From c3940d01d619257ac5829e9c39a89a2443d5a220 Mon Sep 17 00:00:00 2001
From: Aaron Schulz <aschulz@wikimedia.org>
Date: Wed, 8 May 2019 17:21:36 -0700
Subject: [PATCH] Fix stale client caching of history pages with
 $wgShowUpdateMarker enabled

Change-Id: I5b9d986e4f34166f2a1602129d2e01573bfe7ac5
---
 includes/Title.php                 |  2 +-
 includes/actions/HistoryAction.php | 25 +++++++++++++++++++++----
 2 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/includes/Title.php b/includes/Title.php
index a956ca2176..b9b1bb7947 100644
--- a/includes/Title.php
+++ b/includes/Title.php
@@ -4262,7 +4262,7 @@ class Title implements LinkTarget, IDBAccessObject {
 	 * Get the timestamp when this page was updated since the user last saw it.
 	 *
 	 * @param User|null $user
-	 * @return string|null
+	 * @return string|bool|null String timestamp, false if not watched, null if nothing is unseen
 	 */
 	public function getNotificationTimestamp( $user = null ) {
 		global $wgUser;
diff --git a/includes/actions/HistoryAction.php b/includes/actions/HistoryAction.php
index 4df2f563c7..958ec06f47 100644
--- a/includes/actions/HistoryAction.php
+++ b/includes/actions/HistoryAction.php
@@ -148,10 +148,17 @@ class HistoryAction extends FormlessAction {
 		$out = $this->getOutput();
 		$request = $this->getRequest();
 
-		/**
-		 * Allow client caching.
-		 */
-		if ( $out->checkLastModified( $this->page->getTouched() ) ) {
+		// Allow client-side HTTP caching of the history page.
+		// But, always ignore this cache if the (logged-in) user has this page on their watchlist
+		// and has one or more unseen revisions. Otherwise, we might be showing stale update markers.
+		// The Last-Modified for the history page does not change when user's markers are cleared,
+		// so going from "some unseen" to "all seen" would not clear the cache.
+		// But, when all of the revisions are marked as seen, then only way for new unseen revision
+		// markers to appear, is for the page to be edited, which updates page_touched/Last-Modified.
+		if (
+			!$this->hasUnseenRevisionMarkers() &&
+			$out->checkLastModified( $this->page->getTouched() )
+		) {
 			return null; // Client cache fresh and headers sent, nothing more to do.
 		}
 
@@ -305,6 +312,16 @@ class HistoryAction extends FormlessAction {
 		return null;
 	}
 
+	/**
+	 * @return bool Page is watched by and has unseen revision for the user
+	 */
+	private function hasUnseenRevisionMarkers() {
+		return (
+			$this->getContext()->getConfig()->get( 'ShowUpdatedMarker' ) &&
+			$this->getTitle()->getNotificationTimestamp( $this->getUser() )
+		);
+	}
+
 	/**
 	 * Fetch an array of revisions, specified by a given limit, offset and
 	 * direction. This is now only used by the feeds. It was previously
-- 
2.20.1