Cache API: Cache non-existent users in WP_User::get_data_by() to prevent duplicate queries.

When get_userdata() or WP_User::get_data_by('id', $id) is called for a non-existent
user, the result is now cached in a 'notusers' array within the 'users' cache group,
following the same pattern as 'notoptions' in the options API. Subsequent calls return
false immediately without hitting the database. The cache is invalidated by
update_user_caches() whenever a user is created or updated.

Fixes #46388.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

Author: MarcinDudekDev

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

@@ -190,6 +190,7 @@ public function init( $data, $site_id = 0 ) {
 	 *
 	 * @since 3.3.0
 	 * @since 4.4.0 Added 'ID' as an alias of 'id' for the `$field` parameter.
+	 * @since 7.1.0 Non-existent users looked up by ID are now cached to prevent duplicate queries.
 	 *
 	 * @global wpdb $wpdb WordPress database abstraction object.
 	 *
@@ -251,13 +252,27 @@ public static function get_data_by( $field, $value ) {
 			}
 		}
 
+		if ( 'id' === $field ) {
+			$notusers = wp_cache_get( 'notusers', 'users' );
+			if ( is_array( $notusers ) && isset( $notusers[ $user_id ] ) ) {
+				return false;
+			}
+		}
+
 		$user = $wpdb->get_row(
 			$wpdb->prepare(
 				"SELECT * FROM $wpdb->users WHERE $db_field = %s LIMIT 1",
 				$value
 			)
 		);
 		if ( ! $user ) {
+			if ( 'id' === $field ) {
+				if ( ! is_array( $notusers ) ) {
+					$notusers = array();
+				}
+				$notusers[ $user_id ] = true;
+				wp_cache_set( 'notusers', $notusers, 'users' );
+			}
 			return false;
 		}
 

src/wp-includes/user.php

@@ -1985,6 +1985,7 @@ function sanitize_user_field( $field, $value, $user_id, $context ) {
  * Updates all user caches.
  *
  * @since 3.0.0
+ * @since 7.1.0 The `notusers` cache is cleared for the user's ID.
  *
  * @param object|WP_User $user User object or database row to be cached
  * @return void|false Void on success, false on failure.
@@ -2005,6 +2006,12 @@ function update_user_caches( $user ) {
 	if ( ! empty( $user->user_email ) ) {
 		wp_cache_add( $user->user_email, $user->ID, 'useremail' );
 	}
+
+	$notusers = wp_cache_get( 'notusers', 'users' );
+	if ( is_array( $notusers ) && isset( $notusers[ $user->ID ] ) ) {
+		unset( $notusers[ $user->ID ] );
+		wp_cache_set( 'notusers', $notusers, 'users' );
+	}
 }
 
 /**

tests/phpunit/tests/user/getDataBy.php

@@ -0,0 +1,112 @@
+<?php
+
+/**
+ * Tests for WP_User::get_data_by() caching behavior.
+ *
+ * @group user
+ * @group cache
+ *
+ * @coversDefaultClass WP_User
+ */
+class Tests_User_GetDataBy extends WP_UnitTestCase {
+
+	/**
+	 * @ticket 46388
+	 * @covers WP_User::get_data_by
+	 */
+	public function test_nonexistent_user_by_id_does_not_trigger_multiple_queries() {
+		global $wpdb;
+
+		$nonexistent_id = PHP_INT_MAX;
+
+		// Ensure the ID is not cached from a previous test.
+		$notusers = wp_cache_get( 'notusers', 'users' );
+		if ( is_array( $notusers ) ) {
+			unset( $notusers[ $nonexistent_id ] );
+			wp_cache_set( 'notusers', $notusers, 'users' );
+		}
+		wp_cache_delete( $nonexistent_id, 'users' );
+
+		$before = $wpdb->num_queries;
+		get_userdata( $nonexistent_id );
+		$after_first_call = $wpdb->num_queries;
+
+		get_userdata( $nonexistent_id );
+		$after_second_call = $wpdb->num_queries;
+
+		$this->assertSame( 1, $after_first_call - $before, 'First call for non-existent user should trigger one DB query.' );
+		$this->assertSame( 0, $after_second_call - $after_first_call, 'Second call for non-existent user should not trigger any DB queries.' );
+	}
+
+	/**
+	 * @ticket 46388
+	 * @covers WP_User::get_data_by
+	 */
+	public function test_nonexistent_user_by_id_is_added_to_notusers_cache() {
+		$nonexistent_id = PHP_INT_MAX - 1;
+
+		// Clear any existing state.
+		$notusers = wp_cache_get( 'notusers', 'users' );
+		if ( is_array( $notusers ) ) {
+			unset( $notusers[ $nonexistent_id ] );
+			wp_cache_set( 'notusers', $notusers, 'users' );
+		}
+		wp_cache_delete( $nonexistent_id, 'users' );
+
+		get_userdata( $nonexistent_id );
+
+		$notusers = wp_cache_get( 'notusers', 'users' );
+
+		$this->assertIsArray( $notusers, 'notusers cache should be an array after a non-existent user lookup.' );
+		$this->assertArrayHasKey( $nonexistent_id, $notusers, 'Non-existent user ID should be stored in notusers cache.' );
+	}
+
+	/**
+	 * @ticket 46388
+	 * @covers WP_User::get_data_by
+	 */
+	public function test_notusers_cache_is_invalidated_when_user_is_created() {
+		global $wpdb;
+
+		// Manually prime the notusers cache for a specific ID.
+		$user_id  = self::factory()->user->create();
+		$notusers = wp_cache_get( 'notusers', 'users' );
+		if ( ! is_array( $notusers ) ) {
+			$notusers = array();
+		}
+		$notusers[ $user_id ] = true;
+		wp_cache_set( 'notusers', $notusers, 'users' );
+
+		// Also clear the positive users cache to force a real test.
+		wp_cache_delete( $user_id, 'users' );
+
+		// update_user_caches is called by wp_insert_user / wp_update_user.
+		// Simulate what happens when user data is refreshed.
+		$raw_user = $wpdb->get_row( $wpdb->prepare( "SELECT * FROM $wpdb->users WHERE ID = %d", $user_id ) );
+		update_user_caches( $raw_user );
+
+		$notusers = wp_cache_get( 'notusers', 'users' );
+
+		$this->assertFalse(
+			is_array( $notusers ) && isset( $notusers[ $user_id ] ),
+			'User ID should be removed from notusers cache after update_user_caches() is called.'
+		);
+	}
+
+	/**
+	 * @ticket 46388
+	 * @covers WP_User::get_data_by
+	 */
+	public function test_existing_user_by_id_is_not_added_to_notusers_cache() {
+		$user_id = self::factory()->user->create();
+
+		get_userdata( $user_id );
+
+		$notusers = wp_cache_get( 'notusers', 'users' );
+
+		$this->assertFalse(
+			is_array( $notusers ) && isset( $notusers[ $user_id ] ),
+			'Existing user ID should not be stored in notusers cache.'
+		);
+	}
+}