Deliver locally on subdirectory and plain-permalink installs

.github/changelog/fix-local-inbox-rest-url

@@ -0,0 +1,4 @@
+Significance: patch
+Type: fixed
+
+Activities between accounts on the same site are now delivered on sites installed in a subdirectory or using plain permalinks.

includes/class-dispatcher.php

@@ -330,9 +330,9 @@ private static function send_to_inboxes( $inboxes, $outbox_item_id ) {
 	 * @return array|\WP_Error The result in the format of a remote post response, or WP_Error on failure.
 	 */
 	private static function send_to_local_inbox( $inbox_url, $json ) {
-		// Parse the inbox URL to extract the REST route.
-		$path       = \wp_parse_url( $inbox_url, PHP_URL_PATH ) ?? '';
-		$rest_route = \preg_replace( '#^/' . \preg_quote( \rest_get_url_prefix(), '#' ) . '#', '', $path );
+		// Core resolves the route for subdirectory installs and every permalink setting.
+		$url_request = \WP_REST_Request::from_url( $inbox_url );
+		$rest_route  = $url_request ? $url_request->get_route() : '';
 
 		/*
 		 * The inbox URL comes from a remote actor document, and signature verification is deferred below,

tests/phpunit/tests/includes/class-test-dispatcher.php

@@ -831,12 +831,20 @@ public function test_local_inboxes_skip_http() {
 	 * The inbox URL comes from a remote actor document, and signature verification is deferred
 	 * for local deliveries, so any other route on this site must not be dispatched.
 	 *
+	 * @dataProvider data_local_inbox_installs
 	 * @covers ::send_to_inboxes
 	 * @covers ::send_to_local_inbox
+	 *
+	 * @param string $structure The permalink structure.
+	 * @param string $home_path The path WordPress is installed under.
 	 */
-	public function test_local_delivery_only_reaches_inbox_routes() {
-		// Local delivery reads the route from the URL path, which plain permalinks do not have.
-		$this->set_permalink_structure( '/%postname%/' );
+	public function test_local_delivery_only_reaches_inbox_routes( $structure, $home_path ) {
+		$this->set_permalink_structure( $structure );
+		// The test config sets WP_HOME, which core applies on `option_home`, so filter after it.
+		$home = static function ( $url ) use ( $home_path ) {
+			return $url . $home_path;
+		};
+		\add_filter( 'option_home', $home, 11 );
 
 		$post_id     = self::factory()->post->create( array( 'post_author' => self::$user_id ) );
 		$outbox_item = $this->get_latest_outbox_item( \add_query_arg( 'p', $post_id, \home_url( '/' ) ) );
@@ -873,6 +881,7 @@ public function test_local_delivery_only_reaches_inbox_routes() {
 
 		\remove_filter( 'rest_pre_dispatch', $record );
 		\remove_action( 'activitypub_sent_to_inbox', $collect );
+		\remove_filter( 'option_home', $home, 11 );
 
 		$this->assertCount( 2, $dispatched, 'Only the two inboxes should be dispatched.' );
 
@@ -882,6 +891,20 @@ public function test_local_delivery_only_reaches_inbox_routes() {
 		}
 	}
 
+	/**
+	 * Permalink settings and install paths that change the shape of a REST URL.
+	 *
+	 * @return array[] Permalink structure and install path.
+	 */
+	public function data_local_inbox_installs() {
+		return array(
+			'pretty permalinks' => array( '/%postname%/', '' ),
+			'plain permalinks'  => array( '', '' ),
+			'index.php'         => array( '/index.php/%postname%/', '' ),
+			'subdirectory'      => array( '/%postname%/', '/blog' ),
+		);
+	}
+
 	/**
 	 * Test that remote inboxes still use HTTP.
 	 *