（兩百一十七）Android Q WiFi對應用前後臺掃描的限制

1.流程梳理

ScanRequestProxy

        // Check and throttle scan request unless,
        // a) App has either NETWORK_SETTINGS or NETWORK_SETUP_WIZARD permission.
        // b) Throttling has been disabled by user.
        if (!fromSettingsOrSetupWizard && mThrottleEnabledSettingObserver.isEnabled()
                && shouldScanRequestBeThrottledForApp(callingUid, packageName)) {
            Log.i(TAG, "Scan request from " + packageName + " throttled");
            sendScanResultFailureBroadcastToPackage(packageName);
            return false;
        }


    /**
     * Checks if the scan request from the app (specified by callingUid & packageName) needs
     * to be throttled.
     */
    private boolean shouldScanRequestBeThrottledForApp(int callingUid, String packageName) {
        boolean isThrottled;
        if (isRequestFromBackground(callingUid, packageName)) {
            isThrottled = shouldScanRequestBeThrottledForBackgroundApp();
            if (isThrottled) {
                if (mVerboseLoggingEnabled) {
                    Log.v(TAG, "Background scan app request [" + callingUid + ", "
                            + packageName + "]");
                }
                mWifiMetrics.incrementExternalBackgroundAppOneshotScanRequestsThrottledCount();
            }
        } else {
            isThrottled = shouldScanRequestBeThrottledForForegroundApp(callingUid, packageName);
            if (isThrottled) {
                if (mVerboseLoggingEnabled) {
                    Log.v(TAG, "Foreground scan app request [" + callingUid + ", "
                            + packageName + "]");
                }
                mWifiMetrics.incrementExternalForegroundAppOneshotScanRequestsThrottledCount();
            }
        }
        mWifiMetrics.incrementExternalAppOneshotScanRequestsCount();
        return isThrottled;
    }

1.1 後臺掃描限制

    /**
     * Checks if the scan request from a background app needs to be throttled.
     */
    private boolean shouldScanRequestBeThrottledForBackgroundApp() {
        long lastScanMs = mLastScanTimestampForBgApps;
        long elapsedRealtime = mClock.getElapsedSinceBootMillis();
        if (lastScanMs != 0
                && (elapsedRealtime - lastScanMs) < SCAN_REQUEST_THROTTLE_INTERVAL_BG_APPS_MS) {
            return true;
        }
        // Proceed with the scan request and record the time.
        mLastScanTimestampForBgApps = elapsedRealtime;
        return false;
    }

    public static final int SCAN_REQUEST_THROTTLE_INTERVAL_BG_APPS_MS = 30 * 60 * 1000;

也就是說所有後臺應用30min內只能發起一次掃描

 

1.2 前臺掃描限制

    /**
     * Checks if the scan request from the app (specified by packageName) needs
     * to be throttled.
     * The throttle limit allows a max of {@link #SCAN_REQUEST_THROTTLE_MAX_IN_TIME_WINDOW_FG_APPS}
     * in {@link #SCAN_REQUEST_THROTTLE_TIME_WINDOW_FG_APPS_MS} window.
     */
    private boolean shouldScanRequestBeThrottledForForegroundApp(
            int callingUid, String packageName) {
        LinkedList<Long> scanRequestTimestamps =
                getOrCreateScanRequestTimestampsForForegroundApp(callingUid, packageName);
        long currentTimeMillis = mClock.getElapsedSinceBootMillis();
        // First evict old entries from the list.
        trimPastScanRequestTimesForForegroundApp(scanRequestTimestamps, currentTimeMillis);
        if (scanRequestTimestamps.size() >= SCAN_REQUEST_THROTTLE_MAX_IN_TIME_WINDOW_FG_APPS) {
            return true;
        }
        // Proceed with the scan request and record the time.
        scanRequestTimestamps.addLast(currentTimeMillis);
        return false;
    }


    @VisibleForTesting
    public static final int SCAN_REQUEST_THROTTLE_TIME_WINDOW_FG_APPS_MS = 120 * 1000;
    @VisibleForTesting
    public static final int SCAN_REQUEST_THROTTLE_MAX_IN_TIME_WINDOW_FG_APPS = 4;


    private void trimPastScanRequestTimesForForegroundApp(
            List<Long> scanRequestTimestamps, long currentTimeMillis) {
        Iterator<Long> timestampsIter = scanRequestTimestamps.iterator();
        while (timestampsIter.hasNext()) {
            Long scanRequestTimeMillis = timestampsIter.next();
            if ((currentTimeMillis - scanRequestTimeMillis)
                    > SCAN_REQUEST_THROTTLE_TIME_WINDOW_FG_APPS_MS) {
                timestampsIter.remove();
            } else {
                // This list is sorted by timestamps, so we can skip any more checks
                break;
            }
        }
    }

也就是說每個應用2分鐘內只能發起四次掃描

 

2. 總結

 * d) Throttle scan requests from non-setting apps:
 *  a) Each foreground app can request a max of
 *   {@link #SCAN_REQUEST_THROTTLE_MAX_IN_TIME_WINDOW_FG_APPS} scan every
 *   {@link #SCAN_REQUEST_THROTTLE_TIME_WINDOW_FG_APPS_MS}.
 *  b) Background apps combined can request 1 scan every
 *   {@link #SCAN_REQUEST_THROTTLE_INTERVAL_BG_APPS_MS}.

ScanRequestProxy的註釋講的很清楚

• 每個前臺應用2分鐘內只能發起四次掃描
• 所有的後臺應用加起來30分鐘內只能發起一次掃描