CON20-J. Do not make network calls while holding a lock

It is often insecure to hold a lock while performing network transactions. Depending on the speed and reliability of the connection, held locks can stall the program indefinitely causing a huge performance hit. At other times, it can result in temporary or permanent deadlock.

If the JVM is operating on a platform that uses a file system that operates over the network, then file I/O might also suffer the same performance hits as networked I/O. If such a platform is in use, then file I/O to files over the network should also not be performed while locks are held.

Noncompliant Code Example

This noncompliant code example shows the method sendPage() that sends a Page object from a server to a client. The method is synchronized so that the array pageBuff is accessed safely, when multiple threads request concurrent access.

// Class Page is defined separately. It stores and returns the Page name via getName()

public final boolean SUCCESS = true;
public final boolean FAILURE = false;
Page[] pageBuff = new Page[MAX_PAGE_SIZE];

public synchronized boolean sendPage(Socket socket, String pageName) throws IOException {
  // Get the output stream to write the Page to
  ObjectOutputStream out = new ObjectOutputStream(socket.getOutputStream());

  // Find the Page requested by the client (this operation requires synchronization)
  Page targetPage = null;
  for(Page p : pageBuff) {
    if(p.getName().compareTo(pageName) == 0) {
      targetPage = p;
    }
  }

  // Requested Page does not exist
  if(targetPage == null) {
    return FAILURE;
  } 

  // Send the Page to the client (does not require any synchronization)
  out.writeObject(targetPage);

  out.flush();
  out.close();
  return SUCCESS;
}

Calling writeObject() within the synchronized sendPage() method can result in delays and deadlock-like conditions in high latency networks or when network connections are inherently lossy.

Compliant Solution

This compliant solution entails separating the actions into a sequence of steps:

• Perform actions on data structures requiring synchronization
• Create copies of the objects that are required to be sent
• Perform network calls in a separate method that does not require any synchronization

In this compliant solution, the synchronized method getPage() is called from an unsynchronized method sendPage(), to find the requested Page in the pageBuff array. After the Page is retrieved, the method sendPage() calls the unsynchronized method deliverPage() to deliver the Page to the client.

public boolean sendPage(Socket socket, String pageName) { // No synchronization
  Page targetPage = getPage(pageName); 

  if(targetPage == null)
    return FAILURE;

  return deliverPage(socket, targetPage);
}

private synchronized Page getPage(String pageName) { // Requires synchronization
  Page targetPage = null;

  for(Page p : pageBuff) {
    if(p.getName().equals(pageName)) {
      targetPage = p;
    }
  }
  return targetPage;
}

public boolean deliverPage(Socket socket, Page page){
  try{
    // Get the output stream to write the Page to
    ObjectOutputStream out = new ObjectOutputStream(socket.getOutputStream());

    // Send the Page to the client
    out.writeObject(page);

  } catch(IOException io){
    // If recovery is not possible return FAILURE
    return FAILURE;    
  } finally {
    out.flush();
    out.close();
  }  
  return SUCCESS;
}

Risk Assessment

If a lock is held by a block that contains network transactional logic, temporary or permanent deadlocks may result.

Related Vulnerabilities

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

References

[[Grosso 01]] Chapter 10: Serialization
[[JLS 05]] Chapter 17, Threads and Locks
[[Rotem 08]] Falacies of Distributed Computing Explained

CON19-J. Notify all waiting threads instead of a single thread      11. Concurrency (CON)      CON21-J. Facilitate thread reuse by using Thread Pools