Source

File: wp-admin/includes/post.php

function wp_autosave( $post_data ) {
	// Back-compat.
	if ( ! defined( 'DOING_AUTOSAVE' ) ) {
		define( 'DOING_AUTOSAVE', true );
	}

	$post_id              = (int) $post_data['post_id'];
	$post_data['ID']      = $post_id;
	$post_data['post_ID'] = $post_id;

	if ( false === wp_verify_nonce( $post_data['_wpnonce'], 'update-post_' . $post_id ) ) {
		return new WP_Error( 'invalid_nonce', __( 'Error while saving.' ) );
	}

	$post = get_post( $post_id );

	if ( ! current_user_can( 'edit_post', $post->ID ) ) {
		return new WP_Error( 'edit_posts', __( 'Sorry, you are not allowed to edit this item.' ) );
	}

	if ( 'auto-draft' === $post->post_status ) {
		$post_data['post_status'] = 'draft';
	}

	if ( 'page' !== $post_data['post_type'] && ! empty( $post_data['catslist'] ) ) {
		$post_data['post_category'] = explode( ',', $post_data['catslist'] );
	}

	if ( ! wp_check_post_lock( $post->ID ) && get_current_user_id() == $post->post_author
		&& ( 'auto-draft' === $post->post_status || 'draft' === $post->post_status )
	) {
		// Drafts and auto-drafts are just overwritten by autosave for the same user if the post is not locked.
		return edit_post( wp_slash( $post_data ) );
	} else {
		// Non-drafts or other users' drafts are not overwritten.
		// The autosave is stored in a special post revision for each user.
		return wp_create_post_autosave( wp_slash( $post_data ) );
	}
}