ;
    const char *condition = ap_getword_conf(cmd->pool, &args) ;
    const char *match = ap_getword_conf(cmd->pool, &args) ;
    eq = ap_getword_conf(cmd->pool, &args) ;
    if ( !*fname || !*pname || !*match || !*condition || *eq ) {
        return "usage: FilterProvider filter provider condition match" ;
    }

    /* fname has been declared with DeclareFilter, so we can look it up */
    frec = apr_hash_get(cfg->live_filters, fname, APR_HASH_KEY_STRING);

    /* or if provider is mod_filter itself, we can also look it up */
    if (!frec) {
        c = filter_declare(cmd, CFG, fname, NULL);
        if ( c ) {
            return c;
        }
        frec = apr_hash_get(cfg->live_filters, fname, APR_HASH_KEY_STRING);
    }

    if (!frec) {
        return apr_psprintf(cmd->pool, "Undeclared smart filter %s", fname);
    }

    /* if provider has been registered, we can look it up */
    provider_frec = ap_get_output_filter_handle(pname);
    if (!provider_frec) {
        return apr_psprintf(cmd->pool, "Unknown filter provider %s", pname);
    }

    provider = apr_palloc(cmd->pool, sizeof(ap_filter_provider_t));
    if (*match == '!') {
        provider->not = 1;
        ++match;
    }
    else {
        provider->not = 0;
    }

    switch (*match++) {
    case '<':
        if (*match == '=') {
            provider->match_type = INT_LE;
            ++match;
        }
        else {
            provider->match_type = INT_LT;
        }
        provider->match.number = atoi(match);
        break;
    case '>':
        if (*match == '=') {
            provider->match_type = INT_GE;
            ++match;
        }
        else {
            provider->match_type = INT_GT;
        }
        provider->match.number = atoi(match);
        break;
    case '=':
        provider->match_type = INT_EQ;
        provider->match.number = atoi(match);
        break;
    case '/':
        provider->match_type = REGEX_MATCH;
        rxend = ap_strchr_c(match, '/');
        if (!rxend) {
              return "Bad regexp syntax";
        }
        flags = AP_REG_NOSUB;        /* we're not mod_rewrite:-) */
        for (c = rxend+1; *c; ++c) {
            switch (*c) {
            case 'i': flags |= AP_REG_ICASE; break;
            }
        }
        provider->match.regex = ap_pregcomp(cmd->pool,
                                            apr_pstrndup(cmd->pool,
                                                         match,
                                                         rxend-match),
                                            flags);
        if (provider->match.regex == NULL) {
            return "Bad regexp";
        }
        break;
    case '*':
        provider->match_type = DEFINED;
        provider->match.number = -1;
        break;
    case '$':
        provider->match_type = STRING_CONTAINS;
        str = apr_pstrdup(cmd->pool, match);
        ap_str_tolower(str);
        provider->match.string = str;
        break;
    default:
        provider->match_type = STRING_MATCH;
        provider->match.string = apr_pstrdup(cmd->pool, match-1);
        break;
    }
    provider->frec = provider_frec;
    provider->next = frec->providers;
    frec->providers = provider;

    /* determine what a filter will dispatch this provider on */
    eq = ap_strchr_c(condition, '=');
    if (eq) {
        str = apr_pstrdup(cmd->pool, eq+1);
        if (!strncasecmp(condition, "env=", 4)) {
            provider->dispatch = SUBPROCESS_ENV;
        }
        else if (!strncasecmp(condition, "req=", 4)) {
            provider->dispatch = REQUEST_HEADERS;
        }
        else if (!strncasecmp(condition, "resp=", 5)) {
            provider->dispatch = RESPONSE_HEADERS;
        }
        else {
            return "FilterProvider: unrecognized dispatch table";
        }
    }
    else {
        if (!strcasecmp(condition, "handler")) {
            provider->dispatch = HANDLER;
        }
        else {
            provider->dispatch = RESPONSE_HEADERS;
        }
        str = apr_pstrdup(cmd->pool, condition);
        ap_str_tolower(str);
    }

    if (   (provider->dispatch == RESPONSE_HEADERS)
        && !strcasecmp(str, "content-type")) {
        provider->dispatch = CONTENT_TYPE;
    }
    provider->value = str;

    return NULL;
}

static const char *filter_chain(cmd_parms *cmd, void *CFG, const char *arg)
{
    mod_filter_chain *p;
    mod_filter_chain *q;
    mod_filter_cfg *cfg = CFG;

    switch (arg[0]) {
    case '+':        /* add to end of chain */
        p = apr_pcalloc(cmd->pool, sizeof(mod_filter_chain));
        p->fname = arg+1;
        if (cfg->chain) {
            for (q = cfg->chain; q->next; q = q->next);
            q->next = p;
        }
        else {
            cfg->chain = p;
        }
        break;

    case '@':        /* add to start of chain */
        p = apr_palloc(cmd->pool, sizeof(mod_filter_chain));
        p->fname = arg+1;
        p->next = cfg->chain;
        cfg->chain = p;
        break;

    case '-':        /* remove from chain */
        if (cfg->chain) {
            if (strcasecmp(cfg->chain->fname, arg+1)) {
                for (p = cfg->chain; p->next; p = p->next) {
                    if (!strcasecmp(p->next->fname, arg+1)) {
                        p->next = p->next->next;
                    }
                }
            }
            else {
                cfg->chain = cfg->chain->next;
            }
        }
        break;

    case '!':        /* Empty the chain */
                     /** IG: Add a NULL provider to the beginning so that 
                      *  we can ensure that we'll empty everything before 
                      *  this when doing config merges later */
        p = apr_pcalloc(cmd->pool, sizeof(mod_filter_chain));
        p->fname = NULL;
        cfg->chain = p;
        break;

    case '=':        /* initialise chain with this arg */
                     /** IG: Prepend a NULL provider to the beginning as above */
        p = apr_pcalloc(cmd->pool, sizeof(mod_filter_chain));
        p->fname = NULL;
        p->next = apr_pcalloc(cmd->pool, sizeof(mod_filter_chain));
        p->next->fname = arg+1;
        cfg->chain = p;
        break;

    default:        /* add to end */
        p = apr_pcalloc(cmd->pool, sizeof(mod_filter_chain));
        p->fname = arg;
        if (cfg->chain) {
            for (q = cfg->chain; q->next; q = q->next);
            q->next = p;
        }
        else {
            cfg->chain = p;
        }
        break;
    }

    return NULL;
}

static const char *filter_debug(cmd_parms *cmd, void *CFG, const char *fname,
                                const char *level)
{
    mod_filter_cfg *cfg = CFG;
    ap_filter_rec_t *frec = apr_hash_get(cfg->live_filters, fname,
                                         APR_HASH_KEY_STRING);
    if (!frec) {
        return apr_psprintf(cmd->pool, "Undeclared smart filter %s", fname);
    }
    frec->debug = atoi(level);

    return NULL;
}

static void filter_insert(request_rec *r)
{
    mod_filter_chain *p;
    ap_filter_rec_t *filter;
    mod_filter_cfg *cfg = ap_get_module_config(r->per_dir_config,
                                               &filter_module);
#ifndef NO_PROTOCOL
    int ranges = 1;
    mod_filter_ctx *ctx = apr_pcalloc(r->pool, sizeof(mod_filter_ctx));
    ap_set_module_config(r->request_config, &filter_module, ctx);
#endif

    /** IG: Now that we've merged to the final config, go one last time
     *  through the chain, and prune out the NULL filters */

    for (p = cfg->chain; p; p = p->next) {
        if (p->fname == NULL) 
            cfg->chain = p->next;
    }

    for (p = cfg->chain; p; p = p->next) {
        filter = apr_hash_get(cfg->live_filters, p->fname, APR_HASH_KEY_STRING);
        if (filter == NULL) {
            ap_log_rerror(APLOG_MARK, APLOG_WARNING, 0, r,
                          "Unknown filter %s not added", p->fname);
            continue;
        }
        ap_add_output_filter_handle(filter, NULL, r, r->connection);
#ifndef NO_PROTOCOL
        if (ranges && (filter->proto_flags
                       & (AP_FILTER_PROTO_NO_BYTERANGE
                          | AP_FILTER_PROTO_CHANGE_LENGTH))) {
            ctx->range = apr_table_get(r->headers_in, "Range");
            apr_table_unset(r->headers_in, "Range");
            ranges = 0;
        }
#endif
    }

    return;
}

static void filter_hooks(apr_pool_t *pool)
{
    ap_hook_insert_filter(filter_insert, NULL, NULL, APR_HOOK_MIDDLE);
}

static void *filter_config(apr_pool_t *pool, char *x)
{
    mod_filter_cfg *cfg = apr_palloc(pool, sizeof(mod_filter_cfg));
    cfg->live_filters = apr_hash_make(pool);
    cfg->chain = NULL;
    return cfg;
}

static void *filter_merge(apr_pool_t *pool, void *BASE, void *ADD)
{
    mod_filter_cfg *base = BASE;
    mod_filter_cfg *add = ADD;
    mod_filter_chain *savelink = 0;
    mod_filter_chain *newlink;
    mod_filter_chain *p;
    mod_filter_cfg *conf = apr_palloc(pool, sizeof(mod_filter_cfg));

    conf->live_filters = apr_hash_overlay(pool, add->live_filters,
                                          base->live_filters);
    if (base->chain && add->chain) {
        for (p = base->chain; p; p = p->next) {
            newlink = apr_pmemdup(pool, p, sizeof(mod_filter_chain));
            if (newlink->fname == NULL) {
                conf->chain = savelink = newlink;
            }
            else if (savelink) {
                savelink->next = newlink;
                savelink = newlink;
            }
            else {
                conf->chain = savelink = newlink;
            }
        }

        for (p = add->chain; p; p = p->next) {
            newlink = apr_pmemdup(pool, p, sizeof(mod_filter_chain));
            /** Filter out merged chain resets */
            if (newlink->fname == NULL) {
                conf->chain = savelink = newlink;
            }
            else if (savelink) {
                savelink->next = newlink;
                savelink = newlink;
            }
            else {
                conf->chain = savelink = newlink;
            }
        }
    }
    else if (add->chain) {
        conf->chain = add->chain;
    }
    else {
        conf->chain = base->chain;
    }

    return conf;
}

static const command_rec filter_cmds[] = {
    AP_INIT_TAKE12("FilterDeclare", filter_declare, NULL, OR_OPTIONS,
        "filter-name [, filter-type]"),
    /** we don't have a TAKE4, so we have to use RAW_ARGS */
    AP_INIT_RAW_ARGS("FilterProvider", filter_provider, NULL, OR_OPTIONS,
        "filter-name, provider-name, dispatch--criterion, dispatch-match"),
    AP_INIT_ITERATE("FilterChain", filter_chain, NULL, OR_OPTIONS,
        "list of filter names with optional [+-=!@]"),
    AP_INIT_TAKE2("FilterTrace", filter_debug, NULL, RSRC_CONF | ACCESS_CONF,
        "Debug level"),
#ifndef NO_PROTOCOL
    AP_INIT_TAKE23("FilterProtocol", filter_protocol, NULL, OR_OPTIONS,
        "filter-name [provider-name] protocol-args"),
#endif
    { NULL }
};

module AP_MODULE_DECLARE_DATA filter_module = {
    STANDARD20_MODULE_STUFF,
    filter_config,
    filter_merge,
    NULL,
    NULL,
    filter_cmds,
    filter_hooks
};