ChangeTags.php

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<?php
class ChangeTags {
    const MAX_DELETE_USES = 5000;

    public static function formatSummaryRow( $tags, $page, IContextSource $context = null ) {
        if ( !$tags ) {
            return [ '', [] ];
        }
        if ( !$context ) {
            $context = RequestContext::getMain();
        }

        $classes = [];

        $tags = explode( ',', $tags );
        $displayTags = [];
        foreach ( $tags as $tag ) {
            if ( !$tag ) {
                continue;
            }
            $description = self::tagDescription( $tag );
            if ( $description === false ) {
                continue;
            }
            $displayTags[] = Xml::tags(
                'span',
                [ 'class' => 'mw-tag-marker ' .
                                Sanitizer::escapeClass( "mw-tag-marker-$tag" ) ],
                $description
            );
            $classes[] = Sanitizer::escapeClass( "mw-tag-$tag" );
        }

        if ( !$displayTags ) {
            return [ '', [] ];
        }

        $markers = $context->msg( 'tag-list-wrapper' )
            ->numParams( count( $displayTags ) )
            ->rawParams( $context->getLanguage()->commaList( $displayTags ) )
            ->parse();
        $markers = Xml::tags( 'span', [ 'class' => 'mw-tag-markers' ], $markers );

        return [ $markers, $classes ];
    }

    public static function tagDescription( $tag ) {
        $msg = wfMessage( "tag-$tag" );
        if ( !$msg->exists() ) {
            // No such message, so return the HTML-escaped tag name.
            return htmlspecialchars( $tag );
        }
        if ( $msg->isDisabled() ) {
            // The message exists but is disabled, hide the tag.
            return false;
        }

        // Message exists and isn't disabled, use it.
        return $msg->parse();
    }

    public static function addTags( $tags, $rc_id = null, $rev_id = null,
        $log_id = null, $params = null
    ) {
        $result = self::updateTags( $tags, null, $rc_id, $rev_id, $log_id, $params );
        return (bool)$result[0];
    }

    public static function updateTags(
        $tagsToAdd, $tagsToRemove,
        &$rc_id = null, &$rev_id = null, &$log_id = null, $params = null
    ) {

        $tagsToAdd = array_filter( (array)$tagsToAdd ); // Make sure we're submitting all tags...
        $tagsToRemove = array_filter( (array)$tagsToRemove );

        if ( !$rc_id && !$rev_id && !$log_id ) {
            throw new MWException( 'At least one of: RCID, revision ID, and log ID MUST be ' .
                'specified when adding or removing a tag from a change!' );
        }

        $dbw = wfGetDB( DB_MASTER );

        // Might as well look for rcids and so on.
        if ( !$rc_id ) {
            // Info might be out of date, somewhat fractionally, on slave.
            // LogEntry/LogPage and WikiPage match rev/log/rc timestamps,
            // so use that relation to avoid full table scans.
            if ( $log_id ) {
                $rc_id = $dbw->selectField(
                    [ 'logging', 'recentchanges' ],
                    'rc_id',
                    [
                        'log_id' => $log_id,
                        'rc_timestamp = log_timestamp',
                        'rc_logid = log_id'
                    ],
                    __METHOD__
                );
            } elseif ( $rev_id ) {
                $rc_id = $dbw->selectField(
                    [ 'revision', 'recentchanges' ],
                    'rc_id',
                    [
                        'rev_id' => $rev_id,
                        'rc_timestamp = rev_timestamp',
                        'rc_this_oldid = rev_id'
                    ],
                    __METHOD__
                );
            }
        } elseif ( !$log_id && !$rev_id ) {
            // Info might be out of date, somewhat fractionally, on slave.
            $log_id = $dbw->selectField(
                'recentchanges',
                'rc_logid',
                [ 'rc_id' => $rc_id ],
                __METHOD__
            );
            $rev_id = $dbw->selectField(
                'recentchanges',
                'rc_this_oldid',
                [ 'rc_id' => $rc_id ],
                __METHOD__
            );
        }

        if ( $log_id && !$rev_id ) {
            $rev_id = $dbw->selectField(
                'log_search',
                'ls_value',
                [ 'ls_field' => 'associated_rev_id', 'ls_log_id' => $log_id ],
                __METHOD__
            );
        } elseif ( !$log_id && $rev_id ) {
            $log_id = $dbw->selectField(
                'log_search',
                'ls_log_id',
                [ 'ls_field' => 'associated_rev_id', 'ls_value' => $rev_id ],
                __METHOD__
            );
        }

        // update the tag_summary row
        $prevTags = [];
        if ( !self::updateTagSummaryRow( $tagsToAdd, $tagsToRemove, $rc_id, $rev_id,
            $log_id, $prevTags ) ) {

            // nothing to do
            return [ [], [], $prevTags ];
        }

        // insert a row into change_tag for each new tag
        if ( count( $tagsToAdd ) ) {
            $tagsRows = [];
            foreach ( $tagsToAdd as $tag ) {
                // Filter so we don't insert NULLs as zero accidentally.
                // Keep in mind that $rc_id === null means "I don't care/know about the
                // rc_id, just delete $tag on this revision/log entry". It doesn't
                // mean "only delete tags on this revision/log WHERE rc_id IS NULL".
                $tagsRows[] = array_filter(
                    [
                        'ct_tag' => $tag,
                        'ct_rc_id' => $rc_id,
                        'ct_log_id' => $log_id,
                        'ct_rev_id' => $rev_id,
                        'ct_params' => $params
                    ]
                );
            }

            $dbw->insert( 'change_tag', $tagsRows, __METHOD__, [ 'IGNORE' ] );
        }

        // delete from change_tag
        if ( count( $tagsToRemove ) ) {
            foreach ( $tagsToRemove as $tag ) {
                $conds = array_filter(
                    [
                        'ct_tag' => $tag,
                        'ct_rc_id' => $rc_id,
                        'ct_log_id' => $log_id,
                        'ct_rev_id' => $rev_id
                    ]
                );
                $dbw->delete( 'change_tag', $conds, __METHOD__ );
            }
        }

        self::purgeTagUsageCache();
        return [ $tagsToAdd, $tagsToRemove, $prevTags ];
    }

    protected static function updateTagSummaryRow( &$tagsToAdd, &$tagsToRemove,
        $rc_id, $rev_id, $log_id, &$prevTags = [] ) {

        $dbw = wfGetDB( DB_MASTER );

        $tsConds = array_filter( [
            'ts_rc_id' => $rc_id,
            'ts_rev_id' => $rev_id,
            'ts_log_id' => $log_id
        ] );

        // Can't both add and remove a tag at the same time...
        $tagsToAdd = array_diff( $tagsToAdd, $tagsToRemove );

        // Update the summary row.
        // $prevTags can be out of date on slaves, especially when addTags is called consecutively,
        // causing loss of tags added recently in tag_summary table.
        $prevTags = $dbw->selectField( 'tag_summary', 'ts_tags', $tsConds, __METHOD__ );
        $prevTags = $prevTags ? $prevTags : '';
        $prevTags = array_filter( explode( ',', $prevTags ) );

        // add tags
        $tagsToAdd = array_values( array_diff( $tagsToAdd, $prevTags ) );
        $newTags = array_unique( array_merge( $prevTags, $tagsToAdd ) );

        // remove tags
        $tagsToRemove = array_values( array_intersect( $tagsToRemove, $newTags ) );
        $newTags = array_values( array_diff( $newTags, $tagsToRemove ) );

        sort( $prevTags );
        sort( $newTags );
        if ( $prevTags == $newTags ) {
            // No change.
            return false;
        }

        if ( !$newTags ) {
            // no tags left, so delete the row altogether
            $dbw->delete( 'tag_summary', $tsConds, __METHOD__ );
        } else {
            $dbw->replace( 'tag_summary',
                [ 'ts_rev_id', 'ts_rc_id', 'ts_log_id' ],
                array_filter( array_merge( $tsConds, [ 'ts_tags' => implode( ',', $newTags ) ] ) ),
                __METHOD__
            );
        }

        return true;
    }

    protected static function restrictedTagError( $msgOne, $msgMulti, $tags ) {
        $lang = RequestContext::getMain()->getLanguage();
        $count = count( $tags );
        return Status::newFatal( ( $count > 1 ) ? $msgMulti : $msgOne,
            $lang->commaList( $tags ), $count );
    }

    public static function canAddTagsAccompanyingChange( array $tags,
        User $user = null ) {

        if ( !is_null( $user ) ) {
            if ( !$user->isAllowed( 'applychangetags' ) ) {
                return Status::newFatal( 'tags-apply-no-permission' );
            } elseif ( $user->isBlocked() ) {
                return Status::newFatal( 'tags-apply-blocked' );
            }
        }

        // to be applied, a tag has to be explicitly defined
        // @todo Allow extensions to define tags that can be applied by users...
        $allowedTags = self::listExplicitlyDefinedTags();
        $disallowedTags = array_diff( $tags, $allowedTags );
        if ( $disallowedTags ) {
            return self::restrictedTagError( 'tags-apply-not-allowed-one',
                'tags-apply-not-allowed-multi', $disallowedTags );
        }

        return Status::newGood();
    }

    public static function addTagsAccompanyingChangeWithChecks(
        array $tags, $rc_id, $rev_id, $log_id, $params, User $user
    ) {

        // are we allowed to do this?
        $result = self::canAddTagsAccompanyingChange( $tags, $user );
        if ( !$result->isOK() ) {
            $result->value = null;
            return $result;
        }

        // do it!
        self::addTags( $tags, $rc_id, $rev_id, $log_id, $params );

        return Status::newGood( true );
    }

    public static function canUpdateTags( array $tagsToAdd, array $tagsToRemove,
        User $user = null ) {

        if ( !is_null( $user ) ) {
            if ( !$user->isAllowed( 'changetags' ) ) {
                return Status::newFatal( 'tags-update-no-permission' );
            } elseif ( $user->isBlocked() ) {
                return Status::newFatal( 'tags-update-blocked' );
            }
        }

        if ( $tagsToAdd ) {
            // to be added, a tag has to be explicitly defined
            // @todo Allow extensions to define tags that can be applied by users...
            $explicitlyDefinedTags = self::listExplicitlyDefinedTags();
            $diff = array_diff( $tagsToAdd, $explicitlyDefinedTags );
            if ( $diff ) {
                return self::restrictedTagError( 'tags-update-add-not-allowed-one',
                    'tags-update-add-not-allowed-multi', $diff );
            }
        }

        if ( $tagsToRemove ) {
            // to be removed, a tag must not be defined by an extension, or equivalently it
            // has to be either explicitly defined or not defined at all
            // (assuming no edge case of a tag both explicitly-defined and extension-defined)
            $extensionDefinedTags = self::listExtensionDefinedTags();
            $intersect = array_intersect( $tagsToRemove, $extensionDefinedTags );
            if ( $intersect ) {
                return self::restrictedTagError( 'tags-update-remove-not-allowed-one',
                    'tags-update-remove-not-allowed-multi', $intersect );
            }
        }

        return Status::newGood();
    }

    public static function updateTagsWithChecks( $tagsToAdd, $tagsToRemove,
        $rc_id, $rev_id, $log_id, $params, $reason, User $user ) {

        if ( is_null( $tagsToAdd ) ) {
            $tagsToAdd = [];
        }
        if ( is_null( $tagsToRemove ) ) {
            $tagsToRemove = [];
        }
        if ( !$tagsToAdd && !$tagsToRemove ) {
            // no-op, don't bother
            return Status::newGood( (object)[
                'logId' => null,
                'addedTags' => [],
                'removedTags' => [],
            ] );
        }

        // are we allowed to do this?
        $result = self::canUpdateTags( $tagsToAdd, $tagsToRemove, $user );
        if ( !$result->isOK() ) {
            $result->value = null;
            return $result;
        }

        // basic rate limiting
        if ( $user->pingLimiter( 'changetag' ) ) {
            return Status::newFatal( 'actionthrottledtext' );
        }

        // do it!
        list( $tagsAdded, $tagsRemoved, $initialTags ) = self::updateTags( $tagsToAdd,
            $tagsToRemove, $rc_id, $rev_id, $log_id, $params );
        if ( !$tagsAdded && !$tagsRemoved ) {
            // no-op, don't log it
            return Status::newGood( (object)[
                'logId' => null,
                'addedTags' => [],
                'removedTags' => [],
            ] );
        }

        // log it
        $logEntry = new ManualLogEntry( 'tag', 'update' );
        $logEntry->setPerformer( $user );
        $logEntry->setComment( $reason );

        // find the appropriate target page
        if ( $rev_id ) {
            $rev = Revision::newFromId( $rev_id );
            if ( $rev ) {
                $logEntry->setTarget( $rev->getTitle() );
            }
        } elseif ( $log_id ) {
            // This function is from revision deletion logic and has nothing to do with
            // change tags, but it appears to be the only other place in core where we
            // perform logged actions on log items.
            $logEntry->setTarget( RevDelLogList::suggestTarget( 0, [ $log_id ] ) );
        }

        if ( !$logEntry->getTarget() ) {
            // target is required, so we have to set something
            $logEntry->setTarget( SpecialPage::getTitleFor( 'Tags' ) );
        }

        $logParams = [
            '4::revid' => $rev_id,
            '5::logid' => $log_id,
            '6:list:tagsAdded' => $tagsAdded,
            '7:number:tagsAddedCount' => count( $tagsAdded ),
            '8:list:tagsRemoved' => $tagsRemoved,
            '9:number:tagsRemovedCount' => count( $tagsRemoved ),
            'initialTags' => $initialTags,
        ];
        $logEntry->setParameters( $logParams );
        $logEntry->setRelations( [ 'Tag' => array_merge( $tagsAdded, $tagsRemoved ) ] );

        $dbw = wfGetDB( DB_MASTER );
        $logId = $logEntry->insert( $dbw );
        // Only send this to UDP, not RC, similar to patrol events
        $logEntry->publish( $logId, 'udp' );

        return Status::newGood( (object)[
            'logId' => $logId,
            'addedTags' => $tagsAdded,
            'removedTags' => $tagsRemoved,
        ] );
    }

    public static function modifyDisplayQuery( &$tables, &$fields, &$conds,
                                        &$join_conds, &$options, $filter_tag = false ) {
        global $wgRequest, $wgUseTagFilter;

        if ( $filter_tag === false ) {
            $filter_tag = $wgRequest->getVal( 'tagfilter' );
        }

        // Figure out which conditions can be done.
        if ( in_array( 'recentchanges', $tables ) ) {
            $join_cond = 'ct_rc_id=rc_id';
        } elseif ( in_array( 'logging', $tables ) ) {
            $join_cond = 'ct_log_id=log_id';
        } elseif ( in_array( 'revision', $tables ) ) {
            $join_cond = 'ct_rev_id=rev_id';
        } elseif ( in_array( 'archive', $tables ) ) {
            $join_cond = 'ct_rev_id=ar_rev_id';
        } else {
            throw new MWException( 'Unable to determine appropriate JOIN condition for tagging.' );
        }

        $fields['ts_tags'] = wfGetDB( DB_SLAVE )->buildGroupConcatField(
            ',', 'change_tag', 'ct_tag', $join_cond
        );

        if ( $wgUseTagFilter && $filter_tag ) {
            // Somebody wants to filter on a tag.
            // Add an INNER JOIN on change_tag

            $tables[] = 'change_tag';
            $join_conds['change_tag'] = [ 'INNER JOIN', $join_cond ];
            $conds['ct_tag'] = $filter_tag;
        }
    }

    public static function buildTagFilterSelector( $selected = '',
        $fullForm = false, Title $title = null, $ooui = false
    ) {
        global $wgUseTagFilter;

        if ( !$wgUseTagFilter || !count( self::listDefinedTags() ) ) {
            return $fullForm ? '' : [];
        }

        $data = [
            Html::rawElement(
                'label',
                [ 'for' => 'tagfilter' ],
                wfMessage( 'tag-filter' )->parse()
            )
        ];

        if ( $ooui ) {
            $data[] = new OOUI\TextInputWidget( [
                'id' => 'tagfilter',
                'name' => 'tagfilter',
                'value' => $selected,
                'classes' => 'mw-tagfilter-input',
            ] );
        } else {
            $data[] = Xml::input(
                'tagfilter',
                20,
                $selected,
                [ 'class' => 'mw-tagfilter-input mw-ui-input mw-ui-input-inline', 'id' => 'tagfilter' ]
            );
        }

        if ( !$fullForm ) {
            return $data;
        }

        $html = implode( '&#160;', $data );
        $html .= "\n" .
            Xml::element(
                'input',
                [ 'type' => 'submit', 'value' => wfMessage( 'tag-filter-submit' )->text() ]
            );
        $html .= "\n" . Html::hidden( 'title', $title->getPrefixedText() );
        $html = Xml::tags(
            'form',
            [ 'action' => $title->getLocalURL(), 'class' => 'mw-tagfilter-form', 'method' => 'get' ],
            $html
        );

        return $html;
    }

    public static function defineTag( $tag ) {
        $dbw = wfGetDB( DB_MASTER );
        $dbw->replace( 'valid_tag',
            [ 'vt_tag' ],
            [ 'vt_tag' => $tag ],
            __METHOD__ );

        // clear the memcache of defined tags
        self::purgeTagCacheAll();
    }

    public static function undefineTag( $tag ) {
        $dbw = wfGetDB( DB_MASTER );
        $dbw->delete( 'valid_tag', [ 'vt_tag' => $tag ], __METHOD__ );

        // clear the memcache of defined tags
        self::purgeTagCacheAll();
    }

    protected static function logTagManagementAction( $action, $tag, $reason,
        User $user, $tagCount = null ) {

        $dbw = wfGetDB( DB_MASTER );

        $logEntry = new ManualLogEntry( 'managetags', $action );
        $logEntry->setPerformer( $user );
        // target page is not relevant, but it has to be set, so we just put in
        // the title of Special:Tags
        $logEntry->setTarget( Title::newFromText( 'Special:Tags' ) );
        $logEntry->setComment( $reason );

        $params = [ '4::tag' => $tag ];
        if ( !is_null( $tagCount ) ) {
            $params['5:number:count'] = $tagCount;
        }
        $logEntry->setParameters( $params );
        $logEntry->setRelations( [ 'Tag' => $tag ] );

        $logId = $logEntry->insert( $dbw );
        $logEntry->publish( $logId );
        return $logId;
    }

    public static function canActivateTag( $tag, User $user = null ) {
        if ( !is_null( $user ) ) {
            if ( !$user->isAllowed( 'managechangetags' ) ) {
                return Status::newFatal( 'tags-manage-no-permission' );
            } elseif ( $user->isBlocked() ) {
                return Status::newFatal( 'tags-manage-blocked' );
            }
        }

        // defined tags cannot be activated (a defined tag is either extension-
        // defined, in which case the extension chooses whether or not to active it;
        // or user-defined, in which case it is considered active)
        $definedTags = self::listDefinedTags();
        if ( in_array( $tag, $definedTags ) ) {
            return Status::newFatal( 'tags-activate-not-allowed', $tag );
        }

        // non-existing tags cannot be activated
        $tagUsage = self::tagUsageStatistics();
        if ( !isset( $tagUsage[$tag] ) ) { // we already know the tag is undefined
            return Status::newFatal( 'tags-activate-not-found', $tag );
        }

        return Status::newGood();
    }

    public static function activateTagWithChecks( $tag, $reason, User $user,
        $ignoreWarnings = false ) {

        // are we allowed to do this?
        $result = self::canActivateTag( $tag, $user );
        if ( $ignoreWarnings ? !$result->isOK() : !$result->isGood() ) {
            $result->value = null;
            return $result;
        }

        // do it!
        self::defineTag( $tag );

        // log it
        $logId = self::logTagManagementAction( 'activate', $tag, $reason, $user );
        return Status::newGood( $logId );
    }

    public static function canDeactivateTag( $tag, User $user = null ) {
        if ( !is_null( $user ) ) {
            if ( !$user->isAllowed( 'managechangetags' ) ) {
                return Status::newFatal( 'tags-manage-no-permission' );
            } elseif ( $user->isBlocked() ) {
                return Status::newFatal( 'tags-manage-blocked' );
            }
        }

        // only explicitly-defined tags can be deactivated
        $explicitlyDefinedTags = self::listExplicitlyDefinedTags();
        if ( !in_array( $tag, $explicitlyDefinedTags ) ) {
            return Status::newFatal( 'tags-deactivate-not-allowed', $tag );
        }
        return Status::newGood();
    }

    public static function deactivateTagWithChecks( $tag, $reason, User $user,
        $ignoreWarnings = false ) {

        // are we allowed to do this?
        $result = self::canDeactivateTag( $tag, $user );
        if ( $ignoreWarnings ? !$result->isOK() : !$result->isGood() ) {
            $result->value = null;
            return $result;
        }

        // do it!
        self::undefineTag( $tag );

        // log it
        $logId = self::logTagManagementAction( 'deactivate', $tag, $reason, $user );
        return Status::newGood( $logId );
    }

    public static function canCreateTag( $tag, User $user = null ) {
        if ( !is_null( $user ) ) {
            if ( !$user->isAllowed( 'managechangetags' ) ) {
                return Status::newFatal( 'tags-manage-no-permission' );
            } elseif ( $user->isBlocked() ) {
                return Status::newFatal( 'tags-manage-blocked' );
            }
        }

        // no empty tags
        if ( $tag === '' ) {
            return Status::newFatal( 'tags-create-no-name' );
        }

        // tags cannot contain commas (used as a delimiter in tag_summary table) or
        // slashes (would break tag description messages in MediaWiki namespace)
        if ( strpos( $tag, ',' ) !== false || strpos( $tag, '/' ) !== false ) {
            return Status::newFatal( 'tags-create-invalid-chars' );
        }

        // could the MediaWiki namespace description messages be created?
        $title = Title::makeTitleSafe( NS_MEDIAWIKI, "Tag-$tag-description" );
        if ( is_null( $title ) ) {
            return Status::newFatal( 'tags-create-invalid-title-chars' );
        }

        // does the tag already exist?
        $tagUsage = self::tagUsageStatistics();
        if ( isset( $tagUsage[$tag] ) || in_array( $tag, self::listDefinedTags() ) ) {
            return Status::newFatal( 'tags-create-already-exists', $tag );
        }

        // check with hooks
        $canCreateResult = Status::newGood();
        Hooks::run( 'ChangeTagCanCreate', [ $tag, $user, &$canCreateResult ] );
        return $canCreateResult;
    }

    public static function createTagWithChecks( $tag, $reason, User $user,
        $ignoreWarnings = false ) {

        // are we allowed to do this?
        $result = self::canCreateTag( $tag, $user );
        if ( $ignoreWarnings ? !$result->isOK() : !$result->isGood() ) {
            $result->value = null;
            return $result;
        }

        // do it!
        self::defineTag( $tag );

        // log it
        $logId = self::logTagManagementAction( 'create', $tag, $reason, $user );
        return Status::newGood( $logId );
    }

    public static function deleteTagEverywhere( $tag ) {
        $dbw = wfGetDB( DB_MASTER );
        $dbw->startAtomic( __METHOD__ );

        // delete from valid_tag
        self::undefineTag( $tag );

        // find out which revisions use this tag, so we can delete from tag_summary
        $result = $dbw->select( 'change_tag',
            [ 'ct_rc_id', 'ct_log_id', 'ct_rev_id', 'ct_tag' ],
            [ 'ct_tag' => $tag ],
            __METHOD__ );
        foreach ( $result as $row ) {
            // remove the tag from the relevant row of tag_summary
            $tagsToAdd = [];
            $tagsToRemove = [ $tag ];
            self::updateTagSummaryRow( $tagsToAdd, $tagsToRemove, $row->ct_rc_id,
                $row->ct_rev_id, $row->ct_log_id );
        }

        // delete from change_tag
        $dbw->delete( 'change_tag', [ 'ct_tag' => $tag ], __METHOD__ );

        $dbw->endAtomic( __METHOD__ );

        // give extensions a chance
        $status = Status::newGood();
        Hooks::run( 'ChangeTagAfterDelete', [ $tag, &$status ] );
        // let's not allow error results, as the actual tag deletion succeeded
        if ( !$status->isOK() ) {
            wfDebug( 'ChangeTagAfterDelete error condition downgraded to warning' );
            $status->ok = true;
        }

        // clear the memcache of defined tags
        self::purgeTagCacheAll();

        return $status;
    }

    public static function canDeleteTag( $tag, User $user = null ) {
        $tagUsage = self::tagUsageStatistics();

        if ( !is_null( $user ) ) {
            if ( !$user->isAllowed( 'managechangetags' ) ) {
                return Status::newFatal( 'tags-manage-no-permission' );
            } elseif ( $user->isBlocked() ) {
                return Status::newFatal( 'tags-manage-blocked' );
            }
        }

        if ( !isset( $tagUsage[$tag] ) && !in_array( $tag, self::listDefinedTags() ) ) {
            return Status::newFatal( 'tags-delete-not-found', $tag );
        }

        if ( isset( $tagUsage[$tag] ) && $tagUsage[$tag] > self::MAX_DELETE_USES ) {
            return Status::newFatal( 'tags-delete-too-many-uses', $tag, self::MAX_DELETE_USES );
        }

        $extensionDefined = self::listExtensionDefinedTags();
        if ( in_array( $tag, $extensionDefined ) ) {
            // extension-defined tags can't be deleted unless the extension
            // specifically allows it
            $status = Status::newFatal( 'tags-delete-not-allowed' );
        } else {
            // user-defined tags are deletable unless otherwise specified
            $status = Status::newGood();
        }

        Hooks::run( 'ChangeTagCanDelete', [ $tag, $user, &$status ] );
        return $status;
    }

    public static function deleteTagWithChecks( $tag, $reason, User $user,
        $ignoreWarnings = false ) {

        // are we allowed to do this?
        $result = self::canDeleteTag( $tag, $user );
        if ( $ignoreWarnings ? !$result->isOK() : !$result->isGood() ) {
            $result->value = null;
            return $result;
        }

        // store the tag usage statistics
        $tagUsage = self::tagUsageStatistics();
        $hitcount = isset( $tagUsage[$tag] ) ? $tagUsage[$tag] : 0;

        // do it!
        $deleteResult = self::deleteTagEverywhere( $tag );
        if ( !$deleteResult->isOK() ) {
            return $deleteResult;
        }

        // log it
        $logId = self::logTagManagementAction( 'delete', $tag, $reason, $user, $hitcount );
        $deleteResult->value = $logId;
        return $deleteResult;
    }

    public static function listExtensionActivatedTags() {
        return ObjectCache::getMainWANInstance()->getWithSetCallback(
            wfMemcKey( 'active-tags' ),
            300,
            function ( $oldValue, &$ttl, array &$setOpts ) {
                $setOpts += Database::getCacheSetOptions( wfGetDB( DB_SLAVE ) );

                // Ask extensions which tags they consider active
                $extensionActive = [];
                Hooks::run( 'ChangeTagsListActive', [ &$extensionActive ] );
                return $extensionActive;
            },
            [
                'checkKeys' => [ wfMemcKey( 'active-tags' ) ],
                'lockTSE' => 300,
                'pcTTL' => 30
            ]
        );
    }

    public static function listDefinedTags() {
        $tags1 = self::listExplicitlyDefinedTags();
        $tags2 = self::listExtensionDefinedTags();
        return array_values( array_unique( array_merge( $tags1, $tags2 ) ) );
    }

    public static function listExplicitlyDefinedTags() {
        $fname = __METHOD__;

        return ObjectCache::getMainWANInstance()->getWithSetCallback(
            wfMemcKey( 'valid-tags-db' ),
            300,
            function ( $oldValue, &$ttl, array &$setOpts ) use ( $fname ) {
                $dbr = wfGetDB( DB_SLAVE );

                $setOpts += Database::getCacheSetOptions( $dbr );

                $tags = $dbr->selectFieldValues( 'valid_tag', 'vt_tag', [], $fname );

                return array_filter( array_unique( $tags ) );
            },
            [
                'checkKeys' => [ wfMemcKey( 'valid-tags-db' ) ],
                'lockTSE' => 300,
                'pcTTL' => 30
            ]
        );
    }

    public static function listExtensionDefinedTags() {
        return ObjectCache::getMainWANInstance()->getWithSetCallback(
            wfMemcKey( 'valid-tags-hook' ),
            300,
            function ( $oldValue, &$ttl, array &$setOpts ) {
                $setOpts += Database::getCacheSetOptions( wfGetDB( DB_SLAVE ) );

                $tags = [];
                Hooks::run( 'ListDefinedTags', [ &$tags ] );
                return array_filter( array_unique( $tags ) );
            },
            [
                'checkKeys' => [ wfMemcKey( 'valid-tags-hook' ) ],
                'lockTSE' => 300,
                'pcTTL' => 30
            ]
        );
    }

    public static function purgeTagCacheAll() {
        $cache = ObjectCache::getMainWANInstance();

        $cache->touchCheckKey( wfMemcKey( 'active-tags' ) );
        $cache->touchCheckKey( wfMemcKey( 'valid-tags-db' ) );
        $cache->touchCheckKey( wfMemcKey( 'valid-tags-hook' ) );

        self::purgeTagUsageCache();
    }

    public static function purgeTagUsageCache() {
        $cache = ObjectCache::getMainWANInstance();

        $cache->touchCheckKey( wfMemcKey( 'change-tag-statistics' ) );
    }

    public static function tagUsageStatistics() {
        $fname = __METHOD__;
        return ObjectCache::getMainWANInstance()->getWithSetCallback(
            wfMemcKey( 'change-tag-statistics' ),
            300,
            function ( $oldValue, &$ttl, array &$setOpts ) use ( $fname ) {
                $dbr = wfGetDB( DB_SLAVE, 'vslow' );

                $setOpts += Database::getCacheSetOptions( $dbr );

                $res = $dbr->select(
                    'change_tag',
                    [ 'ct_tag', 'hitcount' => 'count(*)' ],
                    [],
                    $fname,
                    [ 'GROUP BY' => 'ct_tag', 'ORDER BY' => 'hitcount DESC' ]
                );

                $out = [];
                foreach ( $res as $row ) {
                    $out[$row->ct_tag] = $row->hitcount;
                }

                return $out;
            },
            [
                'checkKeys' => [ wfMemcKey( 'change-tag-statistics' ) ],
                'lockTSE' => 300,
                'pcTTL' => 30
            ]
        );
    }

    public static function showTagEditingUI( User $user ) {
        return $user->isAllowed( 'changetags' ) && (bool)self::listExplicitlyDefinedTags();
    }
}