tunnel interface (VpnService) in android 4.4 does not block on read()

Question

I am using the VpnService to create a tunnel interface. The interface gets created correctly. I am able to set the routes and MTU on the tunnel interface as well. However, when I perform a read on this interface the read never blocks leading to a busy wait and wasted CPU cycles.

This is the run loop of my code to read from the tunnel interface:` public synchronized void run() { Log.v(TAG, "Launching the stitch tunnel thread");

    FileInputStream in = new FileInputStream(_tunnel_interface.getFileDescriptor());
    byte[] pktBuf = new byte[1024];

    while(true) {
        try {
            int length = in.read(pktBuf);

            Log.v(TAG, "Got a packet of length"+Integer.toString(length));
            if ( length == 0) {
                try {
                    Thread.sleep(1000);
                }catch (InterruptedException e) {
                    Log.w(TAG, "Thread was interrupted:"+e.toString());
                    stopSelf();
                    return;
                }
            }
        } catch (IOException e) {
            Log.w(TAG, "Stitch tunnel interface close unexpectedly:"+ e.toString());
            stopSelf();
            return;
        }
    }

}`

I checked the ToyVPN example and seems like even there the code doesn't expect to block on calling read on the tunnel interface.

Is this a bug? According to JAVA specifications for read on a fileinputstream, the read call should return 0 bytes only if the buffer that is passed is of length zero. This is obviously not the case over here. So a bit confused with the behavior I am observing.

Answer 1

From the VpnService.Builder establish method documentation :

"The file descriptor is put into non-blocking mode by default to avoid blocking Java threads."