edgestash through link headers & a synthesized JSON backend

edgestash-synth-backend.vcl

vcl 4.1;
import cookieplus;
import edgestash;
import std;
import ykey;
import redis;
import synthbackend;

backend default {
    .host = "origin";
    .port = "80";
}

sub vcl_init {
    new db = redis.db(
            location="redis:6379",
            type=master,
            connection_timeout=500,
            shared_connections=false,
            max_connections=1);
}
sub vcl_recv {
    set req.http.X-Login = "false";
    set req.http.Surrogate-Capability="key=ESI/1.0, edgestash=EDGESTASH/2.1";
    set req.http.x-session = cookieplus.get("PHPSESSID","guest");

    if(req.http.x-session != "guest") {
        db.command("EXISTS");
        db.push("sf_s"+cookieplus.get("PHPSESSID"));
        db.execute();
        if(db.get_integer_reply() == 1) {
            set req.http.X-Login = "true";
        }
    }

    if(req.method == "POST") {
        ykey.purge(req.http.x-session);
    }

    if ((req.method != "GET" && req.method != "HEAD") || req.http.Authorization) {
        return (pass);
    }

    return(hash);
}

sub vcl_hash {
    if(req.http.host ~ "^edgestash\.") {
        hash_data(req.http.x-session);
    }
}

sub vcl_backend_fetch
{
    if(bereq.url == "/api/session") {
        db.command("EVAL");
        db.push({"
            local session = redis.call('GET', KEYS[1])
            if session == nil then
                return '{"username":"","liked":[],"somethingCool": false}'
            end
            local result = string.gsub(session, '[%c]', '')
            local username = string.gsub(result,'.+Userusername\";s:[0-9]+:\"([^\"]+)\";.+','%1')
            return '{"username":"'.. username ..'","liked":[],"somethingCool": true}'
        "});
        db.push(1);
        db.push("sf_s"+cookieplus.get("PHPSESSID"));
        db.execute();
        set bereq.backend = synthbackend.from_string(db.get_string_reply());
    } else {
        set bereq.backend = default;
    }

    if (bereq.http.Host ~ "^edgestash\.") {
        set bereq.http.Host = regsub(bereq.http.Host, "^edgestash\.", "");
    }
}

sub vcl_backend_response {

    std.collect(beresp.http.Link);
    set beresp.http.x-edgestash-json-urls = regsuball(beresp.http.Link,"<([^>]+)>; rel=edgestash","\1");
    if(beresp.http.Surrogate-Control~"ESI/1.0") {
        set beresp.do_esi=true;
    }

    if(beresp.http.Surrogate-Control~"EDGESTASH/2\.[0-9]+") {
        edgestash.parse_response();
    }

    if(beresp.http.Surrogate-Control) {
        unset beresp.http.Surrogate-Control;
    }
}

sub vcl_deliver {
   if(edgestash.is_edgestash()) {
        edgestash.add_json_url_csv(resp.http.x-edgestash-json-urls, json_host="edgestash." + req.http.host);
        edgestash.execute();
    }

    unset resp.http.Link;
    unset resp.http.x-edgestash-json-urls;
}