PageEditStash.php

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<?php
namespace MediaWiki\Storage;

use ActorMigration;
use BagOStuff;
use Content;
use Hooks;
use Liuggio\StatsdClient\Factory\StatsdDataFactoryInterface;
use ParserOutput;
use Psr\Log\LoggerInterface;
use stdClass;
use Title;
use User;
use Wikimedia\Rdbms\ILoadBalancer;
use Wikimedia\ScopedCallback;
use WikiPage;

class PageEditStash {
    private $cache;
    private $lb;
    private $logger;
    private $stats;
    private $initiator;

    const ERROR_NONE = 'stashed';
    const ERROR_PARSE = 'error_parse';
    const ERROR_CACHE = 'error_cache';
    const ERROR_UNCACHEABLE = 'uncacheable';
    const ERROR_BUSY = 'busy';

    const PRESUME_FRESH_TTL_SEC = 30;
    const MAX_CACHE_TTL = 300; // 5 minutes
    const MAX_SIGNATURE_TTL = 60;

    const MAX_CACHE_RECENT = 2;

    const INITIATOR_USER = 1;
    const INITIATOR_JOB_OR_CLI = 2;

    public function __construct(
        BagOStuff $cache,
        ILoadBalancer $lb,
        LoggerInterface $logger,
        StatsdDataFactoryInterface $stats,
        $initiator
    ) {
        $this->cache = $cache;
        $this->lb = $lb;
        $this->logger = $logger;
        $this->stats = $stats;
        $this->initiator = $initiator;
    }

    public function parseAndCache( WikiPage $page, Content $content, User $user, $summary ) {
        $logger = $this->logger;

        $title = $page->getTitle();
        $key = $this->getStashKey( $title, $this->getContentHash( $content ), $user );
        $fname = __METHOD__;

        // Use the master DB to allow for fast blocking locks on the "save path" where this
        // value might actually be used to complete a page edit. If the edit submission request
        // happens before this edit stash requests finishes, then the submission will block until
        // the stash request finishes parsing. For the lock acquisition below, there is not much
        // need to duplicate parsing of the same content/user/summary bundle, so try to avoid
        // blocking at all here.
        $dbw = $this->lb->getConnectionRef( DB_MASTER );
        if ( !$dbw->lock( $key, $fname, 0 ) ) {
            // De-duplicate requests on the same key
            return self::ERROR_BUSY;
        }
        $unlocker = new ScopedCallback( function () use ( $dbw, $key, $fname ) {
            $dbw->unlock( $key, $fname );
        } );

        $cutoffTime = time() - self::PRESUME_FRESH_TTL_SEC;

        // Reuse any freshly build matching edit stash cache
        $editInfo = $this->getStashValue( $key );
        if ( $editInfo && wfTimestamp( TS_UNIX, $editInfo->timestamp ) >= $cutoffTime ) {
            $alreadyCached = true;
        } else {
            $format = $content->getDefaultFormat();
            $editInfo = $page->prepareContentForEdit( $content, null, $user, $format, false );
            $editInfo->output->setCacheTime( $editInfo->timestamp );
            $alreadyCached = false;
        }

        $context = [ 'cachekey' => $key, 'title' => $title->getPrefixedText() ];

        if ( $editInfo && $editInfo->output ) {
            // Let extensions add ParserOutput metadata or warm other caches
            Hooks::run( 'ParserOutputStashForEdit',
                [ $page, $content, $editInfo->output, $summary, $user ] );

            if ( $alreadyCached ) {
                $logger->debug( "Parser output for key '{cachekey}' already cached.", $context );

                return self::ERROR_NONE;
            }

            $code = $this->storeStashValue(
                $key,
                $editInfo->pstContent,
                $editInfo->output,
                $editInfo->timestamp,
                $user
            );

            if ( $code === true ) {
                $logger->debug( "Cached parser output for key '{cachekey}'.", $context );

                return self::ERROR_NONE;
            } elseif ( $code === 'uncacheable' ) {
                $logger->info(
                    "Uncacheable parser output for key '{cachekey}' [{code}].",
                    $context + [ 'code' => $code ]
                );

                return self::ERROR_UNCACHEABLE;
            } else {
                $logger->error(
                    "Failed to cache parser output for key '{cachekey}'.",
                    $context + [ 'code' => $code ]
                );

                return self::ERROR_CACHE;
            }
        }

        return self::ERROR_PARSE;
    }

    public function checkCache( Title $title, Content $content, User $user ) {
        if (
            // The context is not an HTTP POST request
            !$user->getRequest()->wasPosted() ||
            // The context is a CLI script or a job runner HTTP POST request
            $this->initiator !== self::INITIATOR_USER ||
            // The editor account is a known bot
            $user->isBot()
        ) {
            // Avoid wasted queries and statsd pollution
            return false;
        }

        $logger = $this->logger;

        $key = $this->getStashKey( $title, $this->getContentHash( $content ), $user );
        $context = [
            'key' => $key,
            'title' => $title->getPrefixedText(),
            'user' => $user->getName()
        ];

        $editInfo = $this->getAndWaitForStashValue( $key );
        if ( !is_object( $editInfo ) || !$editInfo->output ) {
            $this->incrStatsByContent( 'cache_misses.no_stash', $content );
            if ( $this->recentStashEntryCount( $user ) > 0 ) {
                $logger->info( "Empty cache for key '{key}' but not for user.", $context );
            } else {
                $logger->debug( "Empty cache for key '{key}'.", $context );
            }

            return false;
        }

        $age = time() - wfTimestamp( TS_UNIX, $editInfo->output->getCacheTime() );
        $context['age'] = $age;

        $isCacheUsable = true;
        if ( $age <= self::PRESUME_FRESH_TTL_SEC ) {
            // Assume nothing changed in this time
            $this->incrStatsByContent( 'cache_hits.presumed_fresh', $content );
            $logger->debug( "Timestamp-based cache hit for key '{key}'.", $context );
        } elseif ( $user->isAnon() ) {
            $lastEdit = $this->lastEditTime( $user );
            $cacheTime = $editInfo->output->getCacheTime();
            if ( $lastEdit < $cacheTime ) {
                // Logged-out user made no local upload/template edits in the meantime
                $this->incrStatsByContent( 'cache_hits.presumed_fresh', $content );
                $logger->debug( "Edit check based cache hit for key '{key}'.", $context );
            } else {
                $isCacheUsable = false;
                $this->incrStatsByContent( 'cache_misses.proven_stale', $content );
                $logger->info( "Stale cache for key '{key}' due to outside edits.", $context );
            }
        } else {
            if ( $editInfo->edits === $user->getEditCount() ) {
                // Logged-in user made no local upload/template edits in the meantime
                $this->incrStatsByContent( 'cache_hits.presumed_fresh', $content );
                $logger->debug( "Edit count based cache hit for key '{key}'.", $context );
            } else {
                $isCacheUsable = false;
                $this->incrStatsByContent( 'cache_misses.proven_stale', $content );
                $logger->info( "Stale cache for key '{key}'due to outside edits.", $context );
            }
        }

        if ( !$isCacheUsable ) {
            return false;
        }

        if ( $editInfo->output->getFlag( 'vary-revision' ) ) {
            // This can be used for the initial parse, e.g. for filters or doEditContent(),
            // but a second parse will be triggered in doEditUpdates() no matter what
            $logger->info(
                "Cache for key '{key}' has 'vary-revision'; post-insertion parse inevitable.",
                $context
            );
        } else {
            static $flagsMaybeReparse = [
                // Similar to the above if we didn't guess the ID correctly
                'vary-revision-id',
                // Similar to the above if we didn't guess the timestamp correctly
                'vary-revision-timestamp',
                // Similar to the above if we didn't guess the content correctly
                'vary-revision-sha1'
            ];
            foreach ( $flagsMaybeReparse as $flag ) {
                if ( $editInfo->output->getFlag( $flag ) ) {
                    $logger->debug(
                        "Cache for key '{key}' has $flag; post-insertion parse possible.",
                        $context
                    );
                }
            }
        }

        return $editInfo;
    }

    private function incrStatsByContent( $subkey, Content $content ) {
        $this->stats->increment( 'editstash.' . $subkey ); // overall for b/c
        $this->stats->increment( 'editstash_by_model.' . $content->getModel() . '.' . $subkey );
    }

    private function getAndWaitForStashValue( $key ) {
        $editInfo = $this->getStashValue( $key );

        if ( !$editInfo ) {
            $start = microtime( true );
            // We ignore user aborts and keep parsing. Block on any prior parsing
            // so as to use its results and make use of the time spent parsing.
            // Skip this logic if there no master connection in case this method
            // is called on an HTTP GET request for some reason.
            $dbw = $this->lb->getAnyOpenConnection( $this->lb->getWriterIndex() );
            if ( $dbw && $dbw->lock( $key, __METHOD__, 30 ) ) {
                $editInfo = $this->getStashValue( $key );
                $dbw->unlock( $key, __METHOD__ );
            }

            $timeMs = 1000 * max( 0, microtime( true ) - $start );
            $this->stats->timing( 'editstash.lock_wait_time', $timeMs );
        }

        return $editInfo;
    }

    public function fetchInputText( $textHash ) {
        $textKey = $this->cache->makeKey( 'stashedit', 'text', $textHash );

        return $this->cache->get( $textKey );
    }

    public function stashInputText( $text, $textHash ) {
        $textKey = $this->cache->makeKey( 'stashedit', 'text', $textHash );

        return $this->cache->set(
            $textKey,
            $text,
            self::MAX_CACHE_TTL,
            BagOStuff::WRITE_ALLOW_SEGMENTS
        );
    }

    private function lastEditTime( User $user ) {
        $db = $this->lb->getConnectionRef( DB_REPLICA );

        $actorQuery = ActorMigration::newMigration()->getWhere( $db, 'rc_user', $user, false );
        $time = $db->selectField(
            [ 'recentchanges' ] + $actorQuery['tables'],
            'MAX(rc_timestamp)',
            [ $actorQuery['conds'] ],
            __METHOD__,
            [],
            $actorQuery['joins']
        );

        return wfTimestampOrNull( TS_MW, $time );
    }

    private function getContentHash( Content $content ) {
        return sha1( implode( "\n", [
            $content->getModel(),
            $content->getDefaultFormat(),
            $content->serialize( $content->getDefaultFormat() )
        ] ) );
    }

    private function getStashKey( Title $title, $contentHash, User $user ) {
        return $this->cache->makeKey(
            'stashedit-info-v1',
            md5( $title->getPrefixedDBkey() ),
            // Account for the edit model/text
            $contentHash,
            // Account for user name related variables like signatures
            md5( $user->getId() . "\n" . $user->getName() )
        );
    }

    private function getStashValue( $key ) {
        $stashInfo = $this->cache->get( $key );
        if ( is_object( $stashInfo ) && $stashInfo->output instanceof ParserOutput ) {
            return $stashInfo;
        }

        return false;
    }

    private function storeStashValue(
        $key,
        Content $pstContent,
        ParserOutput $parserOutput,
        $timestamp,
        User $user
    ) {
        // If an item is renewed, mind the cache TTL determined by config and parser functions.
        // Put an upper limit on the TTL for sanity to avoid extreme template/file staleness.
        $age = time() - wfTimestamp( TS_UNIX, $parserOutput->getCacheTime() );
        $ttl = min( $parserOutput->getCacheExpiry() - $age, self::MAX_CACHE_TTL );
        // Avoid extremely stale user signature timestamps (T84843)
        if ( $parserOutput->getFlag( 'user-signature' ) ) {
            $ttl = min( $ttl, self::MAX_SIGNATURE_TTL );
        }

        if ( $ttl <= 0 ) {
            return 'uncacheable'; // low TTL due to a tag, magic word, or signature?
        }

        // Store what is actually needed and split the output into another key (T204742)
        $stashInfo = (object)[
            'pstContent' => $pstContent,
            'output'     => $parserOutput,
            'timestamp'  => $timestamp,
            'edits'      => $user->getEditCount()
        ];

        $ok = $this->cache->set( $key, $stashInfo, $ttl, BagOStuff::WRITE_ALLOW_SEGMENTS );
        if ( $ok ) {
            // These blobs can waste slots in low cardinality memcached slabs
            $this->pruneExcessStashedEntries( $user, $key );
        }

        return $ok ? true : 'store_error';
    }

    private function pruneExcessStashedEntries( User $user, $newKey ) {
        $key = $this->cache->makeKey( 'stash-edit-recent', sha1( $user->getName() ) );

        $keyList = $this->cache->get( $key ) ?: [];
        if ( count( $keyList ) >= self::MAX_CACHE_RECENT ) {
            $oldestKey = array_shift( $keyList );
            $this->cache->delete( $oldestKey, BagOStuff::WRITE_PRUNE_SEGMENTS );
        }

        $keyList[] = $newKey;
        $this->cache->set( $key, $keyList, 2 * self::MAX_CACHE_TTL );
    }

    private function recentStashEntryCount( User $user ) {
        $key = $this->cache->makeKey( 'stash-edit-recent', sha1( $user->getName() ) );

        return count( $this->cache->get( $key ) ?: [] );
    }
}