Re: [PATCH] rust/revocable: add try_with() convenience method

[Danilo Krummrich <dakr@kernel.org>]

On Sat, Mar 15, 2025 at 11:07:44PM +0900, Alexandre Courbot wrote:
> On Fri Mar 14, 2025 at 2:50 AM JST, Benno Lossin wrote:
> > On Thu Mar 13, 2025 at 4:48 PM CET, Danilo Krummrich wrote:
> >> On Thu, Mar 13, 2025 at 03:38:55PM +0000, Benno Lossin wrote:
> >>> On Thu Mar 13, 2025 at 4:08 PM CET, Alexandre Courbot wrote:
> >>> > On Thu Mar 13, 2025 at 11:19 PM JST, Benno Lossin wrote:
> >>> >> Would it make sense to not use `Result` here and continue with `Option`?
> >>> >
> >>> > We would have to return an Option<Result<R>> in this case. The current
> >>> > code folds the closure's Result into the one of the guard's acquisition
> >>> > for convenience.
> >>> >
> >>> > Actually, I don't think I have ever used try_access() a single time
> >>> > without converting its returned Option into a Result. Wouldn't it make
> >>> > sense to do the opposite, i.e. make try_access() return Err(ENXIO) when
> >>> > the guard cannot be acquired and document this behavior?
> >>> 
> >>> Sure, if you're always doing
> >>> 
> >>>     let guard = rev.try_access().ok_or(ENXIO)?;
> >>> 
> >>> Then it makes sense from my view, maybe Danilo has some other argument
> >>> for why `Option` is better.
> >>
> >> Most of the time I think we indeed want to derive an Err() if try_access()
> >> fails, but not with a specific error code. The error code depends on the context
> >> of where the revocable is used (e.g. for I/O mappings), but it also depends on
> >> the driver semantics.
> >
> > In that case a single function with this signature would make sense:
> >
> >     fn access_with<R>(&self, f: impl FnOnce(&T) -> R) -> Option<R>;
> >
> > If there are common usages that always return the same error code, then
> > we could add them as functions with `Result`.
> 
> Yeah the more I think about it the more this seems to make sense,
> from a strictly logical point of view.
> 
> Where I am still on the fence is that the goal is also to reduce the
> friction introduced by the Revocable business, which a large driver
> might need to interact with hundreds of times. If the user wants the
> callback to return a Result, then this method will return an
> Option<Result>. One would then need to ok_or the Option, then flatten
> the two results, which is a bit verbose.

I think you see this from the perspective of one specific usecase, i.e.
Devres<T>, where T dereferences to Io, right?

> I suppose drivers could add their own macros to do that automatically
> and reduce code verbosity, at the cost of less cohesion across drivers.
> Guess I'll go with that if I cannot come with anything better.

Maybe we could do something more specific but yet generic on top (for the
use-case above), but we still can't assume the exact error code a driver wants
to derive from failing try_access(). So, maybe a driver specific wrapper is
indeed what you want on top of what this patch provides.