Question

Not really language specific, but I have this in C#:

public static void StartJob() {
    ThreadPool.QueueUserWorkItem(s => {
        if (Monitor.TryEnter(_lock)) {
            ProcessRows();
            Monitor.Exit(_lock);
        }
    }
);

ProcessRows() processes and removes rows in a database until all rows are removed. At program launch and whenever a row is added to the database elsewhere in the program, StartJob is called, to ensure all rows are processed without blocking the program. Now if a row is added at exactly the same time as StartJob is about to release the lock, it will not be processed.

How do I ensure that all rows are processed? I prefer not to have ProcessRows() run unless rows are added.

Answer 1

Don’t lock around the method; lock within the method using a flag that says whether you’re running or not.

One implementation might look like the code below – in this case I’ve moved all the logic into ProcessRows and made the method return immediately if it’s already running.

public static bool _isRunning = false;

public static void StartJob() 
{ 
    ThreadPool.QueueUserWorkItem(s => { ProcessRows(); })
} 

public static void ProcessRows()
{
    Monitor.Enter(_lock);
    if (_isRunning) 
    {
        Monitor.Exit(_lock);
        return;
    }

    _isRunning = true;

    while (rowsToProcess)
    {
        Monitor.Exit(_lock);;

        // ... do your stuff

        Monitor.Enter(_lock);
    }

    _isRunning = false;
    Monitor.Exit(_lock);
}

With this structure, it’s impossible for the while loop to complete without setting _isRunning = false – if this weren’t the case, there would be a race condition if the loop completed just as another instance of the method started. Equally, when the method is called, it will always enter the loop and set _isRunning = true before another instance gets a chance to execute.