[phobos] custom BlkAttr flags

[Andrei Alexandrescu]

I think the flag would be a must for SafeD, so we may as well add it now.

Andrei

On 07/13/2010 10:09 AM, Steve Schveighoffer wrote:
> Currently, there is a problem in the runtime which can result in very odd
> behavior.  Let's say you declare a class like this:
>
> class C
> {
>     int[1] x;
> }
>
> Now, let's say you do something like this:
>
> auto c = new C;
> auto x = c.x[];
> x ~= 1;
>
> What happens here?  Well, the memory for c and  c.x are on the heap, so the
> block allocated by c is considered for appending, and a "length" field is looked
> at, even though that length is possibly garbage.  The result is that it's
> extremely improbable, but possible, that the append could happen in place if
> that "length" happens to be correct (thereby overwriting other members of c).  I
> can't even begin to construct a case which shows this is possible, and it may
> not even be, but I think this needs attention.
>
> In addition, if anyone allocates memory via GC.malloc, and then tries to append
> to such memory, a similar problem occurs, because the allocation did not go
> through the array allocation routine.  This is more provable, but less likely to
> happen (most people don't call GC.malloc).  However, a place where I am most
> concerned is closures, which I think call GC.malloc.
>
> What I propose to fix this is to allocate one of the block attribute flags to
> designate that a block is appendable.  The odd part about it, is that the GC is
> not aware of appending, so it is somewhat of a "user-defined" flag.
>
> My question is, how should we declare the flag?  I currently only defined it in
> lifetime.d, outside the gc, but I'm thinking it should be defined in the GC as
> well.  Also, should I allocate the next bit, or one of the higher order bits?
>
> I'm going to check in my code that defines it in lifetime.d only, and depending
> on the responses to this, I'll add it to the GC definition too.
>
> -Steve
>
>
>
>
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