Source

File: wp-includes/option.php

function get_site_transient( $transient ) {

	/**
	 * Filters the value of an existing site transient before it is retrieved.
	 *
	 * The dynamic portion of the hook name, `$transient`, refers to the transient name.
	 *
	 * Returning a truthy value from the filter will effectively short-circuit retrieval
	 * and return the passed value instead.
	 *
	 * @since 2.9.0
	 * @since 4.4.0 The `$transient` parameter was added.
	 *
	 * @param mixed  $pre_site_transient The default value to return if the site transient does not exist.
	 *                                   Any value other than false will short-circuit the retrieval
	 *                                   of the transient, and return that value.
	 * @param string $transient          Transient name.
	 */
	$pre = apply_filters( "pre_site_transient_{$transient}", false, $transient );

	if ( false !== $pre ) {
		return $pre;
	}

	if ( wp_using_ext_object_cache() ) {
		$value = wp_cache_get( $transient, 'site-transient' );
	} else {
		// Core transients that do not have a timeout. Listed here so querying timeouts can be avoided.
		$no_timeout       = array( 'update_core', 'update_plugins', 'update_themes' );
		$transient_option = '_site_transient_' . $transient;
		if ( ! in_array( $transient, $no_timeout, true ) ) {
			$transient_timeout = '_site_transient_timeout_' . $transient;
			$timeout           = get_site_option( $transient_timeout );
			if ( false !== $timeout && $timeout < time() ) {
				delete_site_option( $transient_option );
				delete_site_option( $transient_timeout );
				$value = false;
			}
		}

		if ( ! isset( $value ) ) {
			$value = get_site_option( $transient_option );
		}
	}

	/**
	 * Filters the value of an existing site transient.
	 *
	 * The dynamic portion of the hook name, `$transient`, refers to the transient name.
	 *
	 * @since 2.9.0
	 * @since 4.4.0 The `$transient` parameter was added.
	 *
	 * @param mixed  $value     Value of site transient.
	 * @param string $transient Transient name.
	 */
	return apply_filters( "site_transient_{$transient}", $value, $transient );
}