Allow custom regex via "vhp-ban-regex" query parameter

[lukasbesch]

My approach for #96.

Similar to query parameter vhp-regex, you can now add a custom regex using the query parameter vhp-ban-regex to the purge_urls.
If it is set, the PURGE request contains an additional header X-Ban-Regex and the X-Purge-Method is ban-regex.
If the VCL contains this functionality (described below), it will be used. Otherwise – to preserve backwards compatibility – only the request url itself is purged.

The VCL now looks like this:

sub vcl_recv {
    if (req.method == "PURGE" || req.method == "BAN") {

        # If the IP isn't a match, we can't flush cache.
        if (!client.ip ~ purge) {
            return (synth(405, "This IP is not allowed to send PURGE requests."));
        }

        # If we're trying to custom regex, then we need to use the ban calls:
        if (req.http.X-Purge-Method == "ban-regex" && req.http.X-Ban-Regex) {
            ban("obj.http.x-url ~ " + req.http.X-Ban-Regex + " && obj.http.x-host ~ " + req.http.host);
            return (synth(200, "Banned"));
        }

        # If we're trying to regex, then we need to use the ban calls:
        if (req.http.X-Purge-Method == "regex") {
            ban("obj.http.x-url ~ " + req.url + " && obj.http.x-host ~ " + req.http.host);
            return (synth(200, "Purged"));
        }

        # Backported catch for exact cache flush calls:
        if (req.http.X-Purge-Method == "exact") {
            ban("obj.http.x-url == " + req.url + " && obj.http.X-Req-Host == " + req.http.host);
            return (synth(200, "Purged"));
        }

        # Otherwise, we strip the query params and flush as is:
        set req.url = regsub(req.url, "\?.*$", "");
        return (purge);
    }
}

What I don't like in this approach is that X-Purge-Method: regex and X-Purge-Method: ban-regex as well as $pregex and $bregex might be confusing (but is needed for backwards compatibility or for people who cannot change their VCL).

[lukasbesch]

#98 will provide a solution for custom invalidation strategies so I close this