I18N: Add an additional caching layer for `_load_textdomain_just_in_time()`.

Previously, if no translation files exist for a text domain, `_load_textdomain_just_in_time()` went through the entire process each time it was called. This results in an increased call to `get_locale()` and its `locale` filter.
This change splits the logic into `_get_path_to_translation()` and `_get_path_to_translation_from_lang_dir()`. The former, which is used by `_load_textdomain_just_in_time()`, caches the result of the latter. It also removes some non-working code from `WP_Locale_Switcher::load_translations()`.

Props jrf, swissspidy, sharkomatic, ocean90.
Fixes #37997.

git-svn-id: https://develop.svn.wordpress.org/trunk@39330 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-locale-switcher.php

@@ -202,14 +202,7 @@ class WP_Locale_Switcher {
 				continue;
 			}
 
-			$mofile = $l10n[ $domain ]->get_filename();
-
 			unload_textdomain( $domain );
-
-			if ( $mofile ) {
-				load_textdomain( $domain, $mofile );
-			}
-
 			get_translations_for_domain( $domain );
 		}
 	}
@@ -228,6 +221,9 @@ class WP_Locale_Switcher {
 	 * @param string $locale The locale to change to.
 	 */
 	private function change_locale( $locale ) {
+		// Reset translation availability information.
+		_get_path_to_translation( null, true );
+
 		$this->load_translations( $locale );
 
 		$GLOBALS['wp_locale'] = new WP_Locale();

src/wp-includes/l10n.php

@@ -827,8 +827,6 @@ function load_child_theme_textdomain( $domain, $path = false ) {
  * the translation file from `wp-content/languages`, removing the need
  * to call load_plugin_texdomain() or load_theme_texdomain().
  *
- * Holds a cached list of available .mo files to improve performance.
- *
  * @since 4.6.0
  * @access private
  *
@@ -843,13 +841,63 @@ function _load_textdomain_just_in_time( $domain ) {
 
 	$l10n_unloaded = (array) $l10n_unloaded;
 
-	static $cached_mofiles = null;
-
 	// Short-circuit if domain is 'default' which is reserved for core.
 	if ( 'default' === $domain || isset( $l10n_unloaded[ $domain ] ) ) {
 		return false;
 	}
 
+	$translation_path = _get_path_to_translation( $domain );
+	if ( false === $translation_path ) {
+		return false;
+	}
+
+	return load_textdomain( $domain, $translation_path );
+}
+
+/**
+ * Gets the path to a translation file for loading a textdomain just in time.
+ *
+ * Caches the retrieved results internally.
+ *
+ * @since 4.7.0
+ * @access private
+ *
+ * @see _load_textdomain_just_in_time()
+ *
+ * @param string $domain Text domain. Unique identifier for retrieving translated strings.
+ * @param bool   $reset  Whether to reset the internal cache. Used by the switch to locale functionality.
+ * @return string|false The path to the translation file or false if no translation file was found.
+ */
+function _get_path_to_translation( $domain, $reset = false ) {
+	static $available_translations = array();
+
+	if ( true === $reset ) {
+		$available_translations = array();
+	}
+
+	if ( ! isset( $available_translations[ $domain ] ) ) {
+		$available_translations[ $domain ] = _get_path_to_translation_from_lang_dir( $domain );
+	}
+
+	return $available_translations[ $domain ];
+}
+
+/**
+ * Gets the path to a translation file in the languages directory for the current locale.
+ *
+ * Holds a cached list of available .mo files to improve performance.
+ *
+ * @since 4.7.0
+ * @access private
+ *
+ * @see _get_path_to_translation()
+ *
+ * @param string $domain Text domain. Unique identifier for retrieving translated strings.
+ * @return string|false The path to the translation file or false if no translation file was found.
+ */
+function _get_path_to_translation_from_lang_dir( $domain ) {
+	static $cached_mofiles = null;
+
 	if ( null === $cached_mofiles ) {
 		$cached_mofiles = array();
 
@@ -869,12 +917,14 @@ function _load_textdomain_just_in_time( $domain ) {
 	$locale = is_admin() ? get_user_locale() : get_locale();
 	$mofile = "{$domain}-{$locale}.mo";
 
-	if ( in_array( WP_LANG_DIR . '/plugins/' . $mofile, $cached_mofiles ) ) {
-		return load_textdomain( $domain, WP_LANG_DIR . '/plugins/' . $mofile );
+	$path = WP_LANG_DIR . '/plugins/' . $mofile;
+	if ( in_array( $path, $cached_mofiles ) ) {
+		return $path;
 	}
 
-	if ( in_array( WP_LANG_DIR . '/themes/' . $mofile, $cached_mofiles ) ) {
-		return load_textdomain( $domain, WP_LANG_DIR . '/themes/' . $mofile );
+	$path = WP_LANG_DIR . '/themes/' . $mofile;
+	if ( in_array( $path, $cached_mofiles ) ) {
+		return $path;
 	}
 
 	return false;

tests/phpunit/tests/l10n/loadTextdomainJustInTime.php

@@ -8,6 +8,7 @@ class Tests_L10n_loadTextdomainJustInTime extends WP_UnitTestCase {
 	protected $orig_theme_dir;
 	protected $theme_root;
 	protected static $user_id;
+	private $locale_count;
 
 	public static function wpSetUpBeforeClass( $factory ) {
 		self::$user_id = $factory->user->create( array(
@@ -21,6 +22,7 @@ class Tests_L10n_loadTextdomainJustInTime extends WP_UnitTestCase {
 
 		$this->theme_root = DIR_TESTDATA . '/themedir1';
 		$this->orig_theme_dir = $GLOBALS['wp_theme_directories'];
+		$this->locale_count = 0;
 
 		// /themes is necessary as theme.php functions assume /themes is the root if there is only one root.
 		$GLOBALS['wp_theme_directories'] = array( WP_CONTENT_DIR . '/themes', $this->theme_root );
@@ -31,6 +33,7 @@ class Tests_L10n_loadTextdomainJustInTime extends WP_UnitTestCase {
 		unset( $GLOBALS['wp_themes'] );
 		unset( $GLOBALS['l10n'] );
 		unset( $GLOBALS['l10n_unloaded'] );
+		_get_path_to_translation( null, true );
 	}
 
 	public function tearDown() {
@@ -42,6 +45,7 @@ class Tests_L10n_loadTextdomainJustInTime extends WP_UnitTestCase {
 		unset( $GLOBALS['wp_themes'] );
 		unset( $GLOBALS['l10n'] );
 		unset( $GLOBALS['l10n_unloaded'] );
+		_get_path_to_translation( null, true );
 
 		parent::tearDown();
 	}
@@ -162,6 +166,24 @@ class Tests_L10n_loadTextdomainJustInTime extends WP_UnitTestCase {
 		$this->assertSame( 'Das ist ein Dummy Plugin', $expected );
 	}
 
+	/**
+	 * @ticket 37997
+	 */
+	public function test_plugin_translation_after_switching_locale_twice() {
+		require_once DIR_TESTDATA . '/plugins/internationalized-plugin.php';
+
+		switch_to_locale( 'de_DE' );
+		$expected_de_DE = i18n_plugin_test();
+
+		switch_to_locale( 'es_ES' );
+		$expected_es_ES = i18n_plugin_test();
+
+		restore_current_locale();
+
+		$this->assertSame( 'Das ist ein Dummy Plugin', $expected_de_DE );
+		$this->assertSame( 'This is a dummy plugin', $expected_es_ES );
+	}
+
 	/**
 	 * @ticket 26511
 	 */
@@ -212,4 +234,30 @@ class Tests_L10n_loadTextdomainJustInTime extends WP_UnitTestCase {
 
 		$this->assertSame( 'Das ist ein Dummy Theme', $expected );
 	}
+
+	/**
+	 * @ticket 37997
+	 */
+	public function test_get_locale_is_called_only_once_per_textdomain() {
+		$textdomain = 'foo-bar-baz';
+
+		add_filter( 'locale', array( $this, '_filter_locale_count' ) );
+
+		__( 'Foo', $textdomain );
+		__( 'Bar', $textdomain );
+		__( 'Baz', $textdomain );
+		__( 'Foo Bar', $textdomain );
+		__( 'Foo Bar Baz', $textdomain );
+
+		remove_filter( 'locale', array( $this, '_filter_locale_count' ) );
+
+		$this->assertFalse( is_textdomain_loaded( $textdomain ) );
+		$this->assertSame( 1, $this->locale_count );
+	}
+
+	public function _filter_locale_count( $locale ) {
+		++$this->locale_count;
+
+		return $locale;
+	}
 }

tests/phpunit/tests/l10n/localeSwitcher.php

@@ -24,11 +24,13 @@ class Tests_Locale_Switcher extends WP_UnitTestCase {
 
 		unset( $GLOBALS['l10n'] );
 		unset( $GLOBALS['l10n_unloaded'] );
+		_get_path_to_translation( null, true );
 	}
 
 	public function tearDown() {
 		unset( $GLOBALS['l10n'] );
 		unset( $GLOBALS['l10n_unloaded'] );
+		_get_path_to_translation( null, true );
 
 		parent::tearDown();
 	}