PackageManagerService.queryIntentReceivers 方法查询 intent 对应的静态广播，请问其中这个 reader synchronized 部分是干什么的

private @NonNull List<ResolveInfo> queryIntentReceiversInternal(Intent intent,
            String resolvedType, int flags, int userId) {
        if (!sUserManager.exists(userId)) return Collections.emptyList();
        flags = updateFlagsForResolve(flags, userId, intent);
        ComponentName comp = intent.getComponent();
        if (comp == null) {
            if (intent.getSelector() != null) {
                intent = intent.getSelector();
                comp = intent.getComponent();
            }
        }
        if (comp != null) {
            List<ResolveInfo> list = new ArrayList<ResolveInfo>(1);
            ActivityInfo ai = getReceiverInfo(comp, flags, userId);
            if (ai != null) {
                ResolveInfo ri = new ResolveInfo();
                ri.activityInfo = ai;
                list.add(ri);
            }
            return list;
        }

        // reader
        synchronized (mPackages) {
            String pkgName = intent.getPackage();
            if (pkgName == null) {
                return mReceivers.queryIntent(intent, resolvedType, flags, userId);
            }
            final PackageParser.Package pkg = mPackages.get(pkgName);
            if (pkg != null) {
                return mReceivers.queryIntentForPackage(intent, resolvedType, flags, pkg.receivers,
                        userId);
            }
            return Collections.emptyList();
        }
    }