Ensure the entire template is passed to the output buffer callback for Optimization Detective to process

plugins/optimization-detective/optimization.php

@@ -31,8 +31,34 @@
  * @return string Unmodified value of $passthrough.
  */
 function od_buffer_output( string $passthrough ): string {
+	/*
+	 * Instead of the default PHP_OUTPUT_HANDLER_STDFLAGS (cleanable, flushable, and removable) being used for flags,
+	 * we need to omit PHP_OUTPUT_HANDLER_FLUSHABLE. If the buffer were flushable, then each time that ob_flush() is
+	 * called, it would send a fragment of the output into the output buffer callback. When buffering the entire
+	 * response as an HTML document, this would result in broken HTML processing.
+	 *
+	 * If this ends up being problematic, then PHP_OUTPUT_HANDLER_FLUSHABLE could be added to the $flags and the
+	 * output buffer callback could check if the phase is PHP_OUTPUT_HANDLER_FLUSH and abort any subsequent
+	 * processing while also emitting a _doing_it_wrong().
+	 *
+	 * The output buffer needs to be removable because WordPress calls wp_ob_end_flush_all() and then calls
+	 * wp_cache_close(). If the buffers are not all flushed before wp_cache_close() is closed, then some output buffer
+	 * handlers (e.g. for caching plugins) may fail to be able to store the page output in the object cache.
+	 * See <https://github.com/WordPress/performance/pull/1317#issuecomment-2271955356>.
+	 */
+	$flags = PHP_OUTPUT_HANDLER_STDFLAGS ^ PHP_OUTPUT_HANDLER_FLUSHABLE;
+
 	ob_start(
-		static function ( string $output ): string {
+		static function ( string $output, ?int $phase ): string {
+			// 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;
+			}
+
+			// Since ob_start() was called without PHP_OUTPUT_HANDLER_FLUSHABLE, at this point the phase should never be flush, and it should always be final.
+			assert( ( $phase & ( PHP_OUTPUT_HANDLER_FLUSH ) ) === 0 );
+			assert( ( $phase & ( PHP_OUTPUT_HANDLER_FINAL ) ) !== 0 );
+
 			/**
 			 * Filters the template output buffer prior to sending to the client.
 			 *
@@ -42,7 +68,9 @@ static function ( string $output ): string {
 			 * @return string Filtered output buffer.
 			 */
 			return (string) apply_filters( 'od_template_output_buffer', $output );
-		}
+		},
+		0, // Unlimited buffer size.
+		$flags
 	);
 	return $passthrough;
 }
@@ -139,7 +167,7 @@ function od_is_response_html_content_type(): bool {
  * @return string Filtered template output buffer.
  */
 function od_optimize_template_output_buffer( string $buffer ): string {
-	if ( ! od_is_response_html_content_type() ) {
+	if ( ! od_is_response_html_content_type() ) { // TODO: This should check to see if there is an HTML tag.
 		return $buffer;
 	}
 

plugins/optimization-detective/tests/test-optimization.php

@@ -54,23 +54,94 @@ public function test_od_buffer_output(): void {
 		// buffer callback. See <https://stackoverflow.com/a/61439514/93579>.
 		ob_start();
 
+		$filter_invoked = false;
 		add_filter(
 			'od_template_output_buffer',
-			function ( $buffer ) use ( $original, $expected ) {
+			function ( $buffer ) use ( $original, $expected, &$filter_invoked ) {
 				$this->assertSame( $original, $buffer );
+				$filter_invoked = true;
 				return $expected;
 			}
 		);
 
 		$original_ob_level = ob_get_level();
-		od_buffer_output( '' );
+		$template          = sprintf( 'page-%s.php', wp_generate_uuid4() );
+		$this->assertSame( $template, od_buffer_output( $template ), 'Expected value to be passed through.' );
 		$this->assertSame( $original_ob_level + 1, ob_get_level(), 'Expected call to ob_start().' );
 		echo $original;
 
 		ob_end_flush(); // Flushing invokes the output buffer callback.
 
 		$buffer = ob_get_clean(); // Get the buffer from our wrapper output buffer.
 		$this->assertSame( $expected, $buffer );
+		$this->assertTrue( $filter_invoked );
+	}
+
+	/**
+	 * Test that calling ob_flush() will not result in the buffer being processed and that ob_clean() will successfully prevent content from being processed.
+	 *
+	 * @covers ::od_buffer_output
+	 */
+	public function test_od_buffer_with_cleaning_and_attempted_flushing(): void {
+		$template_aborted = 'Before time began!';
+		$template_start   = 'The beginning';
+		$template_middle  = ', the middle';
+		$template_end     = ', and the end!';
+
+		// In order to test, a wrapping output buffer is required because ob_get_clean() does not invoke the output
+		// buffer callback. See <https://stackoverflow.com/a/61439514/93579>.
+		$initial_level = ob_get_level();
+		$this->assertTrue( ob_start() );
+		$this->assertSame( $initial_level + 1, ob_get_level() );
+
+		$filter_count = 0;
+		add_filter(
+			'od_template_output_buffer',
+			function ( $buffer ) use ( $template_start, $template_middle, $template_end, &$filter_count ) {
+				$filter_count++;
+				$this->assertSame( $template_start . $template_middle . $template_end, $buffer );
+				return '<filtered>' . $buffer . '</filtered>';
+			}
+		);
+
+		od_buffer_output( '' );
+		$this->assertSame( $initial_level + 2, ob_get_level() );
+
+		echo $template_aborted;
+		$this->assertTrue( ob_clean() ); // By cleaning, the above should never be seen by the filter.
+
+		// This is the start of what will end up getting filtered.
+		echo $template_start;
+
+		// Attempt to flush the output, which will fail because the output buffer was opened without the flushable flag.
+		$this->assertFalse( ob_flush() );
+
+		// This will also be sent into the filter.
+		echo $template_middle;
+		$this->assertFalse( ob_flush() );
+		$this->assertSame( $initial_level + 2, ob_get_level() );
+
+		// Start a nested output buffer which will also end up getting sent into the filter.
+		$this->assertTrue( ob_start() );
+		echo $template_end;
+		$this->assertSame( $initial_level + 3, ob_get_level() );
+		$this->assertTrue( ob_flush() );
+		$this->assertTrue( ob_end_flush() );
+		$this->assertSame( $initial_level + 2, ob_get_level() );
+
+		// Close the output buffer opened by od_buffer_output(). This only works in the unit test because the removable flag was passed.
+		$this->assertTrue( ob_end_flush() );
+		$this->assertSame( $initial_level + 1, ob_get_level() );
+
+		$buffer = ob_get_clean(); // Get the buffer from our wrapper output buffer and close it.
+		$this->assertSame( $initial_level, ob_get_level() );
+
+		$this->assertSame( 1, $filter_count, 'Expected filter to be called once.' );
+		$this->assertSame(
+			'<filtered>' . $template_start . $template_middle . $template_end . '</filtered>',
+			$buffer,
+			'Excepted return value of filter to be the resulting value for the buffer.'
+		);
 	}
 
 	/**