Source

File: wp-admin/includes/post.php

function wp_create_post_autosave( $post_data ) {
	if ( is_numeric( $post_data ) ) {
		$post_id   = $post_data;
		$post_data = $_POST;
	} else {
		$post_id = (int) $post_data['post_ID'];
	}

	$post_data = _wp_translate_postdata( true, $post_data );
	if ( is_wp_error( $post_data ) ) {
		return $post_data;
	}
	$post_data = _wp_get_allowed_postdata( $post_data );

	$post_author = get_current_user_id();

	// Store one autosave per author. If there is already an autosave, overwrite it.
	$old_autosave = wp_get_post_autosave( $post_id, $post_author );
	if ( $old_autosave ) {
		$new_autosave                = _wp_post_revision_data( $post_data, true );
		$new_autosave['ID']          = $old_autosave->ID;
		$new_autosave['post_author'] = $post_author;

		$post = get_post( $post_id );

		// If the new autosave has the same content as the post, delete the autosave.
		$autosave_is_different = false;
		foreach ( array_intersect( array_keys( $new_autosave ), array_keys( _wp_post_revision_fields( $post ) ) ) as $field ) {
			if ( normalize_whitespace( $new_autosave[ $field ] ) !== normalize_whitespace( $post->$field ) ) {
				$autosave_is_different = true;
				break;
			}
		}

		if ( ! $autosave_is_different ) {
			wp_delete_post_revision( $old_autosave->ID );
			return 0;
		}

		/**
		 * Fires before an autosave is stored.
		 *
		 * @since 4.1.0
		 *
		 * @param array $new_autosave Post array - the autosave that is about to be saved.
		 */
		do_action( 'wp_creating_autosave', $new_autosave );

		return wp_update_post( $new_autosave );
	}

	// _wp_put_post_revision() expects unescaped.
	$post_data = wp_unslash( $post_data );

	// Otherwise create the new autosave as a special post revision.
	return _wp_put_post_revision( $post_data, true );
}