Fix mMobileIfaces is not protected by lock

Currently, mMobileIfaces is accessed from multiple threads, and
should be protected from concurrent accessing. However, since the
variable could be accessed frequently, holding the mStatsLock
would make this be blocked by network stats I/O operations.
Thus, protect the variable by making it volatile.

Test: Wifi on/off stress test
Bug: 192758557
Change-Id: Ie7694a63f5203ee7c83830ca13d97219b7949fd7

services/core/java/com/android/server/net/NetworkStatsService.java

@@ -289,8 +289,7 @@ public class NetworkStatsService extends INetworkStatsService.Stub {
     private String mActiveIface;
 
     /** Set of any ifaces associated with mobile networks since boot. */
-    @GuardedBy("mStatsLock")
-    private String[] mMobileIfaces = new String[0];
+    private volatile String[] mMobileIfaces = new String[0];
 
     /** Set of all ifaces currently used by traffic that does not explicitly specify a Network. */
     @GuardedBy("mStatsLock")
@@ -935,7 +934,7 @@ public class NetworkStatsService extends INetworkStatsService.Stub {
 
     @Override
     public String[] getMobileIfaces() {
-        return mMobileIfaces;
+        return mMobileIfaces.clone();
     }
 
     @Override
@@ -1084,7 +1083,8 @@ public class NetworkStatsService extends INetworkStatsService.Stub {
     }
 
     @Override
-    public long getIfaceStats(String iface, int type) {
+    public long getIfaceStats(@NonNull String iface, int type) {
+        Objects.requireNonNull(iface);
         long nativeIfaceStats = nativeGetIfaceStat(iface, type, checkBpfStatsEnable());
         if (nativeIfaceStats == -1) {
             return nativeIfaceStats;
@@ -1382,7 +1382,7 @@ public class NetworkStatsService extends INetworkStatsService.Stub {
             }
         }
 
-        mMobileIfaces = mobileIfaces.toArray(new String[mobileIfaces.size()]);
+        mMobileIfaces = mobileIfaces.toArray(new String[0]);
     }
 
     private static int getSubIdForMobile(@NonNull NetworkStateSnapshot state) {