Open Radar

 
Summary:
I think it would be a great addition to GCD if I could elevate a block to become a barrier block after it started to execute.

Given a concurrent queue, dispatching a block on it, at some point I want to call something like dispatch_barrier_wait(); or something similar that elevates the current block to a barrier block, waits for other concurrently executing blocks to finish and than continues like a regular barrier block. There is the case where multiple blocks try this at the same time though, so this method would need to return a BOOL, communicating if this worked or if the block is still a concurrently executing block.

This would be very helpful for cases like lazy getters in a parallel environment. It would be beneficial in a lot of cases that use the pattern to read synchronously and parallel but write in a barrier block.

For example:

```
- (NSUInteger)someValue {
    __block NSUInteger value;
    dispatch_sync(self.lockQueue, ^{
        if (self->_value == NSUIntegerMax) {
            BOOL success = dispatch_barrier_wait();
            if (success) {
                self->_value = [self calculateSomeValue];
            } else {
                // fall back to current behavior: leave queue, dispatch into it again with a barrier block
            }
        }
        value = self->_value;
    });
    return value;
}
```

In cases like this it is acceptable to wait for the barrier to take effect because the other option would be to leave the queue and dispatch into it again in a barrier block, which presumably would take more time.

Another benefit of this approach is: Due to the business logic in cases like these I know for sure that when this succeeds no other barrier block could have been executed in the meantime; so for my use case I know for sure that the ivar has not been altered in the meantime. This means I save the extra time to read the ivar again, which I would need to do if I leave the block and dispatch into it again in a barrier block to alter the ivar.

Steps to Reproduce:
 

Expected Results:
 

Actual Results:
 

Version:
11

Notes: