Re: simplehash: preserve consistency in case of OOM

On Fri, Nov 17, 2023 at 12:13 PM Andres Freund <andres@anarazel.de> wrote:
>
> On 2023-11-17 10:42:54 -0800, Jeff Davis wrote:
> > Right now, if allocation fails while growing a hashtable, it's left in
> > an inconsistent state and can't be used again.

+1 to the patch.

> I'm not against allowing this - but I am curious, in which use cases is this
> useful?

I don't have an answer to that, but a guess would be when the server
is dealing with memory pressure. In my view the patch has merit purely
on the grounds of increasing robustness.

> > @@ -446,10 +459,11 @@ SH_CREATE(MemoryContext ctx, uint32 nelements, void *private_data)
> >       /* increase nelements by fillfactor, want to store nelements elements */
> >       size = Min((double) SH_MAX_SIZE, ((double) nelements) / SH_FILLFACTOR);
> >
> > -     SH_COMPUTE_PARAMETERS(tb, size);
> > +     size = SH_COMPUTE_SIZE(size);
> >
> > -     tb->data = (SH_ELEMENT_TYPE *) SH_ALLOCATE(tb, sizeof(SH_ELEMENT_TYPE) * tb->size);
> > +     tb->data = (SH_ELEMENT_TYPE *) SH_ALLOCATE(tb, sizeof(SH_ELEMENT_TYPE) * size);
> >
> > +     SH_UPDATE_PARAMETERS(tb, size);
> >       return tb;
> >  }
>
> Maybe add a comment explaining why it's important to update parameters after
> allocating?

Perhaps something like this:

+     /*
+     * Update parameters _after_ allocation succeeds; prevent
+     * bogus/corrupted state.
+     */
+     SH_UPDATE_PARAMETERS(tb, size);

Best regards,
Gurjeet
http://Gurje.et