[Haskell] Re: ANN: HDBC (Haskell Database Connectivity)

[Benjamin Franksen]

On Wednesday 04 January 2006 20:13, John Goerzen wrote:
> On 2006-01-04, Krasimir Angelov <kr.angelov at gmail.com> wrote:
> >> I use ForeignPtrs everywhere in HDBC to try to make sure that
> >> nothing like this happens, and also that The Right Thing happens
> >> if a database handle gets garbage collected without being
> >> explicitly closed first.
> >
> > I prefer not to rely on ForeignPtrs. It isn't guaranteed that they
> > will be run. Another problem is that the order in which finalizers
> > are executed isn't specified. In this case the connection handle
> > can be
>
> Well, yes and no.  It would be impossible to garbage collect (and
> thus finalize) any object for which references to it still exist. 
> Statement handles in HDBC maintain references to the database handle
> pointers, either directly or indirectly, so I can't see how it is
> possible for a database handle to be finalized before the statement
> handle in this situation.

Hi John,

I fear it /is/ possible. This is a very unfortunate situation and one I 
had quite some difficulties to understand, when Simon Marlow explained 
it to me.

The problem is that finalization of the statement handle might be 
delayed indefinitely. The data dependencies between statement and 
connection handle only ensures that whenever the statement handle is 
alive, then too is the connection handle. But it does not say anything 
about what happens in which order after /both/ are dead (garbage). As 
soon as the connection handle to garbage, too, bothe handles can be 
finalized in /any/ order.

As I pointed out before, this is a very bad thing, because it makes 
finalizers a whole lot less useful than they could be if an order 
between finalizations could be specified (directly or indirectly). The 
arguments against such a solution are mostly: (1) it is difficult to 
implement efficienty and (2) the programmer could accidentally cause 
finalizer deadlocks by specifying circular dependencies.

Ben