Re: [Firebird-devel] Providers, yvalve, external engines and object lifecycle

[Alex P. <pes...@ma...>]

 On 03/24/11 17:50, Adriano dos Santos Fernandes wrote:
> On 24-03-2011 11:36, Alex Peshkoff wrote:
>> Well, an example.
>>
>> I have database attachment and want to use it in some background thread.
>> Attachment may be closed at some random moment. I have a choice - check
>> some flags before each use of it (and anyway have problems - may be it
>> was closed at some point before that check and actual use) or add
>> reference to it and use without risk of segfault. Certainly, I must
>> check status vector after each operation, and after getting 'invalid
>> handle value' return from the thread. But checking status is anyway good
>> style, therefore with reference counter in attachment handle I write
>> code for that thread as if I'm the owner of an attachment. And I'm
>> really one of it's owners after addRef() call!
>>
> And let me say why it don't work.
>
> User first get a pointer (refcount = 1) from Firebird.
>
> Then he may use it and "close at some random moment". So let say, he has
> two threads:
>
> T1: globalObj->addRef();
> T1: globalObj->doSomething();
> T1: globalObj->release();
>
> T2: globalObj->release();
>
> Nothing prevent the random moment to call T2 release first than T1
> addRef. So it will segfault, because the release will wipe toe object
> from memory.
>
> So you may do some extra addRef and fake the system. But then you don't
> need any of them, and would just handle it without a public interface
> baggage that serves nothing.
>
> You see, reference counting alone don't solve the user problem.

When used wrong - yes, do not. But if:

T1: globalObj->addRef();
T1: start T2 /* that's using globalObj */

It's OK. BTW, it's present in MS docs - "You must also call AddRef on a
pointer before passing it ....".