Source

File: wp-includes/class-wp-hook.php

public function do_action( $args ) {
		$this->doing_action = true;
		$this->apply_filters( '', $args );

		// If there are recursive calls to the current action, we haven't finished it until we get to the last one.
		if ( ! $this->nesting_level ) {
			$this->doing_action = false;
		}
	}