Source

File: wp-includes/revision.php

function _wp_put_post_revision( $post = null, $autosave = false ) {
	if ( is_object( $post ) ) {
		$post = get_object_vars( $post );
	} elseif ( ! is_array( $post ) ) {
		$post = get_post( $post, ARRAY_A );
	}

	if ( ! $post || empty( $post['ID'] ) ) {
		return new WP_Error( 'invalid_post', __( 'Invalid post ID.' ) );
	}

	if ( isset( $post['post_type'] ) && 'revision' === $post['post_type'] ) {
		return new WP_Error( 'post_type', __( 'Cannot create a revision of a revision' ) );
	}

	$post = _wp_post_revision_data( $post, $autosave );
	$post = wp_slash( $post ); // Since data is from DB.

	$revision_id = wp_insert_post( $post );
	if ( is_wp_error( $revision_id ) ) {
		return $revision_id;
	}

	if ( $revision_id ) {
		/**
		 * Fires once a revision has been saved.
		 *
		 * @since 2.6.0
		 *
		 * @param int $revision_id Post revision ID.
		 */
		do_action( '_wp_put_post_revision', $revision_id );
	}

	return $revision_id;
}