TLBB: The Last Big Breakage

[deadalnix]

On Sunday, 16 March 2014 at 04:08:15 UTC, Andrei Alexandrescu 
wrote:
> D1's approach to multithreading was wanting. D2 executed a big 
> departure from that with the shared qualifier and the 
> default-thread-local approach to data.
>
> We think this is a win, but D2 inherited a lot of D1's 
> thread-related behavior by default, and some of the rules 
> introduced by TDPL (http://goo.gl/9gtH0g) remained in the "I 
> have a dream" stage.
>
> Fixing that has not gained focus until recently, when e.g. 
> https://github.com/D-Programming-Language/dmd/pull/3067 has 
> come about. There is other more stringent control of shared 
> members, e.g. "synchronized" is all or none, "synchronized" 
> only makes direct member variables unshared, and more.
>
> This will statically break code. It will refuse to compile code 
> that is incorrect, but also plenty of code that is correct; the 
> compiler will demand extra guarantees from user code, be they 
> in the form of casts and stated assumptions.
>
> I believe this is a bridge we do need to cross. One question is 
> how we go about it: all at once, or gradually?
>
>
> Andrei

We should probably relax the restriction.

synchronized classes can have public fields, but they can't be 
accessed on shared instances. That way no code is broken when 
shared isn't used. That should dramatically decrease the 
difficulty of introducing the change, and that doesn't make the 
construct any less safe.

Or expressed as simpler rule: when a synchronized class is 
shared, public field become protected.