Add support for fetchpriority bumping in WP_Scripts

src/wp-includes/class-wp-script-modules.php

@@ -289,6 +289,7 @@ public function add_hooks() {
 	 *
 	 * @since 6.9.0
 	 * @see WP_Scripts::filter_eligible_strategies()
+	 * @see WP_Scripts::get_highest_fetchpriority_with_dependents()
 	 *
 	 * @param string              $id      Script module ID.
 	 * @param array<string, true> $checked Optional. An array of already checked script IDs, used to avoid recursive loops.

src/wp-includes/class-wp-scripts.php

@@ -425,8 +425,19 @@ public function do_item( $handle, $group = false ) {
 		if ( $intended_strategy ) {
 			$attr['data-wp-strategy'] = $intended_strategy;
 		}
-		if ( isset( $obj->extra['fetchpriority'] ) && 'auto' !== $obj->extra['fetchpriority'] && $this->is_valid_fetchpriority( $obj->extra['fetchpriority'] ) ) {
-			$attr['fetchpriority'] = $obj->extra['fetchpriority'];
+
+		if ( isset( $obj->extra['fetchpriority'] ) ) {
+			$original_fetchpriority = $obj->extra['fetchpriority'];
+			if ( ! $this->is_valid_fetchpriority( $original_fetchpriority ) ) {
+				$original_fetchpriority = 'auto';
+			}
+			$actual_fetchpriority = $this->get_highest_fetchpriority_with_dependents( $handle );
+			if ( is_string( $actual_fetchpriority ) && 'auto' !== $actual_fetchpriority ) {
+				$attr['fetchpriority'] = $actual_fetchpriority;
+			}
+			if ( $original_fetchpriority !== $actual_fetchpriority ) {
+				$attr['data-wp-fetchpriority'] = $original_fetchpriority;
+			}
 		}
 		$tag  = $translations . $ie_conditional_prefix . $before_script;
 		$tag .= wp_get_script_tag( $attr );
@@ -1023,6 +1034,62 @@ private function filter_eligible_strategies( $handle, $eligible_strategies = nul
 		return $eligible_strategies;
 	}
 
+	/**
+	 * Gets the highest fetch priority for a given script and all of its dependent scripts.
+	 *
+	 * @since 6.9.0
+	 * @see self::filter_eligible_strategies()
+	 * @see WP_Script_Modules::get_highest_fetchpriority_with_dependents()
+	 *
+	 * @param string              $handle  Script module ID.
+	 * @param array<string, true> $checked Optional. An array of already checked script handles, used to avoid recursive loops.
+	 * @return string|null Highest fetch priority for the script and its dependents.
+	 */
+	private function get_highest_fetchpriority_with_dependents( string $handle, array $checked = array() ): ?string {
+		// If there is a recursive dependency, return early.
+		if ( isset( $checked[ $handle ] ) ) {
+			return null;
+		}
+
+		// Mark this handle as checked to guard against infinite recursion.
+		$checked[ $handle ] = true;
+
+		// If the handle is not enqueued, don't filter anything and return.
+		if ( ! $this->query( $handle, 'enqueued' ) ) {
+			return null;
+		}
+
+		$fetchpriority = $this->get_data( $handle, 'fetchpriority' );
+		if ( ! $this->is_valid_fetchpriority( $fetchpriority ) ) {
+			$fetchpriority = 'auto';
+		}
+
+		static $priority_mapping = array(
+			'low'  => 0,
+			'auto' => 1,
+			'high' => 2,
+		);
+
+		$highest_priority_index = $priority_mapping[ $fetchpriority ];
+
+		foreach ( $this->get_dependents( $handle ) as $dependent_handle ) {
+			$dependent_priority = $this->get_highest_fetchpriority_with_dependents( $dependent_handle, $checked );
+			if ( is_string( $dependent_priority ) ) {
+				$highest_priority_index = max(
+					$highest_priority_index,
+					$priority_mapping[ $dependent_priority ]
+				);
+			}
+		}
+
+		$highest_priority = array_search( $highest_priority_index, $priority_mapping, true );
+		if ( is_string( $highest_priority ) ) {
+			return $highest_priority;
+		}
+
+		return null;
+	}
+
 	/**
 	 * Gets data for inline scripts registered for a specific handle.
 	 *

tests/phpunit/tests/dependencies/scripts.php

@@ -1172,8 +1172,8 @@ public function test_defer_with_async_dependent() {
 		);
 		$output    = get_echo( 'wp_print_scripts' );
 		$expected  = "<script type='text/javascript' src='/main-script-d4.js' id='main-script-d4-js' defer='defer' data-wp-strategy='defer'></script>\n";
-		$expected .= "<script type='text/javascript' src='/dependent-script-d4-1.js' id='dependent-script-d4-1-js' defer='defer' data-wp-strategy='defer'></script>\n";
-		$expected .= "<script type='text/javascript' src='/dependent-script-d4-2.js' id='dependent-script-d4-2-js' defer='defer' data-wp-strategy='async' fetchpriority='low'></script>\n";
+		$expected .= "<script type='text/javascript' src='/dependent-script-d4-1.js' id='dependent-script-d4-1-js' defer='defer' data-wp-strategy='defer' fetchpriority='high' data-wp-fetchpriority='auto'></script>\n";
+		$expected .= "<script type='text/javascript' src='/dependent-script-d4-2.js' id='dependent-script-d4-2-js' defer='defer' data-wp-strategy='async' fetchpriority='high' data-wp-fetchpriority='low'></script>\n";
 		$expected .= "<script type='text/javascript' src='/dependent-script-d4-3.js' id='dependent-script-d4-3-js' defer='defer' data-wp-strategy='defer' fetchpriority='high'></script>\n";
 
 		$this->assertEqualHTML( $expected, $output, '<body>', 'Scripts registered as defer but that have dependents that are async are expected to have said dependents deferred.' );
@@ -1281,6 +1281,59 @@ public function test_invalid_fetchpriority_on_alias() {
 		$this->assertArrayNotHasKey( 'fetchpriority', wp_scripts()->registered['alias']->extra );
 	}
 
+	/**
+	 * Data provider.
+	 *
+	 * @return array<string, array{enqueues: string[], expected: string}>
+	 */
+	public function data_provider_to_test_fetchpriority_bumping(): array {
+		return array(
+			'enqueue_bajo' => array(
+				'enqueues' => array( 'bajo' ),
+				'expected' => '<script fetchpriority="low" id="bajo-js" src="/bajo.js" type="text/javascript"></script>',
+			),
+			'enqueue_auto' => array(
+				'enqueues' => array( 'auto' ),
+				'expected' => '
+					<script type="text/javascript" src="/bajo.js" id="bajo-js" data-wp-fetchpriority="low"></script>
+					<script type="text/javascript" src="/auto.js" id="auto-js"></script>
+				',
+			),
+			'enqueue_alto' => array(
+				'enqueues' => array( 'alto' ),
+				'expected' => '
+					<script type="text/javascript" src="/bajo.js" id="bajo-js" fetchpriority="high" data-wp-fetchpriority="low"></script>
+					<script type="text/javascript" src="/auto.js" id="auto-js" fetchpriority="high" data-wp-fetchpriority="auto"></script>
+					<script type="text/javascript" src="/alto.js" id="alto-js" fetchpriority="high"></script>
+				',
+			),
+		);
+	}
+
+	/**
+	 * Tests a higher fetchpriority on a dependent script module causes the fetchpriority of a dependency script module to be bumped.
+	 *
+	 * @ticket 61734
+	 *
+	 * @covers WP_Scripts::get_dependents
+	 * @covers WP_Scripts::get_highest_fetchpriority_with_dependents
+	 * @covers WP_Scripts::do_item
+	 *
+	 * @dataProvider data_provider_to_test_fetchpriority_bumping
+	 */
+	public function test_fetchpriority_bumping( array $enqueues, string $expected ) {
+		wp_register_script( 'bajo', '/bajo.js', array(), null, array( 'fetchpriority' => 'low' ) );
+		wp_register_script( 'auto', '/auto.js', array( 'bajo' ), null, array( 'fetchpriority' => 'auto' ) );
+		wp_register_script( 'alto', '/alto.js', array( 'auto' ), null, array( 'fetchpriority' => 'high' ) );
+
+		foreach ( $enqueues as $enqueue ) {
+			wp_enqueue_script( $enqueue );
+		}
+
+		$actual = get_echo( 'wp_print_scripts' );
+		$this->assertEqualHTML( $expected, $actual, '<body>', "Snapshot:\n$actual" );
+	}
+
 	/**
 	 * Tests that scripts registered as defer become blocking when their dependents chain are all blocking.
 	 *