RCArray is unsafe

[Manu via Digitalmars-d]

On 3 March 2015 at 06:37, Walter Bright via Digitalmars-d
<digitalmars-d at puremagic.com> wrote:
> On 3/1/2015 12:51 PM, Michel Fortin wrote:
>>
>> That's actually not enough. You'll have to block access to global
>> variables too:
>>
>>      S s;
>>
>>      void main() {
>>          s.array = RCArray!T([T()]);   // s.array's refcount is now 1
>>          foo(s.array[0]);           // pass by ref
>>      }
>>      void foo(ref T t) {
>>          s.array = RCArray!T([]);      // drop the old s.array
>>          t.doSomething();              // oops, t is gone
>>      }
>
>
> Thinking about it, there are many other ways this can happen. At the moment,
> I'm stuck thinking of a solution other than requiring foo() to be pure.
> Anyone have ideas?

My immediate impression on this problem:

s.array[0] is being passed to foo from main. s does not belong to main
(is global), and main does not hold have a reference to s.array.
Shouldn't main just need to inc/dec array around the call to foo when
passing un-owned references down the call tree.
It seems to me that there always needs to be a reference _somewhere_
on the stack for anything being passed down the call tree (unless the
function is pure). Seems simplest to capture a stack ref at the top
level, then as it's received as arguments to each callee, it's
effectively owned by those functions and they don't need to worry
anymore.

So, passing global x to some function; inc/dec x around the function
call that it's passed to...? Then the stack has its own reference, and
the global reference can go away safely.