So I'm working on a SMT infrastructure, and expression of
thread-safety is a core design mechanic... but I'm really struggling
to express it in terms of the type system.
I figure I'll throw some design challenges out here and see if anyone
can offer some good ideas.

The thing I'm trying to model is an attribute along the lines of
`shared`, but actually useful ;)
I'll use the attribute `threadsafe` in place of `shared`, and see
where that goes.

Consider:
struct Bob
{
  int x;
  threadsafe Atomic!int y;

  void m1();
  void m2() threadsafe;;

  void overloaded();
  void overloaded() threadsafe;
}

void func(Bob x, threadsafe Bob y)
{
  x.x = 10; // fine
  x.y = 10; // fine
  x.m1(); // fine
  x.m2(); // fine
  x.overloaded(); // fine, use the un-threadsafe overload

  y.x = 10; // ERROR, a threadsafe reference can NOT modify an
un-threadsafe member
  y.y = 10; // fine
  x.m1(); // ERROR, method not threadsafe
  x.m2(); // fine
  x.overloaded(); // fine, use the threadsafe overload

  threadsafe Bob* p = &x; // can take threadsafe reference to
thread-local object
}

This is loosely what `shared` models, but there's a few differences:
1. thread-local can NOT promote to shared
2. shared `this` applies to members

For `shared` to be useful, it should be that a shared reference to something inhibits access to it's thread-local stuff. And in that world, then I believe that thread-local promotion to shared would work like const does.

I guess I'm wondering; should `shared` be transitive? Perhaps that's what's wrong with it...?

On Sunday, 7 October 2018 at 01:59:21 UTC, Manu wrote:
> So I'm working on a SMT infrastructure, and expression of
> thread-safety is a core design mechanic... but I'm really struggling
> to express it in terms of the type system.
> I figure I'll throw some design challenges out here and see if anyone
> can offer some good ideas.
>
> The thing I'm trying to model is an attribute along the lines of
> `shared`, but actually useful ;)
> I'll use the attribute `threadsafe` in place of `shared`, and see
> where that goes.
>
> Consider:
> struct Bob
> {
>   int x;
>   threadsafe Atomic!int y;
>
>   void m1();
>   void m2() threadsafe;;
>
>   void overloaded();
>   void overloaded() threadsafe;
> }
>
> void func( ref Bob x, ref threadsafe Bob y)
> {
>   x.x = 10; // fine
>   x.y = 10; // fine
>   x.m1(); // fine
>   x.m2(); // fine
>   x.overloaded(); // fine, use the un-threadsafe overload
>
>   y.x = 10; // ERROR, a threadsafe reference can NOT modify an
> un-threadsafe member
>   y.y = 10; // fine
>   x.m1(); // ERROR, method not threadsafe
>   x.m2(); // fine
>   x.overloaded(); // fine, use the threadsafe overload
>
>   threadsafe Bob* p = &x; // can take threadsafe reference to
> thread-local object
> }
>
> This is loosely what `shared` models, but there's a few differences:
> 1. thread-local can NOT promote to shared
> 2. shared `this` applies to members
>
> For `shared` to be useful, it should be that a shared reference to something inhibits access to it's thread-local stuff. And in that world, then I believe that thread-local promotion to shared would work like const does.
>
> I guess I'm wondering; should `shared` be transitive? Perhaps that's what's wrong with it...?

A delta comparison with shared

void func( ref Bob x, ref threadshared /* either shared or threadsafe*/ Bob y)
{
 // threadsafe / shared
  x.x = 10; // fine / fine
  x.y = 10; // fine / fine uses atomics
  x.m1(); // fine / fine
  x.m2(); // fine / error cannot call shared method on unshared object
  x.overloaded(); // fine, use the un-threadsafe overload / fine

  y.x = 10; // ERROR, a threadsafe reference can NOT modify an un-threadsafe member / error
  y.y = 10; // fine / fine (using atomics)
// Assuming these are supposed to be y not x
  y.m1(); // ERROR, method not threadsafe / error
  y.m2(); // fine / fine
  y.overloaded(); // fine, use the threadsafe overload / fine

  threadsafe Bob* p = &x; // can take threadsafe reference to thread-local object / error
}

Differences:
Can call threadsafe method on thread local / unshared
Can take threadsafe reference to thread-local object.

One thing that occurred to me is that _objects_ are shared, whereas _functions/methods_ (and their parameters) are thread safe .

Theadsafe is kind of like a const (as to mutable/immutable) to threading, a promise to behave correctly in the presence of threading. thread safe references therefore must not escape.

On Sat, Oct 6, 2018 at 7:40 PM Nicholas Wilson via Digitalmars-d <digitalmars-d@puremagic.com> wrote:
>
> [...]
>
> One thing that occurred to me is that _objects_ are shared, whereas _functions/methods_ (and their parameters) are thread safe .
>
> Theadsafe is kind of like a const (as to mutable/immutable) to threading, a promise to behave correctly in the presence of threading. thread safe references therefore must not escape.

Right, that's kinda what I want to model... but the more I think of it, the more I think that experience can fit into `shared`, because it's almost there, and the current incarnation of shared is objectively useless.

Consider shared as is today;
struct Bob
{
  int x;
  void f() shared
  {
    x = 10; // <- this compiles... WAT?!
  }
}

Right now, if you have a shared instance, you can read/write to the
members... and that makes *absolutely no sense* no matter how you look
at it.
There is no reality where you have a shared thing, and accessing
members un-controlled can be safe.

Conventional wisdom is that when you have a shared thing, and you want
to do stuff with it, you must acquire locks (or whatever) and case
shared away. That should apply to f() above.

struct Bob
{
  int x;
  void f() shared
  {
    auto lock = getLock();
    auto unshared = shared_cast(&this);
    unshared.x = 10; // <- this is now okay.
  }
}

If we made a change were `shared` lost the ability to access non-`shared` members, I don't think that would interfere with current or proposed uses of shared in any way whatsoever... and we would make shared useful in the process.

On Sunday, 7 October 2018 at 02:59:12 UTC, Manu wrote:
> On Sat, Oct 6, 2018 at 7:40 PM Nicholas Wilson via Digitalmars-d <digitalmars-d@puremagic.com> wrote:
>>
>> [...]
>>
>> One thing that occurred to me is that _objects_ are shared, whereas _functions/methods_ (and their parameters) are thread safe .
>>
>> Theadsafe is kind of like a const (as to mutable/immutable) to threading, a promise to behave correctly in the presence of threading. thread safe references therefore must not escape.
>
> Right, that's kinda what I want to model... but the more I think of it, the more I think that experience can fit into `shared`, because it's almost there, and the current incarnation of shared is objectively useless.
>
> Consider shared as is today;
> struct Bob
> {
>   int x;
>   void f() shared
>   {
>     x = 10; // <- this compiles... WAT?!
>   }
> }
>
> Right now, if you have a shared instance, you can read/write to the
> members... and that makes *absolutely no sense* no matter how you look
> at it.
> There is no reality where you have a shared thing, and accessing
> members un-controlled can be safe.
>
> Conventional wisdom is that when you have a shared thing, and you want
> to do stuff with it, you must acquire locks (or whatever) and case
> shared away. That should apply to f() above.
>
> struct Bob
> {
>   int x;
>   void f() shared
>   {
>     auto lock = getLock();
>     auto unshared = shared_cast(&this);
>     unshared.x = 10; // <- this is now okay.
>   }
> }
>
> If we made a change were `shared` lost the ability to access non-`shared` members, I don't think that would interfere with current or proposed uses of shared in any way whatsoever... and we would make shared useful in the process.

I think it should be more like this
shared struct Bob
{
    shared int x;
    void f() shared
    {
      Laquire:
      auto owned_bob = try_aquire(&this, pthread_self());
      // type will be the same as Bob* but without shared stripped from variables
      // and without any functions
      if (owned_bob is null)
      {
          __mmPause();
          goto Laquire;
      }
      *owned_bob.x = 10;
    }

}

struct Bob
{
  threadsafe Atomic!(string[string]) y;
}

void f(ref threadsafe Bob b)
{
  string[string] aa=b.y;
  aa["b"]="c";
}

Like this?