Trigger wp_send_late_headers even when the output buffer is not HTML

Author: westonruter

src/wp-includes/class-wp.php

@@ -588,6 +588,9 @@ public function send_headers() {
 		/**
 		 * Fires once the requested HTTP headers for caching, content type, etc. have been sent.
 		 *
+		 * The {@see 'wp_send_late_headers'} action may be used to send headers after rendering the template into an
+		 * output buffer.
+		 *
 		 * @since 2.1.0
 		 *
 		 * @param WP $wp Current WordPress environment instance (passed by reference).

src/wp-includes/template.php

@@ -922,6 +922,21 @@ function wp_start_template_enhancement_output_buffer(): bool {
  * @return string Finalized output buffer.
  */
 function wp_finalize_template_enhancement_output_buffer( string $output, int $phase ): string {
+	$do_send_late_headers = static function ( string $output ): void {
+		/**
+		 * Fires at the last moment HTTP headers may be sent.
+		 *
+		 * This happens immediately before the template enhancement output buffer is flushed. This is in contrast with
+		 * the {@see 'send_headers'} action which fires after the initial headers have been sent before the template
+		 * has begun rendering, and thus does not depend on output buffering.
+		 *
+		 * @since 6.9.0
+		 *
+		 * @param string $output Output buffer.
+		 */
+		do_action( 'wp_send_late_headers', $output );
+	};
+
 	// When the output is being cleaned (e.g. pending template is replaced with error page), do not send it through the filter.
 	if ( ( $phase & PHP_OUTPUT_HANDLER_CLEAN ) !== 0 ) {
 		return $output;
@@ -958,6 +973,7 @@ function wp_finalize_template_enhancement_output_buffer( string $output, int $ph
 
 	// If the content type is not HTML, short-circuit since it is not relevant for enhancement.
 	if ( ! $is_html_content_type ) {
+		$do_send_late_headers( $output );
 		return $output;
 	}
 
@@ -980,16 +996,7 @@ function wp_finalize_template_enhancement_output_buffer( string $output, int $ph
 	 */
 	$filtered_output = (string) apply_filters( 'wp_template_enhancement_output_buffer', $filtered_output, $output );
 
-	/**
-	 * Fires at the last moment HTTP headers may be sent.
-	 *
-	 * This happens immediately after the template enhancement output buffer has been processed.
-	 *
-	 * @since 6.9.0
-	 *
-	 * @param string $filtered_output Filtered output buffer.
-	 */
-	do_action( 'wp_send_late_headers', $filtered_output );
+	$do_send_late_headers( $filtered_output );
 
 	return $filtered_output;
 }