Source

File: wp-includes/rest-api/endpoints/class-wp-rest-autosaves-controller.php

public function create_post_autosave( $post_data ) {

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

		if ( is_wp_error( $post ) ) {
			return $post;
		}

		$user_id = 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, $user_id );

		if ( $old_autosave ) {
			$new_autosave                = _wp_post_revision_data( $post_data, true );
			$new_autosave['ID']          = $old_autosave->ID;
			$new_autosave['post_author'] = $user_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 new WP_Error(
					'rest_autosave_no_changes',
					__( 'There is nothing to save. The autosave and the post content are the same.' ),
					array( 'status' => 400 )
				);
			}

			/** This filter is documented in wp-admin/post.php */
			do_action( 'wp_creating_autosave', $new_autosave );

			// wp_update_post() expects escaped array.
			return wp_update_post( wp_slash( $new_autosave ) );
		}

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