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

[Andrei Alexandrescu]

On 5/30/12 12:03 PM, Steven Schveighoffer wrote:
> On Wed, 30 May 2012 14:32:59 -0400, Andrei Alexandrescu
> <SeeWebsiteForEmail at erdani.org> wrote:
>
>> On 5/30/12 10:47 AM, Steven Schveighoffer wrote:
>>> Yes, you can just use a private mutex. But doesn't that just lead to
>>> recommending not using a feature of the language?
>>
>> I don't think so. Synchronized classes are the unit of scoped locking
>> in D. If you want to do all scoped locking internally, make the class
>> private.
>
> Maybe you didn't read thoroughly the first part of my post (the example
> that shows a deadlock that is easily preventable if the mutex isn't
> exposed).

The mutex is not exposed.

> The issue is that it's possible to hijack the mutex that you
> are using to protect the class data in order to protect *external* data.
> Hijacking is not possible if the mutex is private (i.e. a private member).
>
> So great! Just don't use the builtin object mutex, because it exposes
> possible race conditions. But then why do we have it (the object hidden
> mutex)? And now I can't use synchronized classes to ensure the whole
> thing is protected, I have to write all my functions like this:
>
> void foo()
> {
> synchronized(this.mutex) // lock a private mutex
> {
> }
> }

This is a good example. But many synchronized classes are actually 
meaningful (e.g. the classic synchronized queue etc).


Andrei