From c99e3d537ede1628f1bc2d833e5285a2dd083399 Mon Sep 17 00:00:00 2001
From: "This, that and the other" <at.light@live.com.au>
Date: Fri, 29 Jan 2016 01:03:18 +1100
Subject: [PATCH] Linker: Avoid passing false to Title::newFromText

This is needed because the call to substr() returns false when the string
is only 1 character long, which occurs when parsing degenerate links like
[[:]] and [[::|foo]].

The seemingly unnecessary test for $match[1] !== '' is for forwards
compatibility with PHP 7.

Bug: T121706
Change-Id: Icc19ee990d01958d64b938d298e9a7e1df7181b5
---
 includes/Linker.php | 43 +++++++++++++++++++++++--------------------
 1 file changed, 23 insertions(+), 20 deletions(-)

diff --git a/includes/Linker.php b/includes/Linker.php
index 8e79d9c38e..43df8397cf 100644
--- a/includes/Linker.php
+++ b/includes/Linker.php
@@ -1449,31 +1449,34 @@ class Linker {
 					}
 				} else {
 					# Other kind of link
-					if ( preg_match( $wgContLang->linkTrail(), $match[3], $submatch ) ) {
-						$trail = $submatch[1];
-					} else {
-						$trail = "";
-					}
-					$linkRegexp = '/\[\[(.*?)\]\]' . preg_quote( $trail, '/' ) . '/';
+					# Make sure its target is non-empty
 					if ( isset( $match[1][0] ) && $match[1][0] == ':' ) {
 						$match[1] = substr( $match[1], 1 );
 					}
-					list( $inside, $trail ) = Linker::splitTrail( $trail );
-
-					$linkText = $text;
-					$linkTarget = Linker::normalizeSubpageLink( $title, $match[1], $linkText );
-
-					$target = Title::newFromText( $linkTarget );
-					if ( $target ) {
-						if ( $target->getText() == '' && !$target->isExternal()
-							&& !$local && $title
-						) {
-							$newTarget = clone $title;
-							$newTarget->setFragment( '#' . $target->getFragment() );
-							$target = $newTarget;
+					if ( $match[1] !== false && $match[1] !== '' ) {
+						if ( preg_match( $wgContLang->linkTrail(), $match[3], $submatch ) ) {
+							$trail = $submatch[1];
+						} else {
+							$trail = "";
 						}
+						$linkRegexp = '/\[\[(.*?)\]\]' . preg_quote( $trail, '/' ) . '/';
+						list( $inside, $trail ) = Linker::splitTrail( $trail );
+
+						$linkText = $text;
+						$linkTarget = Linker::normalizeSubpageLink( $title, $match[1], $linkText );
+
+						$target = Title::newFromText( $linkTarget );
+						if ( $target ) {
+							if ( $target->getText() == '' && !$target->isExternal()
+								&& !$local && $title
+							) {
+								$newTarget = clone $title;
+								$newTarget->setFragment( '#' . $target->getFragment() );
+								$target = $newTarget;
+							}
 
-						$thelink = Linker::makeCommentLink( $target, $linkText . $inside, $wikiId ) . $trail;
+							$thelink = Linker::makeCommentLink( $target, $linkText . $inside, $wikiId ) . $trail;
+						}
 					}
 				}
 				if ( $thelink ) {
-- 
2.20.1