CON36-J. Ensure that tasks submitted to a thread pool are interruptible

According to the Java API interface java.util.concurrent.ExecutorService, method shutdownNow() documentation [[API 06]]:

Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the tasks that were awaiting execution. There are no guarantees beyond best-effort attempts to stop processing actively executing tasks. For example, typical implementations will cancel via Thread.interrupt(), so any task that fails to respond to interrupts may never terminate.

Consequently, care must be taken so that only interruptible tasks are submitted to the thread pool. When using worker threads that run untrusted code or dynamically loaded extension code, a facility to interrupt the worker should be provided. For instance, a worker thread in a thread pool should generally have the following structure [[Goetz 06]]:

public void run() {
  Throwable thrown = null;
  try {
    while (!isInterrupted())
      runTask(getTaskFromWorkQueue());
  } catch (Throwable e) {
    thrown = e;
  } finally {
    threadExited(this, thrown); // Notify upper layers
  }
}

The worker thread based architecture that uses interruptible workers permits the caller to cleanly terminate misbehaving sub-tasks. If the sub-tasks terminate because of runtime exceptions, then the upper layers can take some suitable action such as replacing the worker thread with a new one.

Noncompliant Code Example (shutting down thread pools)

This noncompliant code example uses the SocketReader class defined earlier in the Compliant Solution (close socket connection) of the guideline [CON24-J. Ensure that threads and tasks performing blocking operations can be terminated] and submits it as a task to a thread pool defined in class PoolService.

public final class PoolService {
  private final ExecutorService pool;

  public PoolService(int poolSize) {
    pool = Executors.newFixedThreadPool(poolSize);
  }
	  
  public void doSomething() throws InterruptedException, IOException {	   
    pool.submit(new SocketReader());
    // ...
    List<Runnable> awaitingTasks = pool.shutdownNow();	      
  }

  public static void main(String[] args) throws InterruptedException, IOException {
    PoolService service = new PoolService(5);
    service.doSomething();
  }
}

public final class SocketReader implements Runnable {
  private final Socket socket;
  // ...
}

Because the task does not support interruption through the use of Thread.interrupted(), there is no guarantee that the shutdownNow() method will shutdown the thread pool. Using the shutdown() method does not fix the problem either, because it waits until all executing tasks have finished.

Similarly, tasks that check a volatile flag to determine whether it is safe to shutdown will also be unresponsive to these methods. The guideline [CON13-J. Ensure that threads are stopped cleanly] provides more information on using a flag to terminate threads.

Compliant Solution (submit interruptible tasks)

Tasks that do not support interruption using Thread.interrupt() should not be submitted to a thread pool. This compliant solution submits the interruptible version of SocketReader discussed in the Compliant Solution (interruptible channel) of the guideline [CON24-J. Ensure that threads and tasks performing blocking operations can be terminated], to the thread pool.

public final class PoolService {
  // ...
}

public final class SocketReader implements Runnable {
  private final SocketChannel sc;
  // ...
}

Similarly, when attempting to cancel individual tasks within the thread pool using the Future.cancel() method, ensure that the tasks support interruption. If they do, pass a boolean argument true to cancel(), otherwise pass false. The value false indicates that the corresponding task will be canceled if it has not already started.

Exceptions

EX1: Tasks that execute without blocking may violate this guideline.

Risk Assessment

Submitting tasks that are not interruptible may preclude the thread pool from shutting down and cause denial of service.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[API 06]] interface ExecutorService
[[Goetz 06]] Chapter 7: Cancellation and shutdown

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