Fix misuse of System.Random in ExponentialRetry.cs

[jon-couloir]

Changed the random instance to be created at the class level. This prevents multiple calls to the ShouldRetry instance in the same Environment.TickCount period (~15 milliseconds) receiving the exact same GetNext() values from the random instance. This random needs to be protected with a lock, as System.Random is not threadsafe (this is a very simple threadsafe implementation, for high concurrency there are more performant implementations of threadsafe random access at the cost of higher complexity)

e.g. see https://blogs.msdn.microsoft.com/pfxteam/2009/02/19/getting-random-numbers-in-a-thread-safe-way/

[danielmarbach]

Why not use

public static class StaticRandom
{
    static int seed = Environment.TickCount;

    static readonly ThreadLocal<Random> random =
        new ThreadLocal<Random>(() => new Random(Interlocked.Increment(ref seed)));

    public static int Rand()
    {
        return random.Value.Next();
    }
}

https://stackoverflow.com/questions/19270507/correct-way-to-use-random-in-multithread-application

to avoid lock?

[kfarmer-msft]

Howdy --

Good catch!

I'll be adding this, with the ThreadLocal incrementing seed suggested by Daniel.

[kfarmer-msft]

Please try v9.4.2