Script Loader: Harden removal of script tag wrappers.

* Add `wp_remove_surrounding_empty_script_tags()` to more precisely remove script tag wrappers and warn when doing it wrong.
* Add clarifying comments for XML escaping logic in `wp_get_inline_script_tag()`.
* Leverage `WP_HTML_Tag_Processor` in `test_remove_frameless_preview_messenger_channel`.
* Reuse `assertEqualMarkup` in `test_blocking_dependent_with_delayed_dependency`.
* Normalize whitespace in `parse_markup_fragment` for `assertEqualMarkup`.

Follow-up to [56687].
Props dmsnell, westonruter, flixos90.
See #58664.


git-svn-id: https://develop.svn.wordpress.org/trunk@56748 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/script-loader.php

@@ -2853,9 +2853,43 @@ function wp_get_inline_script_tag( $javascript, $attributes = array() ) {
 		);
 	}
 
-	// Ensure markup is XHTML compatible if not HTML5.
+	/*
+	 * XHTML extracts the contents of the SCRIPT element and then the XML parser
+	 * decodes character references and other syntax elements. This can lead to
+	 * misinterpretation of the script contents or invalid XHTML documents.
+	 *
+	 * Wrapping the contents in a CDATA section instructs the XML parser not to
+	 * transform the contents of the SCRIPT element before passing them to the
+	 * JavaScript engine.
+	 *
+	 * Example:
+	 *
+	 *     <script>console.log('&hellip;');</script>
+	 *
+	 *     In an HTML document this would print "&hellip;" to the console,
+	 *     but in an XHTML document it would print "…" to the console.
+	 *
+	 *     <script>console.log('An image is <img> in HTML');</script>
+	 *
+	 *     In an HTML document this would print "An image is <img> in HTML",
+	 *     but it's an invalid XHTML document because it interprets the `<img>`
+	 *     as an empty tag missing its closing `/`.
+	 *
+	 * @see https://www.w3.org/TR/xhtml1/#h-4.8
+	 */
 	if ( ! $is_html5 ) {
-		$javascript = str_replace( ']]>', ']]]]><![CDATA[>', $javascript ); // Escape any existing CDATA section.
+		/*
+		 * If the string `]]>` exists within the JavaScript it would break
+		 * out of any wrapping CDATA section added here, so to start, it's
+		 * necessary to escape that sequence which requires splitting the
+		 * content into two CDATA sections wherever it's found.
+		 *
+		 * Note: it's only necessary to escape the closing `]]>` because
+		 * an additional `<![CDATA[` leaves the contents unchanged.
+		 */
+		$javascript = str_replace( ']]>', ']]]]><![CDATA[>', $javascript );
+
+		// Wrap the entire escaped script inside a CDATA section.
 		$javascript = sprintf( "/* <![CDATA[ */\n%s\n/* ]]> */", $javascript );
 	}
 
@@ -3299,3 +3333,51 @@ function wp_add_editor_classic_theme_styles( $editor_settings ) {
 
 	return $editor_settings;
 }
+
+/**
+ * Removes leading and trailing _empty_ script tags.
+ *
+ * This is a helper meant to be used for literal script tag construction
+ * within `wp_get_inline_script_tag()` or `wp_print_inline_script_tag()`.
+ * It removes the literal values of "<script>" and "</script>" from
+ * around an inline script after trimming whitespace. Typlically this
+ * is used in conjunction with output buffering, where `ob_get_clean()`
+ * is passed as the `$contents` argument.
+ *
+ * Example:
+ *
+ *     // Strips exact literal empty SCRIPT tags.
+ *     $js = '<script>sayHello();</script>;
+ *     'sayHello();' === wp_remove_surrounding_empty_script_tags( $js );
+ *
+ *     // Otherwise if anything is different it warns in the JS console.
+ *     $js = '<script type="text/javascript">console.log( "hi" );</script>';
+ *     'console.error( ... )' === wp_remove_surrounding_empty_script_tags( $js );
+ *
+ * @private
+ * @since 6.4.0
+ *
+ * @see wp_print_inline_script_tag()
+ * @see wp_get_inline_script_tag()
+ *
+ * @param string $contents Script body with manually created SCRIPT tag literals.
+ * @return string Script body without surrounding script tag literals, or
+ *                original contents if both exact literals aren't present.
+ */
+function wp_remove_surrounding_empty_script_tags( $contents ) {
+	$contents = trim( $contents );
+	$opener   = '<SCRIPT>';
+	$closer   = '</SCRIPT>';
+
+	if (
+		strlen( $contents ) > strlen( $opener ) + strlen( $closer ) &&
+		strtoupper( substr( $contents, 0, strlen( $opener ) ) ) === $opener &&
+		strtoupper( substr( $contents, -strlen( $closer ) ) ) === $closer
+	) {
+		return substr( $contents, strlen( $opener ), -strlen( $closer ) );
+	} else {
+		$error_message = __( 'Expected string to start with script tag (without attributes) and end with script tag, with optional whitespace.' );
+		_doing_it_wrong( __FUNCTION__, $error_message, '6.4' );
+		return sprintf( 'console.error(%s)', wp_json_encode( __( 'Function wp_remove_surrounding_empty_script_tags() used incorrectly in PHP.' ) . ' ' . $error_message ) );
+	}
+}