EtcdConfig.php

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<?php
use Psr\Log\LoggerAwareInterface;
use Psr\Log\LoggerInterface;
use Wikimedia\ObjectFactory;
use Wikimedia\WaitConditionLoop;
 
class EtcdConfig implements Config, LoggerAwareInterface {
    private $http;
    private $srvCache;
    private $procCache;
    private $logger;
 
    private $host;
    private $protocol;
    private $directory;
    private $encoding;
    private $baseCacheTTL;
    private $skewCacheTTL;
    private $timeout;
 
    public function __construct( array $params ) {
        $params += [
            'protocol' => 'http',
            'encoding' => 'JSON',
            'cacheTTL' => 10,
            'skewTTL' => 1,
            'timeout' => 2
        ];
 
        $this->host = $params['host'];
        $this->protocol = $params['protocol'];
        $this->directory = trim( $params['directory'], '/' );
        $this->encoding = $params['encoding'];
        $this->skewCacheTTL = $params['skewTTL'];
        $this->baseCacheTTL = max( $params['cacheTTL'] - $this->skewCacheTTL, 0 );
        $this->timeout = $params['timeout'];
 
        if ( !isset( $params['cache'] ) ) {
            $this->srvCache = new HashBagOStuff();
        } elseif ( $params['cache'] instanceof BagOStuff ) {
            $this->srvCache = $params['cache'];
        } else {
            $this->srvCache = ObjectFactory::getObjectFromSpec( $params['cache'] );
        }
 
        $this->logger = new Psr\Log\NullLogger();
        $this->http = new MultiHttpClient( [
            'connTimeout' => $this->timeout,
            'reqTimeout' => $this->timeout,
            'logger' => $this->logger
        ] );
    }
 
    public function setLogger( LoggerInterface $logger ) {
        $this->logger = $logger;
        $this->http->setLogger( $logger );
    }
 
    public function has( $name ) {
        $this->load();
 
        return array_key_exists( $name, $this->procCache['config'] );
    }
 
    public function get( $name ) {
        $this->load();
 
        if ( !array_key_exists( $name, $this->procCache['config'] ) ) {
            throw new ConfigException( "No entry found for '$name'." );
        }
 
        return $this->procCache['config'][$name];
    }
 
    public function getModifiedIndex() {
        $this->load();
        return $this->procCache['modifiedIndex'];
    }
 
    private function load() {
        if ( $this->procCache !== null ) {
            return; // already loaded
        }
 
        $now = microtime( true );
        $key = $this->srvCache->makeGlobalKey(
            __CLASS__,
            $this->host,
            $this->directory
        );
 
        // Get the cached value or block until it is regenerated (by this or another thread)...
        $data = null; // latest config info
        $error = null; // last error message
        $loop = new WaitConditionLoop(
            function () use ( $key, $now, &$data, &$error ) {
                // Check if the values are in cache yet...
                $data = $this->srvCache->get( $key );
                if ( is_array( $data ) && $data['expires'] > $now ) {
                    $this->logger->debug( "Found up-to-date etcd configuration cache." );
 
                    return WaitConditionLoop::CONDITION_REACHED;
                }
 
                // Cache is either empty or stale;
                // refresh the cache from etcd, using a mutex to reduce stampedes...
                if ( $this->srvCache->lock( $key, 0, $this->baseCacheTTL ) ) {
                    try {
                        $etcdResponse = $this->fetchAllFromEtcd();
                        $error = $etcdResponse['error'];
                        if ( is_array( $etcdResponse['config'] ) ) {
                            // Avoid having all servers expire cache keys at the same time
                            $expiry = microtime( true ) + $this->baseCacheTTL;
                            $expiry += mt_rand( 0, 1e6 ) / 1e6 * $this->skewCacheTTL;
                            $data = [
                                'config' => $etcdResponse['config'],
                                'expires' => $expiry,
                                'modifiedIndex' => $etcdResponse['modifiedIndex']
                            ];
                            $this->srvCache->set( $key, $data, BagOStuff::TTL_INDEFINITE );
 
                            $this->logger->info( "Refreshed stale etcd configuration cache." );
 
                            return WaitConditionLoop::CONDITION_REACHED;
                        } else {
                            $this->logger->error( "Failed to fetch configuration: $error" );
                            if ( !$etcdResponse['retry'] ) {
                                // Fail fast since the error is likely to keep happening
                                return WaitConditionLoop::CONDITION_FAILED;
                            }
                        }
                    } finally {
                        $this->srvCache->unlock( $key ); // release mutex
                    }
                }
 
                if ( is_array( $data ) ) {
                    $this->logger->info( "Using stale etcd configuration cache." );
 
                    return WaitConditionLoop::CONDITION_REACHED;
                }
 
                return WaitConditionLoop::CONDITION_CONTINUE;
            },
            $this->timeout
        );
 
        if ( $loop->invoke() !== WaitConditionLoop::CONDITION_REACHED ) {
            // No cached value exists and etcd query failed; throw an error
            throw new ConfigException( "Failed to load configuration from etcd: $error" );
        }
 
        $this->procCache = $data;
    }
 
    public function fetchAllFromEtcd() {
        // TODO: inject DnsSrvDiscoverer in order to be able to test this method
        $dsd = new DnsSrvDiscoverer( $this->host );
        $servers = $dsd->getServers();
        if ( !$servers ) {
            return $this->fetchAllFromEtcdServer( $this->host );
        }
 
        do {
            // Pick a random etcd server from dns
            $server = $dsd->pickServer( $servers );
            $host = IP::combineHostAndPort( $server['target'], $server['port'] );
            // Try to load the config from this particular server
            $response = $this->fetchAllFromEtcdServer( $host );
            if ( is_array( $response['config'] ) || $response['retry'] ) {
                break;
            }
 
            // Avoid the server next time if that failed
            $servers = $dsd->removeServer( $server, $servers );
        } while ( $servers );
 
        return $response;
    }
 
    protected function fetchAllFromEtcdServer( $address ) {
        // Retrieve all the values under the MediaWiki config directory
        list( $rcode, $rdesc, /* $rhdrs */, $rbody, $rerr ) = $this->http->run( [
            'method' => 'GET',
            'url' => "{$this->protocol}://{$address}/v2/keys/{$this->directory}/?recursive=true",
            'headers' => [ 'content-type' => 'application/json' ]
        ] );
 
        $response = [ 'config' => null, 'error' => null, 'retry' => false, 'modifiedIndex' => 0 ];
 
        static $terminalCodes = [ 404 => true ];
        if ( $rcode < 200 || $rcode > 399 ) {
            $response['error'] = strlen( $rerr ) ? $rerr : "HTTP $rcode ($rdesc)";
            $response['retry'] = empty( $terminalCodes[$rcode] );
            return $response;
        }
 
        try {
            $parsedResponse = $this->parseResponse( $rbody );
        } catch ( EtcdConfigParseError $e ) {
            $parsedResponse = [ 'error' => $e->getMessage() ];
        }
        return array_merge( $response, $parsedResponse );
    }
 
    protected function parseResponse( $rbody ) {
        $info = json_decode( $rbody, true );
        if ( $info === null ) {
            throw new EtcdConfigParseError( "Error unserializing JSON response." );
        }
        if ( !isset( $info['node'] ) || !is_array( $info['node'] ) ) {
            throw new EtcdConfigParseError(
                "Unexpected JSON response: Missing or invalid node at top level." );
        }
        $config = [];
        $lastModifiedIndex = $this->parseDirectory( '', $info['node'], $config );
        return [ 'modifiedIndex' => $lastModifiedIndex, 'config' => $config ];
    }
 
    protected function parseDirectory( $dirName, $dirNode, &$config ) {
        $lastModifiedIndex = 0;
        if ( !isset( $dirNode['nodes'] ) ) {
            throw new EtcdConfigParseError(
                "Unexpected JSON response in dir '$dirName'; missing 'nodes' list." );
        }
        if ( !is_array( $dirNode['nodes'] ) ) {
            throw new EtcdConfigParseError(
                "Unexpected JSON response in dir '$dirName'; 'nodes' is not an array." );
        }
 
        foreach ( $dirNode['nodes'] as $node ) {
            $baseName = basename( $node['key'] );
            $fullName = $dirName === '' ? $baseName : "$dirName/$baseName";
            if ( !empty( $node['dir'] ) ) {
                $lastModifiedIndex = max(
                    $this->parseDirectory( $fullName, $node, $config ),
                    $lastModifiedIndex );
            } else {
                $value = $this->unserialize( $node['value'] );
                if ( !is_array( $value ) || !array_key_exists( 'val', $value ) ) {
                    throw new EtcdConfigParseError( "Failed to parse value for '$fullName'." );
                }
                $lastModifiedIndex = max( $node['modifiedIndex'], $lastModifiedIndex );
                $config[$fullName] = $value['val'];
            }
        }
        return $lastModifiedIndex;
    }
 
    private function unserialize( $string ) {
        if ( $this->encoding === 'YAML' ) {
            return yaml_parse( $string );
        } else {
            return json_decode( $string, true );
        }
    }
}