Source

File: wp-includes/class-wp-xmlrpc-server.php

public function set_custom_fields( $post_id, $fields ) {
		$post_id = (int) $post_id;

		foreach ( (array) $fields as $meta ) {
			if ( isset( $meta['id'] ) ) {
				$meta['id'] = (int) $meta['id'];
				$pmeta      = get_metadata_by_mid( 'post', $meta['id'] );

				if ( ! $pmeta || $pmeta->post_id != $post_id ) {
					continue;
				}

				if ( isset( $meta['key'] ) ) {
					$meta['key'] = wp_unslash( $meta['key'] );
					if ( $meta['key'] !== $pmeta->meta_key ) {
						continue;
					}
					$meta['value'] = wp_unslash( $meta['value'] );
					if ( current_user_can( 'edit_post_meta', $post_id, $meta['key'] ) ) {
						update_metadata_by_mid( 'post', $meta['id'], $meta['value'] );
					}
				} elseif ( current_user_can( 'delete_post_meta', $post_id, $pmeta->meta_key ) ) {
					delete_metadata_by_mid( 'post', $meta['id'] );
				}
			} elseif ( current_user_can( 'add_post_meta', $post_id, wp_unslash( $meta['key'] ) ) ) {
				add_post_meta( $post_id, $meta['key'], $meta['value'] );
			}
		}
	}