Provide a standard way to get the target of a redirect page

The new Scribunto_LuaTitleLibrary::redirectTarget() method is
used by mw.title objects as read-only attribute 'redirectTarget'.

If the page does not exist or it is not a redirect, the value
of the attribute is `false`; otherwise, it is the target of the
redirect page, as mw.title object.

This is a proper alternative to parsing wikitext as it is done in:
https://en.wikipedia.org/wiki/Module:Redirect

Bug: T68974
Change-Id: Id4d9b0f8c1cd09ebc42c031d4d3fc0c33eea44aa

engines/LuaCommon/TitleLibrary.php

@@ -18,6 +18,7 @@ class Scribunto_LuaTitleLibrary extends Scribunto_LuaLibraryBase {
 			'getFileInfo' => array( $this, 'getFileInfo' ),
 			'protectionLevels' => array( $this, 'protectionLevels' ),
 			'cascadingProtection' => array( $this, 'cascadingProtection' ),
+			'redirectTarget' => array( $this, 'redirectTarget' ),
 		);
 		return $this->getEngine()->registerInterface( 'mw.title.lua', $lib, array(
 			'thisTitle' => $this->getInexpensiveTitleData( $this->getTitle() ),
@@ -241,11 +242,18 @@ class Scribunto_LuaTitleLibrary extends Scribunto_LuaLibraryBase {
 		return array( call_user_func_array( array( $title, $func ), $args ) );
 	}
 
-	function getContent( $text ) {
-		$this->checkType( 'getContent', 1, $text, 'string' );
+	/**
+	 * Utility to get a Content object from a title
+	 *
+	 * The title is counted as a transclusion.
+	 *
+	 * @param $text string Title text
+	 * @return Content|null The Content object of the title, null if missing
+	 */
+	private function getContentInternal( $text ) {
 		$title = Title::newFromText( $text );
 		if ( !$title ) {
-			return array( null );
+			return null;
 		}
 
 		// Record in templatelinks, so edits cause the page to be refreshed
@@ -261,14 +269,13 @@ class Scribunto_LuaTitleLibrary extends Scribunto_LuaLibraryBase {
 		} else {
 			$rev = Revision::newFromTitle( $title );
 		}
-		if ( !$rev ) {
-			return array( null );
+		return $rev ? $rev->getContent() : null;
 	}
-		$content = $rev->getContent();
-		if ( !$content ) {
-			return array( null );
-		}
-		return array( $content->serialize() );
+
+	function getContent( $text ) {
+		$this->checkType( 'getContent', 1, $text, 'string' );
+		$content = $this->getContentInternal( $text );
+		return array( $content ? $content->serialize() : null );
 	}
 
 	function getFileInfo( $text ) {
@@ -355,4 +362,11 @@ class Scribunto_LuaTitleLibrary extends Scribunto_LuaLibraryBase {
 			'restrictions' => array_map( 'Scribunto_LuaTitleLibrary::makeArrayOneBased', $restrictions )
 		) );
 	}
+
+	public function redirectTarget( $text ) {
+		$this->checkType( 'redirectTarget', 1, $text, 'string' );
+		$content = $this->getContentInternal( $text );
+		$redirTitle = $content ? $content->getRedirectTarget() : null;
+		return array( $redirTitle ? $this->getInexpensiveTitleData( $redirTitle ) : null );
+	}
 }

engines/LuaCommon/lualib/mw.title.lua

@@ -173,6 +173,7 @@ local function makeTitleObject( data )
 		isRedirect = 'e',
 		contentModel = 'e',
 		id = 'e',
+		redirectTarget = true,
 	}
 	for k in pairs( data ) do
 		readOnlyFields[k] = true
@@ -259,6 +260,12 @@ local function makeTitleObject( data )
 				end
 				return data.cascadingProtection
 			end
+			if k == 'redirectTarget' then
+				if data.redirectTarget == nil then
+					data.redirectTarget = makeTitleObject( php.redirectTarget( data.prefixedText ) ) or false
+				end
+				return data.redirectTarget
+			end
 
 			return data[k]
 		end,

tests/engines/LuaCommon/TitleLibraryTest.php

@@ -48,6 +48,19 @@ class Scribunto_LuaTitleLibraryTests extends Scribunto_LuaEngineTestBase {
 			new WikitextContent( '{{int:mainpage}}<includeonly>...</includeonly><noinclude>...</noinclude>' ),
 			'Summary'
 		);
+		$testPageId = $page->getId();
+
+		// Pages for redirectTarget tests
+		$page = WikiPage::factory( Title::newFromText( 'ScribuntoTestRedirect' ) );
+		$page->doEditContent(
+			new WikitextContent( '#REDIRECT [[ScribuntoTestTarget]]' ),
+			'Summary'
+		);
+		$page = WikiPage::factory( Title::newFromText( 'ScribuntoTestNonRedirect' ) );
+		$page->doEditContent(
+			new WikitextContent( 'Not a redirect.' ),
+			'Summary'
+		);
 
 		// Set restrictions for protectionLevels and cascadingProtection tests
 		// Since mRestrictionsLoaded is true, they don't count as expensive
@@ -88,7 +101,7 @@ class Scribunto_LuaTitleLibraryTests extends Scribunto_LuaEngineTestBase {
 		// Indicate to the tests that it's safe to create the title objects
 		$interpreter = $this->getEngine()->getInterpreter();
 		$interpreter->callFunction(
-			$interpreter->loadString( "mw.title.testPageId = {$page->getId()}", 'fortest' )
+			$interpreter->loadString( "mw.title.testPageId = $testPageId", 'fortest' )
 		);
 
 		$this->setMwGlobals( array(

tests/engines/LuaCommon/TitleLibraryTests.lua

@@ -71,7 +71,25 @@ local function test_inexpensive()
 end
 
 local function test_getContent()
-	return mw.title.new( 'ScribuntoTestPage' ):getContent()
+	return mw.title.new( 'ScribuntoTestPage' ):getContent(),
+		mw.title.new( 'ScribuntoTestNonExistingPage' ):getContent()
+end
+
+local function test_redirectTarget()
+	local targets = {}
+	local titles = {
+		'ScribuntoTestRedirect',
+		'ScribuntoTestNonRedirect',
+		'ScribuntoTestNonExistingPage'
+	}
+	for _, title in ipairs( titles ) do
+		local target = mw.title.new( title ).redirectTarget
+		if title.prefixedText ~= nil then
+			target = title.prefixedText
+		end
+		table.insert( targets, target )
+	end
+	return unpack( targets )
 end
 
 local function test_getCurrentTitle_fragment()
@@ -372,7 +390,14 @@ local tests = {
 	},
 
 	{ name = '.getContent()', func = test_getContent,
-	  expect = { '{{int:mainpage}}<includeonly>...</includeonly><noinclude>...</noinclude>' }
+	  expect = {
+		  '{{int:mainpage}}<includeonly>...</includeonly><noinclude>...</noinclude>',
+		  nil,
+	  }
+	},
+
+	{ name = '.redirectTarget', func = test_redirectTarget, type = 'ToString',
+	  expect = { 'ScribuntoTestTarget', false, false }
 	},
 
 	{ name = 'not quite too many expensive functions', func = test_expensive_10,