This has come up several times on the mailing list. Looking for something like this to
be provided in sync.Mutex:

func (m *Mutex) TryLock() (locked bool) {
    locked = atomic.CompareAndSwapInt32(&m.state, 0, mutexLocked)
    if locked && raceenabled {
        raceAcquire(unsafe.Pointer(m))
    }
    return
}

Now you could use CompareAndSwapInt32 yourself. But it gets tricky when you need to use
Lock in one function, and TryLock in another on the same mutex.

The typical use-case is something like this:

if m.TryLock() {
    // do something
} else {
    // do something else
}

It's very similar to select default for channels:

select {
case <-c:
// do something
default:
// do something else
}

The difference being that with such a function you can do this with a primitive.

Other languages provide this functionality:

Java:
http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/locks/Lock.html#tryLock()
Objective-C:
https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSLock_Class/Reference/Reference.html#//apple_ref/occ/instm/NSLock/tryLock

A "LockBeforeTime" would also be useful, but I understand that's more
difficult to implement.