Re: [PATCH v7 06/10] rust: id_pool: take a NonZero capacity in with_capacity

[Alice Ryhl]

On Tue, Aug 25, 2026 at 08:09:13PM +0900, Eliot Courtney wrote:
> On Fri Aug 21, 2026 at 5:39 PM JST, Alexandre Courbot wrote:
> > On Mon Aug 17, 2026 at 4:04 PM JST, Eliot Courtney wrote:
> >> There is no good reason to allocate an IdPool with zero capacity.
> >> Reflect this in IdPool::with_capacity.
> >>
> >> Signed-off-by: Eliot Courtney <ecourtney@xxxxxxxxxx>
> >
> > I am not sure this one is justifiable; `KVec::with_capacity(0)` is
> > doable, so why not here? As long as it doesn't introduce soundness
> > issues I'd say this is the caller's business; a driver with a legitimate
> > empty IdPool use-case would now need to special-case it.
> >
> > Now we do have an actual soundness issue with zero-sized IdPools, which
> > is that `find_unused_id` would panic with `CONFIG_RUST_BITMAP_HARDENED`,
> > but as I said on patch 5 I don't think it should anyway. Another
> > potential issue is that `grow_request` would not grow anything; but that
> > should be fixed there by handling the `capacity == 0` case. Actually
> > that would give justification for empty IdPools to exist: just like a
> > vector can start empty and grow, so can an IdPool.
> 
> I don't have a very strong opinion here but I can't really think of a
> use case for a zero capacity IdPool. Unlike an empty vector, since
> IdPool doesn't automatically grow (there is a notion of a fixed ID
> space), the only thing you can do with a zero capacity IdPool is grow it
> to non-zero. All the other operations don't do anything useful.

It may not grow automatically, but that's only because Binder (which
will grow its IdPool) holds it in a spinlock and needs to use the
PoolResizer and so on to grow it without allocating under said spinlock.

> If such a use case exists, maybe it'd have to be something like you are
> using the capacity to identify your ID space size (and the ID space size
> is important otherwise you would just use IdPool::new() with the
> MAX_INLINE_LEN capacity) but then the only way you can grow it is via
> grow_request() which doesn't grow the ID space in caller controllable
> way.
> 
> Anyway, let me know if you feel strongly about this one. FWIW, previous
> to this patch series you couldn't construct a 0 capacity IdPool either.

I feel strongly.

Using NonZero to prevent passing zero is a very strong mitigation due to
its big ergonomic cost. There's nothing really wrong about a
zero-capacity IdPool, so let's not pay the ergonomics cost when we don't
need to.

Alice