linux-kernel - Re: [PATCH] rust/revocable: add try

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Message-ID: <D8FBUHBNPIEL.5A8GOEMPJSEA@proton.me>
Date: Thu, 13 Mar 2025 17:50:40 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Danilo Krummrich <dakr@...nel.org>
Cc: Alexandre Courbot <acourbot@...dia.com>, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust/revocable: add try_with() convenience method

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`.

---
Cheers,
Benno