Merge "DatabaseMssql: Don't duplicate body of makeList()"

[mediawiki.git] / includes / ChangeTags.php

<?php
/**
 * Recent changes tagging.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 */

class ChangeTags {
        /**
         * Can't delete tags with more than this many uses. Similar in intent to
         * the bigdelete user right
         * @todo Use the job queue for tag deletion to avoid this restriction
         */
        const MAX_DELETE_USES = 5000;

        /**
         * Creates HTML for the given tags
         *
         * @param string $tags Comma-separated list of tags
         * @param string $page A label for the type of action which is being displayed,
         *   for example: 'history', 'contributions' or 'newpages'
         * @return array Array with two items: (html, classes)
         *   - html: String: HTML for displaying the tags (empty string when param $tags is empty)
         *   - classes: Array of strings: CSS classes used in the generated html, one class for each tag
         */
        public static function formatSummaryRow( $tags, $page ) {
                global $wgLang;

                $tags = explode( ',', $tags );

                // XXX(Ori Livneh, 2014-11-08): remove once bug 73181 is resolved.
                $tags = array_diff( $tags, array( 'HHVM', '' ) );

                if ( !$tags ) {
                        return array( '', array() );
                }

                $classes = array();

                $displayTags = array();
                foreach ( $tags as $tag ) {
                        $displayTags[] = Xml::tags(
                                'span',
                                array( 'class' => 'mw-tag-marker ' .
                                                                Sanitizer::escapeClass( "mw-tag-marker-$tag" ) ),
                                self::tagDescription( $tag )
                        );
                        $classes[] = Sanitizer::escapeClass( "mw-tag-$tag" );
                }
                $markers = wfMessage( 'tag-list-wrapper' )
                        ->numParams( count( $displayTags ) )
                        ->rawParams( $wgLang->commaList( $displayTags ) )
                        ->parse();
                $markers = Xml::tags( 'span', array( 'class' => 'mw-tag-markers' ), $markers );

                return array( $markers, $classes );
        }

        /**
         * Get a short description for a tag
         *
         * @param string $tag Tag
         *
         * @return string Short description of the tag from "mediawiki:tag-$tag" if this message exists,
         *   html-escaped version of $tag otherwise
         */
        public static function tagDescription( $tag ) {
                $msg = wfMessage( "tag-$tag" );
                return $msg->exists() ? $msg->parse() : htmlspecialchars( $tag );
        }

        /**
         * Add tags to a change given its rc_id, rev_id and/or log_id
         *
         * @param string|array $tags Tags to add to the change
         * @param int|null $rc_id The rc_id of the change to add the tags to
         * @param int|null $rev_id The rev_id of the change to add the tags to
         * @param int|null $log_id The log_id of the change to add the tags to
         * @param string $params Params to put in the ct_params field of table 'change_tag'
         *
         * @throws MWException
         * @return bool False if no changes are made, otherwise true
         *
         * @exception MWException When $rc_id, $rev_id and $log_id are all null
         */
        public static function addTags( $tags, $rc_id = null, $rev_id = null,
                $log_id = null, $params = null
        ) {
                if ( !is_array( $tags ) ) {
                        $tags = array( $tags );
                }

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

                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 a tag to 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.
                        if ( $log_id ) {
                                $rc_id = $dbw->selectField(
                                        'recentchanges',
                                        'rc_id',
                                        array( 'rc_logid' => $log_id ),
                                        __METHOD__
                                );
                        } elseif ( $rev_id ) {
                                $rc_id = $dbw->selectField(
                                        'recentchanges',
                                        'rc_id',
                                        array( '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',
                                array( 'rc_id' => $rc_id ),
                                __METHOD__
                        );
                        $rev_id = $dbw->selectField(
                                'recentchanges',
                                'rc_this_oldid',
                                array( 'rc_id' => $rc_id ),
                                __METHOD__
                        );
                }

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

                // 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 ) );
                $newTags = array_unique( array_merge( $prevTags, $tags ) );
                sort( $prevTags );
                sort( $newTags );

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

                $dbw->replace(
                        'tag_summary',
                        array( 'ts_rev_id', 'ts_rc_id', 'ts_log_id' ),
                        array_filter( array_merge( $tsConds, array( 'ts_tags' => implode( ',', $newTags ) ) ) ),
                        __METHOD__
                );

                // Insert the tags rows.
                $tagsRows = array();
                foreach ( $tags as $tag ) { // Filter so we don't insert NULLs as zero accidentally.
                        $tagsRows[] = array_filter(
                                array(
                                        '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__, array( 'IGNORE' ) );

                self::purgeTagUsageCache();
                return true;
        }

        /**
         * Applies all tags-related changes to a query.
         * Handles selecting tags, and filtering.
         * Needs $tables to be set up properly, so we can figure out which join conditions to use.
         *
         * @param string|array $tables Table names, see DatabaseBase::select
         * @param string|array $fields Fields used in query, see DatabaseBase::select
         * @param string|array $conds Conditions used in query, see DatabaseBase::select
         * @param array $join_conds Join conditions, see DatabaseBase::select
         * @param array $options Options, see Database::select
         * @param bool|string $filter_tag Tag to select on
         *
         * @throws MWException When unable to determine appropriate JOIN condition for tagging
         */
        public static function modifyDisplayQuery( &$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'] = array( 'INNER JOIN', $join_cond );
                        $conds['ct_tag'] = $filter_tag;
                }
        }

        /**
         * Build a text box to select a change tag
         *
         * @param string $selected Tag to select by default
         * @param bool $fullForm
         *        - if false, then it returns an array of (label, form).
         *        - if true, it returns an entire form around the selector.
         * @param Title $title Title object to send the form to.
         *        Used when, and only when $fullForm is true.
         * @return string|array
         *        - if $fullForm is false: Array with
         *        - if $fullForm is true: String, html fragment
         */
        public static function buildTagFilterSelector( $selected = '',
                $fullForm = false, Title $title = null
        ) {
                global $wgUseTagFilter;

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

                $data = array(
                        Html::rawElement(
                                'label',
                                array( 'for' => 'tagfilter' ),
                                wfMessage( 'tag-filter' )->parse()
                        ),
                        Xml::input(
                                'tagfilter',
                                20,
                                $selected,
                                array( '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',
                                array( 'type' => 'submit', 'value' => wfMessage( 'tag-filter-submit' )->text() )
                        );
                $html .= "\n" . Html::hidden( 'title', $title->getPrefixedText() );
                $html = Xml::tags(
                        'form',
                        array( 'action' => $title->getLocalURL(), 'class' => 'mw-tagfilter-form', 'method' => 'get' ),
                        $html
                );

                return $html;
        }

        /**
         * Defines a tag in the valid_tag table, without checking that the tag name
         * is valid.
         * Extensions should NOT use this function; they can use the ListDefinedTags
         * hook instead.
         *
         * @param string $tag Tag to create
         * @since 1.25
         */
        public static function defineTag( $tag ) {
                $dbw = wfGetDB( DB_MASTER );
                $dbw->replace( 'valid_tag',
                        array( 'vt_tag' ),
                        array( 'vt_tag' => $tag ),
                        __METHOD__ );

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

        /**
         * Removes a tag from the valid_tag table. The tag may remain in use by
         * extensions, and may still show up as 'defined' if an extension is setting
         * it from the ListDefinedTags hook.
         *
         * @param string $tag Tag to remove
         * @since 1.25
         */
        public static function undefineTag( $tag ) {
                $dbw = wfGetDB( DB_MASTER );
                $dbw->delete( 'valid_tag', array( 'vt_tag' => $tag ), __METHOD__ );

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

        /**
         * Writes a tag action into the tag management log.
         *
         * @param string $action
         * @param string $tag
         * @param string $reason
         * @param User $user Who to attribute the action to
         * @param int $tagCount For deletion only, how many usages the tag had before
         * it was deleted.
         * @since 1.25
         */
        protected static function logTagAction( $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 = array( '4::tag' => $tag );
                if ( !is_null( $tagCount ) ) {
                        $params['5:number:count'] = $tagCount;
                }
                $logEntry->setParameters( $params );
                $logEntry->setRelations( array( 'Tag' => $tag ) );

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

        /**
         * Is it OK to allow the user to activate this tag?
         *
         * @param string $tag Tag that you are interested in activating
         * @param User|null $user User whose permission you wish to check, or null if
         * you don't care (e.g. maintenance scripts)
         * @return Status
         * @since 1.25
         */
        public static function canActivateTag( $tag, User $user = null ) {
                if ( !is_null( $user ) && !$user->isAllowed( 'managechangetags' ) ) {
                        return Status::newFatal( 'tags-manage-no-permission' );
                }

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

                // 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 );
                }

                return Status::newGood();
        }

        /**
         * Activates a tag, checking whether it is allowed first, and adding a log
         * entry afterwards.
         *
         * Includes a call to ChangeTag::canActivateTag(), so your code doesn't need
         * to do that.
         *
         * @param string $tag
         * @param string $reason
         * @param User $user Who to give credit for the action
         * @param bool $ignoreWarnings Can be used for API interaction, default false
         * @return Status If successful, the Status contains the ID of the added log
         * entry as its value
         * @since 1.25
         */
        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::logTagAction( 'activate', $tag, $reason, $user );
                return Status::newGood( $logId );
        }

        /**
         * Is it OK to allow the user to deactivate this tag?
         *
         * @param string $tag Tag that you are interested in deactivating
         * @param User|null $user User whose permission you wish to check, or null if
         * you don't care (e.g. maintenance scripts)
         * @return Status
         * @since 1.25
         */
        public static function canDeactivateTag( $tag, User $user = null ) {
                if ( !is_null( $user ) && !$user->isAllowed( 'managechangetags' ) ) {
                        return Status::newFatal( 'tags-manage-no-permission' );
                }

                // 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();
        }

        /**
         * Deactivates a tag, checking whether it is allowed first, and adding a log
         * entry afterwards.
         *
         * Includes a call to ChangeTag::canDeactivateTag(), so your code doesn't need
         * to do that.
         *
         * @param string $tag
         * @param string $reason
         * @param User $user Who to give credit for the action
         * @param bool $ignoreWarnings Can be used for API interaction, default false
         * @return Status If successful, the Status contains the ID of the added log
         * entry as its value
         * @since 1.25
         */
        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::logTagAction( 'deactivate', $tag, $reason, $user );
                return Status::newGood( $logId );
        }

        /**
         * Is it OK to allow the user to create this tag?
         *
         * @param string $tag Tag that you are interested in creating
         * @param User|null $user User whose permission you wish to check, or null if
         * you don't care (e.g. maintenance scripts)
         * @return Status
         * @since 1.25
         */
        public static function canCreateTag( $tag, User $user = null ) {
                if ( !is_null( $user ) && !$user->isAllowed( 'managechangetags' ) ) {
                        return Status::newFatal( 'tags-manage-no-permission' );
                }

                // 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] ) ) {
                        return Status::newFatal( 'tags-create-already-exists', $tag );
                }

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

        /**
         * Creates a tag by adding a row to the `valid_tag` table.
         *
         * Includes a call to ChangeTag::canDeleteTag(), so your code doesn't need to
         * do that.
         *
         * @param string $tag
         * @param string $reason
         * @param User $user Who to give credit for the action
         * @param bool $ignoreWarnings Can be used for API interaction, default false
         * @return Status If successful, the Status contains the ID of the added log
         * entry as its value
         * @since 1.25
         */
        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::logTagAction( 'create', $tag, $reason, $user );
                return Status::newGood( $logId );
        }

        /**
         * Permanently removes all traces of a tag from the DB. Good for removing
         * misspelt or temporary tags.
         *
         * This function should be directly called by maintenance scripts only, never
         * by user-facing code. See deleteTagWithChecks() for functionality that can
         * safely be exposed to users.
         *
         * @param string $tag Tag to remove
         * @return Status The returned status will be good unless a hook changed it
         * @since 1.25
         */
        public static function deleteTagEverywhere( $tag ) {
                $dbw = wfGetDB( DB_MASTER );
                $dbw->begin( __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',
                        array( 'ct_rc_id', 'ct_log_id', 'ct_rev_id', 'ct_tag' ),
                        array( 'ct_tag' => $tag ),
                        __METHOD__ );
                foreach ( $result as $row ) {
                        if ( $row->ct_rev_id ) {
                                $field = 'ts_rev_id';
                                $fieldValue = $row->ct_rev_id;
                        } elseif ( $row->ct_log_id ) {
                                $field = 'ts_log_id';
                                $fieldValue = $row->ct_log_id;
                        } elseif ( $row->ct_rc_id ) {
                                $field = 'ts_rc_id';
                                $fieldValue = $row->ct_rc_id;
                        } else {
                                // don't know what's up; just skip it
                                continue;
                        }

                        // remove the tag from the relevant row of tag_summary
                        $tsResult = $dbw->selectField( 'tag_summary',
                                'ts_tags',
                                array( $field => $fieldValue ),
                                __METHOD__ );
                        $tsValues = explode( ',', $tsResult );
                        $tsValues = array_values( array_diff( $tsValues, array( $tag ) ) );
                        if ( !$tsValues ) {
                                // no tags left, so delete the row altogether
                                $dbw->delete( 'tag_summary',
                                        array( $field => $fieldValue ),
                                        __METHOD__ );
                        } else {
                                $dbw->update( 'tag_summary',
                                        array( 'ts_tags' => implode( ',', $tsValues ) ),
                                        array( $field => $fieldValue ),
                                        __METHOD__ );
                        }
                }

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

                $dbw->commit( __METHOD__ );

                // give extensions a chance
                $status = Status::newGood();
                Hooks::run( 'ChangeTagAfterDelete', array( $tag, &$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;
        }

        /**
         * Is it OK to allow the user to delete this tag?
         *
         * @param string $tag Tag that you are interested in deleting
         * @param User|null $user User whose permission you wish to check, or null if
         * you don't care (e.g. maintenance scripts)
         * @return Status
         * @since 1.25
         */
        public static function canDeleteTag( $tag, User $user = null ) {
                $tagUsage = self::tagUsageStatistics();

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

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

                if ( $tagUsage[$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', array( $tag, $user, &$status ) );
                return $status;
        }

        /**
         * Deletes a tag, checking whether it is allowed first, and adding a log entry
         * afterwards.
         *
         * Includes a call to ChangeTag::canDeleteTag(), so your code doesn't need to
         * do that.
         *
         * @param string $tag
         * @param string $reason
         * @param User $user Who to give credit for the action
         * @param bool $ignoreWarnings Can be used for API interaction, default false
         * @return Status If successful, the Status contains the ID of the added log
         * entry as its value
         * @since 1.25
         */
        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();

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

                // log it
                $logId = self::logTagAction( 'delete', $tag, $reason, $user, $tagUsage[$tag] );
                $deleteResult->value = $logId;
                return $deleteResult;
        }

        /**
         * Lists those tags which extensions report as being "active".
         *
         * @return array
         * @since 1.25
         */
        public static function listExtensionActivatedTags() {
                // Caching...
                global $wgMemc;
                $key = wfMemcKey( 'active-tags' );
                $tags = $wgMemc->get( $key );
                if ( $tags ) {
                        return $tags;
                }

                // ask extensions which tags they consider active
                $extensionActive = array();
                Hooks::run( 'ChangeTagsListActive', array( &$extensionActive ) );

                // Short-term caching.
                $wgMemc->set( $key, $extensionActive, 300 );
                return $extensionActive;
        }

        /**
         * Basically lists defined tags which count even if they aren't applied to anything.
         * It returns a union of the results of listExplicitlyDefinedTags() and
         * listExtensionDefinedTags().
         *
         * @return string[] Array of strings: tags
         */
        public static function listDefinedTags() {
                $tags1 = self::listExplicitlyDefinedTags();
                $tags2 = self::listExtensionDefinedTags();
                return array_values( array_unique( array_merge( $tags1, $tags2 ) ) );
        }

        /**
         * Lists tags explicitly defined in the `valid_tag` table of the database.
         * Tags in table 'change_tag' which are not in table 'valid_tag' are not
         * included.
         *
         * Tries memcached first.
         *
         * @return string[] Array of strings: tags
         * @since 1.25
         */
        public static function listExplicitlyDefinedTags() {
                // Caching...
                global $wgMemc;
                $key = wfMemcKey( 'valid-tags-db' );
                $tags = $wgMemc->get( $key );
                if ( $tags ) {
                        return $tags;
                }

                $emptyTags = array();

                // Some DB stuff
                $dbr = wfGetDB( DB_SLAVE );
                $res = $dbr->select( 'valid_tag', 'vt_tag', array(), __METHOD__ );
                foreach ( $res as $row ) {
                        $emptyTags[] = $row->vt_tag;
                }

                $emptyTags = array_filter( array_unique( $emptyTags ) );

                // Short-term caching.
                $wgMemc->set( $key, $emptyTags, 300 );
                return $emptyTags;
        }

        /**
         * Lists tags defined by extensions using the ListDefinedTags hook.
         * Extensions need only define those tags they deem to be in active use.
         *
         * Tries memcached first.
         *
         * @return string[] Array of strings: tags
         * @since 1.25
         */
        public static function listExtensionDefinedTags() {
                // Caching...
                global $wgMemc;
                $key = wfMemcKey( 'valid-tags-hook' );
                $tags = $wgMemc->get( $key );
                if ( $tags ) {
                        return $tags;
                }

                $emptyTags = array();
                Hooks::run( 'ListDefinedTags', array( &$emptyTags ) );
                $emptyTags = array_filter( array_unique( $emptyTags ) );

                // Short-term caching.
                $wgMemc->set( $key, $emptyTags, 300 );
                return $emptyTags;
        }

        /**
         * Invalidates the short-term cache of defined tags used by the
         * list*DefinedTags functions, as well as the tag statistics cache.
         * @since 1.25
         */
        public static function purgeTagCacheAll() {
                global $wgMemc;
                $wgMemc->delete( wfMemcKey( 'active-tags' ) );
                $wgMemc->delete( wfMemcKey( 'valid-tags-db' ) );
                $wgMemc->delete( wfMemcKey( 'valid-tags-hook' ) );
                self::purgeTagUsageCache();
        }

        /**
         * Invalidates the tag statistics cache only.
         * @since 1.25
         */
        public static function purgeTagUsageCache() {
                global $wgMemc;
                $wgMemc->delete( wfMemcKey( 'change-tag-statistics' ) );
        }

        /**
         * Returns a map of any tags used on the wiki to number of edits
         * tagged with them, ordered descending by the hitcount.
         *
         * Keeps a short-term cache in memory, so calling this multiple times in the
         * same request should be fine.
         *
         * @return array Array of string => int
         */
        public static function tagUsageStatistics() {
                // Caching...
                global $wgMemc;
                $key = wfMemcKey( 'change-tag-statistics' );
                $stats = $wgMemc->get( $key );
                if ( $stats ) {
                        return $stats;
                }

                $out = array();

                $dbr = wfGetDB( DB_SLAVE );
                $res = $dbr->select(
                        'change_tag',
                        array( 'ct_tag', 'hitcount' => 'count(*)' ),
                        array(),
                        __METHOD__,
                        array( 'GROUP BY' => 'ct_tag', 'ORDER BY' => 'hitcount DESC' )
                );

                foreach ( $res as $row ) {
                        $out[$row->ct_tag] = $row->hitcount;
                }
                foreach ( self::listDefinedTags() as $tag ) {
                        if ( !isset( $out[$tag] ) ) {
                                $out[$tag] = 0;
                        }
                }

                // Cache for a very short time
                $wgMemc->set( $key, $out, 300 );
                return $out;
        }
}