fix: 🐛 #188 (android): Add null checks to prevent NullPointerException…

[louiscavalcante]

This PR closes Issue #188

Problem

The app crashes with a NullPointerException when the socket becomes null during receiver task operations:

java.lang.NullPointerException: Attempt to invoke virtual method
'java.io.InputStream java.net.Socket.getInputStream()' on a null object reference
    at com.asterinet.react.tcpsocket.TcpSocketClient$TcpReceiverTask.run(TcpSocketClient.java:251)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1154)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:652)
    at java.lang.Thread.run(Thread.java:1563)

Root Cause Analysis

This is a race condition in TcpReceiverTask.run(). The socket can become null between retrieval and usage due to:

1. destroy() called concurrently: If destroy() is called while the receiver task is starting or running, it sets socket = null (line 190) while the receiver thread may be accessing it.

2. Timing window: The receiver task runs in a separate thread (listenExecutor). There's a window between when getSocket() returns and when socket.getInputStream() is called where another thread can call destroy().

3. No synchronization: The socket field is accessed from multiple threads without synchronization:

   • Main thread: connect(), destroy(), startTLS()
   • Receiver thread: TcpReceiverTask.run()
   • Write thread: write()

Triggering Scenario

In my case, this occurred when using react-native-zeroconf for mDNS service discovery followed by immediate TCP connection:

1. mDNS scan discovers a network service
2. User selects the service → scan stops
3. App immediately creates TCP socket to connect to the discovered service
4. Race condition: network stack instability from scan cleanup causes socket creation issues
5. Receiver task starts but socket is null → NPE

Changes

1. Add null guard at start of TcpReceiverTask.run()

@Override
public void run() {
    int socketId = clientSocket.getId();
    Socket socket = clientSocket.getSocket();

    // Guard against null socket - can happen if destroy() is called
    // before the receiver task starts, or if socket creation failed
    if (socket == null) {
        return;
    }
    // ... rest of method
}

2. Add null check in catch block

The catch block also accessed socket.isClosed() without null checking:

} catch (IOException | InterruptedException ioe) {
    if (receiverListener != null && socket != null && !socket.isClosed() && !clientSocket.closed) {
        receiverListener.onError(socketId, ioe);
    }
}

3. Add null checks in pause() and resume() methods

These methods access receiverTask without null checking. If called before startListening() completes, they would throw NPE:

public void pause() {
    if (receiverTask != null) {
        receiverTask.pause();
    }
}

public void resume() {
    if (receiverTask != null) {
        receiverTask.resume();
    }
}

Why This Fix

This is a defensive fix that prevents the crash without changing the library's behavior. The alternative would be to add proper thread synchronization (synchronized blocks or locks), but that would be a more invasive change with potential performance implications.

The write() method already has this pattern (lines 157-159):

if (socket == null) {
    receiverListener.onError(getId(), new IOException("Attempted to write to closed socket"));
    return;
}

This PR applies the same defensive pattern to the receiver task.

Testing

Tested by:

1. Starting mDNS discovery for network services
2. Selecting a discovered service (stops discovery, immediately creates TCP connection)
3. Verifying no NullPointerException occurs
4. Confirming normal socket operations work correctly

Impact

• Minimal: Only adds null checks, no behavioral changes
• No breaking changes: Existing code continues to work identically
• No performance impact: Single null check is negligible

[louiscavalcante]

Currently testing these changes.

[louiscavalcante]

Tested and ready for review. Happy new year! @Rapsssito

[louiscavalcante]

@Rapsssito the semantic release of this PR failed. Something about your NPM credentials.

[Rapsssito]

@louiscavalcante, Wops! My bad. NPM just removed classic tokens. I will create a new one ASAP.

[github-actions]

🎉 This PR is included in version 6.3.1 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

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