synchronized (this[.classinfo]) in druntime and phobos

[Andrei Alexandrescu]

On 5/30/12 5:43 AM, Regan Heath wrote:
> Not in all cases. If you have a synchronized class, say a thread-safe
> container, which has a synchronized lookup and synchronized insert
> function, code like..
>
> if (!o.lookup(...))
> o.insert(...)
>
> obtains and releases the mutex twice each, and in between another thread
> could call o.insert() and insert the missing object - resulting in 2
> copies being inserted (or a duplicate insert exception being throw,
> or....).
>
> Now, the above is bad design, but the only other choice is to expose
> lock() and unlock() methods (or the mutex - back to square 1one) to
> allow locking around multiple class, but now the caller has to be
> careful to call unlock in all code paths. scope helps us in D, but this
> is actually the whole point of synchronized blocks - 1. ensuring the
> unlock always happens and 2. making the scope of the locking visible in
> the code.

There's a hybrid design available - a lock() operation may return a 
Locked!Type object which has a richer set of primitives. The implied 
understanding is that for the lifetime of that object, the underlying 
mutex is locked.

Andrei