[dmd-concurrency] draft 7

[Andrei Alexandrescu]

Robert Jacques wrote:
> On Mon, 01 Feb 2010 15:30:47 -0500, Andrei Alexandrescu 
> <andrei at erdani.com> wrote:
>> Michel Fortin wrote:
>>>> The program would compile successfully with the following changes
>>>> to nyukNyuk and sneaky:
>>>>  shared(double)* nyukNyuk; // or: shared(double*) nyukNyuk; void
>>>> sneaky(ref shared(double) r) { nyukNyuk = &r; }
>>>  How can that be safe? Leaking the reference, even as 'shared',
>>> doesn't prevent someone from accessing the variable without acquiring
>>> the lock. Are you suggesting that mixing atomic operations and mutex
>>> synchronization for the same variable is okay? Because it's not.
>>
>> You are making a good point. I thought of banning escapes altogether. 
>> After all, if someone has access to a shared data member and the 
>> synchronized method assumes it can temporarily optimize access to that 
>> member as if it's unshared, things can get messed up.
>>
>> On the other hand, if you hand out a reference to a member, you're 
>> kind of setting yourself up for surprises. Far as the type system is 
>> concerned, it's doing the right thing.
>>
>> One related thought: a long time ago, Walter, Bartosz and I briefly 
>> coquetted with the idea of using "scope" to denote an unescaping 
>> parameter:
>>
>> void print(scope int[] array) { ... }
>>
>> With this signature print guarantees it won't escape anything in 
>> array. Then synchronized functions can pass (addresses of) members to 
>> functions like print.
>>
>> It's not an easy feature to implement though.
> 
> What's difficult to implement specifically? Full escape analysis is 
> difficult, but you can provide the same guarantees by being a bit 
> conservative.

Nah, it's the details that are difficult:

* most functions do take scope parameters, so probably @escaping should 
be the explicit attribute

* interaction of scope with templates

* what annotation do you use for saying that "this" is scoped, e.g.

class A {
    void foo() scope { ... }
}

(and in fact most of the time this is scoped, so again @escaping...)

* figuring out how all of this aggravation will be received by users 
already displeased by the size of the language


Andrei