Limit number of topic subscriptions per user

Per Manuel Arostegui in T263817#7033384. The limit is 5000.
(I picked it arbitrarily, there's no real rationale for it.)

Also log a warning when any user reaches half of the limit,
so that we might make a decision about changing this mechanism
before it starts affecting users. Maybe at that time we'll
have data to show that it's safe to remove the limit.

Bug: T263817
Change-Id: I18a8ee0ad7383759229c5721d5253fb591457d4d

i18n/api/en.json

@@ -5,6 +5,8 @@
 	"apierror-discussiontools-commentid-notfound": "Comment with the ID '$1' not found.",
 	"apierror-discussiontools-commentname-ambiguous": "Multiple comments with the name '$1' found, use ID instead.",
 	"apierror-discussiontools-commentname-notfound": "Comment with the name '$1' not found.",
+	"apierror-discussiontools-subscription-failed-add": "Could not subscribe to this topic.",
+	"apierror-discussiontools-subscription-failed-remove": "Could not unsubscribe from this topic.",
 	"apihelp-discussiontools-param-oldid": "The revision number to use (defaults to latest revision).",
 	"apihelp-discussiontools-paramvalue-paction-transcludedfrom": "Return titles of pages from which each comment on the given page is transcluded.",
 	"apihelp-discussiontools-summary": "Returns metadata required to initialize the discussion tools.",

i18n/api/qqq.json

@@ -7,6 +7,8 @@
 	"apierror-discussiontools-commentid-notfound": "{{doc-apierror}}\n\nParameters:\n* $1 - Comment ID",
 	"apierror-discussiontools-commentname-ambiguous": "{{doc-apierror}}\n\nParameters:\n* $1 - Comment name",
 	"apierror-discussiontools-commentname-notfound": "{{doc-apierror}}\n\nParameters:\n* $1 - Comment name",
+	"apierror-discussiontools-subscription-failed-add": "{{doc-apierror}}",
+	"apierror-discussiontools-subscription-failed-remove": "{{doc-apierror}}",
 	"apihelp-discussiontools-param-oldid": "{{doc-apihelp-param|discussiontools|oldid}}",
 	"apihelp-discussiontools-paramvalue-paction-transcludedfrom": "{{doc-apihelp-paramvalue|discussiontools|paction|transcludedfrom}}",
 	"apihelp-discussiontools-summary": "{{doc-apihelp-summary|discussiontools}}",

includes/ApiDiscussionToolsSubscribe.php

@@ -58,16 +58,22 @@ class ApiDiscussionToolsSubscribe extends ApiBase {
 		$subscribe = $params['subscribe'];
 
 		if ( $subscribe ) {
-			$this->subscriptionStore->addSubscriptionForUser(
+			$success = $this->subscriptionStore->addSubscriptionForUser(
 				$user,
 				$title,
 				$commentName
 			);
+			if ( !$success ) {
+				$this->dieWithError( 'apierror-discussiontools-subscription-failed-add', 'subscription-failed' );
+			}
 		} else {
-			$this->subscriptionStore->removeSubscriptionForUser(
+			$success = $this->subscriptionStore->removeSubscriptionForUser(
 				$user,
 				$commentName
 			);
+			if ( !$success ) {
+				$this->dieWithError( 'apierror-discussiontools-subscription-failed-remove', 'subscription-failed' );
+			}
 		}
 		// TODO: Subscribe should be tri-state:
 		// * subscribe (add row)
@@ -77,7 +83,7 @@ class ApiDiscussionToolsSubscribe extends ApiBase {
 		$result = [
 			'page' => $title,
 			'commentname' => $commentName,
-			'subscribe' => $subscribe
+			'subscribe' => $subscribe,
 		];
 
 		$this->getResult()->addValue( null, $this->getModuleName(), $result );

includes/SubscriptionStore.php

@@ -3,6 +3,7 @@
 namespace MediaWiki\Extension\DiscussionTools;
 
 use MediaWiki\Linker\LinkTarget;
+use MediaWiki\Logger\LoggerFactory;
 use MediaWiki\User\UserFactory;
 use MediaWiki\User\UserIdentity;
 use ReadOnlyMode;
@@ -18,6 +19,11 @@ use Wikimedia\Rdbms\IResultWrapper;
 // use Wikimedia\Timestamp\ConvertibleTimestamp;
 
 class SubscriptionStore {
+	/**
+	 * Maximum number of subscriptions that we can store for each user.
+	 */
+	private const USER_SUBSCRIPTION_LIMIT = 5000;
+
 	/** @var ILBFactory */
 	private $lbFactory;
 
@@ -196,6 +202,36 @@ class SubscriptionStore {
 		);
 	}
 
+	/**
+	 * @param UserIdentity $user
+	 * @return bool
+	 */
+	private function userExceedsSubscriptionLimit( UserIdentity $user ) : bool {
+		$logger = LoggerFactory::getInstance( 'DiscussionTools' );
+		// This is always queried before updating
+		$db = $this->getConnectionRef( DB_PRIMARY );
+
+		$rowCount = $db->selectRowCount(
+			'discussiontools_subscription',
+			'*',
+			[ 'sub_user' => $user->getId() ],
+			__METHOD__,
+			[ 'LIMIT' => self::USER_SUBSCRIPTION_LIMIT ]
+		);
+
+		if ( $rowCount >= self::USER_SUBSCRIPTION_LIMIT / 2 ) {
+			$logger->warning(
+				"User {user} has {rowCount} subscriptions, approaching the limit",
+				[
+					'user' => $user->getId(),
+					'rowCount' => $rowCount,
+				]
+			);
+		}
+
+		return $rowCount >= self::USER_SUBSCRIPTION_LIMIT;
+	}
+
 	/**
 	 * @param UserIdentity $user
 	 * @param LinkTarget $target
@@ -214,6 +250,9 @@ class SubscriptionStore {
 		if ( !$user->isRegistered() ) {
 			return false;
 		}
+		if ( $this->userExceedsSubscriptionLimit( $user ) ) {
+			return false;
+		}
 		$dbw = $this->getConnectionRef( DB_PRIMARY );
 		$dbw->upsert(
 			'discussiontools_subscription',

modules/controller.js

@@ -285,6 +285,8 @@ function initTopicSubscriptions( $container ) {
 					)
 				}
 			);
+		}, function ( code, data ) {
+			mw.notify( api.getErrorMessage( data ), { type: 'error' } );
 		} );
 	} );
 }