Make sure we do not try to process more than one analysis.json update at a time.

[remoteglot] / www / serve-analysis.js

// node.js version of analysis.pl; hopefully scales a bit better
// for this specific kind of task.

// Modules.
var http = require('http');
var fs = require('fs');
var url = require('url');
var querystring = require('querystring');
var path = require('path');
var zlib = require('zlib');
var delta = require('./js/json_delta.js');

// Constants.
var JSON_FILENAME = '/srv/analysis.sesse.net/www/analysis.json';
var HISTORY_TO_KEEP = 5;

// If set to 1, we are already processing a JSON update and should not
// start a new one. If set to 2, we are _also_ having one in the queue.
var json_lock = 0;

// The current contents of the file to hand out, and its last modified time.
var json = undefined;

// The last five timestamps, and diffs from them to the latest version.
var historic_json = [];
var diff_json = {};

// The list of clients that are waiting for new data to show up.
// Uniquely keyed by request_id so that we can take them out of
// the queue if they close the socket.
var sleeping_clients = {};
var request_id = 0;

// List of when clients were last seen, keyed by their unique ID.
// Used to show a viewer count to the user.
var last_seen_clients = {};

// The timer used to touch the file every 30 seconds if nobody
// else does it for us. This makes sure we don't have clients
// hanging indefinitely (which might have them return errors).
var touch_timer = undefined;

// If we are behind Varnish, we can't count the number of clients
// ourselves, so some external log-tailing daemon needs to tell us.
var viewer_count_override = undefined;

var replace_json = function(new_json_contents, mtime) {
        // Generate the list of diffs from the last five versions.
        if (json !== undefined) {
                // If two versions have the same mtime, clients could have either.
                // Note the fact, so that we never insert it.
                if (json.last_modified == mtime) {
                        json.invalid_base = true;
                }
                if (!json.invalid_base) {
                        historic_json.push(json);
                        if (historic_json.length > HISTORY_TO_KEEP) {
                                historic_json.shift();
                        }
                }
        }

        var new_json = {
                parsed: JSON.parse(new_json_contents),
                plain: new_json_contents,
                last_modified: mtime
        };
        create_json_historic_diff(new_json, historic_json.slice(0), {}, function(new_diff_json) {
                // gzip the new version (non-delta), and put it into place.
                zlib.gzip(new_json_contents, function(err, buffer) {
                        if (err) throw err;

                        new_json.gzip = buffer;
                        json = new_json;
                        diff_json = new_diff_json;
                        json_lock = 0;

                        // Finally, wake up any sleeping clients.
                        possibly_wakeup_clients();
                });
        });
}

var create_json_historic_diff = function(new_json, history_left, new_diff_json, cb) {
        if (history_left.length == 0) {
                cb(new_diff_json);
                return;
        }

        var histobj = history_left.shift();
        var diff = delta.JSON_delta.diff(histobj.parsed, new_json.parsed);
        var diff_text = JSON.stringify(diff);
        zlib.gzip(diff_text, function(err, buffer) {
                if (err) throw err;
                new_diff_json[histobj.last_modified] = {
                        plain: diff,
                        text: diff_text,
                        gzip: buffer,
                        last_modified: new_json.last_modified,
                };
                create_json_historic_diff(new_json, history_left, new_diff_json, cb);
        });
}

var reread_file = function(event, filename) {
        if (filename != path.basename(JSON_FILENAME)) {
                return;
        }
        if (json_lock >= 2) {
                return;
        }
        if (json_lock == 1) {
                // Already processing; wait a bit.
                json_lock = 2;
                setTimeout(function() { json_lock = 1; reread_file(event, filename); }, 100);
                return;
        }
        json_lock = 1;

        console.log("Rereading " + JSON_FILENAME);
        fs.open(JSON_FILENAME, 'r+', function(err, fd) {
                if (err) throw err;
                fs.fstat(fd, function(err, st) {
                        if (err) throw err;
                        var buffer = new Buffer(1048576);
                        fs.read(fd, buffer, 0, 1048576, 0, function(err, bytesRead, buffer) {
                                if (err) throw err;
                                fs.close(fd, function() {
                                        var new_json_contents = buffer.toString('utf8', 0, bytesRead);
                                        replace_json(new_json_contents, st.mtime.getTime());
                                });
                        });
                });
        });

        if (touch_timer !== undefined) {
                clearTimeout(touch_timer);
        }
        touch_timer = setTimeout(function() {
                console.log("Touching analysis.json due to no other activity");
                var now = Date.now() / 1000;
                fs.utimes(JSON_FILENAME, now, now);
        }, 30000);
}
var possibly_wakeup_clients = function() {
        var num_viewers = count_viewers();
        for (var i in sleeping_clients) {
                mark_recently_seen(sleeping_clients[i].unique);
                send_json(sleeping_clients[i].response,
                          sleeping_clients[i].ims,
                          sleeping_clients[i].accept_gzip,
                          num_viewers);
        }
        sleeping_clients = {};
}
var send_404 = function(response) {
        response.writeHead(404, {
                'Content-Type': 'text/plain',
        });
        response.write('Something went wrong. Sorry.');
        response.end();
}
var handle_viewer_override = function(request, u, response) {
        // Only accept requests from localhost.
        var peer = request.socket.localAddress;
        if ((peer != '127.0.0.1' && peer != '::1') || request.headers['x-forwarded-for']) {
                console.log("Refusing viewer override from " + peer);
                send_404(response);
        } else {
                viewer_count_override = (u.query)['num'];
                response.writeHead(200, {
                        'Content-Type': 'text/plain',
                });
                response.write('OK.');
                response.end();
        }
}
var send_json = function(response, ims, accept_gzip, num_viewers) {
        var this_json = diff_json[ims] || json;

        var headers = {
                'Content-Type': 'text/json',
                'X-Remoteglot-Last-Modified': this_json.last_modified,
                'X-Remoteglot-Num-Viewers': num_viewers,
                'Access-Control-Expose-Headers': 'X-Remoteglot-Last-Modified, X-Remoteglot-Num-Viewers',
                'Expires': 'Mon, 01 Jan 1970 00:00:00 UTC',
                'Vary': 'Accept-Encoding',
        };

        if (accept_gzip) {
                headers['Content-Encoding'] = 'gzip';
                response.writeHead(200, headers);
                response.write(this_json.gzip);
        } else {
                response.writeHead(200, headers);
                response.write(this_json.text);
        }
        response.end();
}
var mark_recently_seen = function(unique) {
        if (unique) {
                last_seen_clients[unique] = (new Date).getTime();
        }
}
var count_viewers = function() {
        if (viewer_count_override !== undefined) {
                return viewer_count_override;
        }

        var now = (new Date).getTime();

        // Go through and remove old viewers, and count them at the same time.
        var new_last_seen_clients = {};
        var num_viewers = 0;
        for (var unique in last_seen_clients) {
                if (now - last_seen_clients[unique] < 5000) {
                        ++num_viewers;
                        new_last_seen_clients[unique] = last_seen_clients[unique];
                }
        }

        // Also add sleeping clients that we would otherwise assume timed out.
        for (var request_id in sleeping_clients) {
                var unique = sleeping_clients[request_id].unique;
                if (unique && !(unique in new_last_seen_clients)) {
                        ++num_viewers;
                }
        }

        last_seen_clients = new_last_seen_clients;
        return num_viewers;
}

// Set up a watcher to catch changes to the file, then do an initial read
// to make sure we have a copy.
fs.watch(path.dirname(JSON_FILENAME), reread_file);
reread_file(null, path.basename(JSON_FILENAME));

var server = http.createServer();
server.on('request', function(request, response) {
        var u = url.parse(request.url, true);
        var ims = (u.query)['ims'];
        var unique = (u.query)['unique'];

        console.log((new Date).getTime()*1e-3 + " " + request.url);
        if (u.pathname === '/override-num-viewers') {
                handle_viewer_override(request, u, response);
                return;
        }
        if (u.pathname !== '/analysis.pl') {
                // This is not the request you are looking for.
                send_404(response);
                return;
        }

        mark_recently_seen(unique);

        var accept_encoding = request.headers['accept-encoding'];
        var accept_gzip;
        if (accept_encoding !== undefined && accept_encoding.match(/\bgzip\b/)) {
                accept_gzip = true;
        } else {
                accept_gzip = false;
        }

        // If we already have something newer than what the user has,
        // just send it out and be done with it.
        if (json !== undefined && (!ims || json.last_modified > ims)) {
                send_json(response, ims, accept_gzip, count_viewers());
                return;
        }

        // OK, so we need to hang until we have something newer.
        // Put the user on the wait list.
        var client = {};
        client.response = response;
        client.request_id = request_id;
        client.accept_gzip = accept_gzip;
        client.unique = unique;
        client.ims = ims;
        sleeping_clients[request_id++] = client;

        request.socket.client = client;
});
server.on('connection', function(socket) {
        socket.on('close', function() {
                var client = socket.client;
                if (client) {
                        mark_recently_seen(client.unique);
                        delete sleeping_clients[client.request_id];
                }
        });
});
server.listen(5000);