Add waited return var to Lock

[yonderblue]

In cases where you might have a GetThing() that concurrently does:

Check cache for k
If hit, return the val
If miss,
     do expensive work
     set in cache
     return the cached val

Using this pkg to put a Lock around the miss part can make sure we don't repeat expensive work from concurrent calls both seeing a miss. With the Lock though you'd want to check after the lock is entered if we are already cached since we could have just waited on a concurrent caller that set the cache.

So it would be:

Check cache for k
If hit, return the val
If miss,
     Lock(k)
     defer Lock(k)
     Check cache for k
        if hit, return the val
     do expensive work
     set in cache
     return the cached val

The added waited return var can allow a skipping of the extra check if waited=false.

[yonderblue]

I should note that I see the argument that doing the second cache check should always be fast enough not to warrant the skip. So feel free to close :)

[im7mortal]

@yonderblue Thank you for PR

I saw immediately your PR but I am still analyzing it.

I can immediately say that it's not possible to change function signature as then it will break the standard Locker interface
But I will think if it worth to wrap in other way