shared: Has anyone used it without a lot of pain?

[deadalnix via Digitalmars-d]

On Wednesday, 5 April 2017 at 14:01:24 UTC, Guillaume Piolat 
wrote:
> Do we have a missed opportunity with shared?
>

Yes we do.

The #1 problem is that it lack a bridge to and from the "normal" 
thread local world. there is literally no way to use shared in a 
correct way, you always need to bypass part of the language ATM.

The 3 main way data are shared go as follow :
1/ Producer/consumer.
Thread 1 create some object, send it to thread 2 for processing. 
This is common in server applications for instance, where a 
thread will accept request and then dispatch it to worker threads.

Right now, there are no way to transfers ownership from one 
thread to another, so you pretty much got to cast to shared, move 
data to the other thread and then cast back to not shared. 
(People who followed closely will notice that this is somewhat 
entangled with other problem D has such as nogc exception, see 
http://forum.dlang.org/post/ikzzvbtvwhqweqlzxytz@forum.dlang.org 
).

2/ Actually shared objects with temporal ownership via a mutex.

Once again, this is about ownership. One can get a temporary 
ownership of some shared object which one can manipulate as if it 
was thread local for the duration a mutex is held.

The current way to do this is to take the mutex and cast away 
shared. This is bad as it breaks all type system guarantees.

To be done safely, this is needs to specify what is actually 
owned by the object that is protected by the mutex and what 
isn't. A phobos facility could, granted ownership was known, take 
an object, take the mutex and then allow to access it in a 
"scope" manner based on the lifetime of the mutex's lock. the 
accessed object can even be const in case of RWlock and it works 
beautifully because const is transitive.

3/ Actually shared object providing method which use atomics and 
alike to be thread safe.

This use case is actually decently served by shared today, except 
the construction of the object in the first place.