From 54626e5ce172ea469942b01e0ea73f34fb5de07a Mon Sep 17 00:00:00 2001
From: daniel <dkinzler@wikimedia.org>
Date: Fri, 12 Jul 2019 12:21:44 +0200
Subject: [PATCH] Title: Title::getSubpage should not lose the interwiki prefix

This issue was discovered while investigating T227700, and added some
confusion. This patch is necessary for Special:MyLanguage to behave
correctly in all cases, but it's not necessary for fixing the primary
critical problem.

Bug: T227700
Change-Id: Ib4cbeec47a877c473cbd501cc964cc66d169b99e
---
 includes/Title.php                   |  7 +++++-
 tests/phpunit/includes/TitleTest.php | 36 ++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/includes/Title.php b/includes/Title.php
index 6e75102c92..079f2c8a55 100644
--- a/includes/Title.php
+++ b/includes/Title.php
@@ -1887,7 +1887,12 @@ class Title implements LinkTarget, IDBAccessObject {
 	 * @since 1.20
 	 */
 	public function getSubpage( $text ) {
-		return self::makeTitleSafe( $this->mNamespace, $this->getText() . '/' . $text );
+		return self::makeTitleSafe(
+			$this->mNamespace,
+			$this->getText() . '/' . $text,
+			'',
+			$this->mInterwiki
+		);
 	}
 
 	/**
diff --git a/tests/phpunit/includes/TitleTest.php b/tests/phpunit/includes/TitleTest.php
index 4ffef02d19..d0a95c252f 100644
--- a/tests/phpunit/includes/TitleTest.php
+++ b/tests/phpunit/includes/TitleTest.php
@@ -1,5 +1,6 @@
 <?php
 
+use MediaWiki\Interwiki\InterwikiLookup;
 use MediaWiki\Linker\LinkTarget;
 use MediaWiki\MediaWikiServices;
 use Wikimedia\TestingAccessWrapper;
@@ -577,6 +578,41 @@ class TitleTest extends MediaWikiTestCase {
 		];
 	}
 
+	public function provideSubpage() {
+		// NOTE: avoid constructing Title objects in the provider, since it may access the database.
+		return [
+			[ 'Foo', 'x', new TitleValue( NS_MAIN, 'Foo/x' ) ],
+			[ 'Foo#bar', 'x', new TitleValue( NS_MAIN, 'Foo/x' ) ],
+			[ 'User:Foo', 'x', new TitleValue( NS_USER, 'Foo/x' ) ],
+			[ 'wiki:User:Foo', 'x', new TitleValue( NS_MAIN, 'User:Foo/x', '', 'wiki' ) ],
+		];
+	}
+
+	/**
+	 * @dataProvider provideSubpage
+	 * @covers Title::getSubpage
+	 */
+	public function testSubpage( $title, $sub, LinkTarget $expected ) {
+		$interwikiLookup = $this->getMock( InterwikiLookup::class );
+		$interwikiLookup->expects( $this->any() )
+			->method( 'isValidInterwiki' )
+			->willReturnCallback(
+				function ( $prefix ) {
+					return $prefix == 'wiki';
+				}
+			);
+
+		$this->setService( 'InterwikiLookup', $interwikiLookup );
+
+		$title = Title::newFromText( $title );
+		$expected = Title::newFromLinkTarget( $expected );
+		$actual = $title->getSubpage( $sub );
+
+		// NOTE: convert to string for comparison
+		$this->assertSame( $expected->getPrefixedText(), $actual->getPrefixedText(), 'text form' );
+		$this->assertTrue( $expected->equals( $actual ), 'Title equality' );
+	}
+
 	public static function provideNewFromTitleValue() {
 		return [
 			[ new TitleValue( NS_MAIN, 'Foo' ) ],
-- 
2.20.1