gistfile1.txt

Pseudo Code:

// Create table:

CREATE TABLE LOCK (who Text, locked Bool);

// You have to use PGCKit to be able to query this table and create instances of:

struct Lock {
  NSString *who;
  BOOL locked;
}

// Let's assume Process A wants to do something critical:

doSomethingCiritial() {
  // 1. Get the current lock
  Lock l = getLock()
  if(l.locked == NO) {
    l.locked = YES;
    l.who = getIDFromUserDefaults()
    db.save(l)
  }
   
  // Do the actual work
  criticalStuff()

  // When done:
  l.who = NULL;
  l.locked = NO;
  db.save(l)
}

// Lets assume Process A crashes before it was able to unlock the lock…

cloudAppDidFinishLaunching() {
  // Get Lock 
  Lock l = db.getLock();
  if(l.locked && l.who == getIDFromUserDefaults()) {
    // The lock is still in place AND it was created by the current process
    // unlock:
    l.locked = NO;
    l.who = NULL:
    db.save(l);
  } else {
    // normal work
  }
}