It seems like what's needed here is a way to register a callback on
`kj::Canceler` which is invoked when cancel() is called. Then the
MembraneHook can register interest in being notified of cancellation,
wherein it can permanently disable itself.
Actually, there is a tricky way this can be done now:
canceler.wrap(kj::Promise<void>(kj::NEVER_DONE).attach(kj::defer([]() {
// This code runs when canceler.cancel() is called.
}));
This is kind of hacky, but it's enough to build what you want without
having to modify kj::Canceler. That said, modifying kj::Canceler directly
is also an option here.
Meanwhile, perhaps `MembraneHook::getCanceler()` should itself throw an
exception if cancellation has already occurred, so that we don't need a
separate `getRevocationReason()`?
Alternatively, maybe `kj::Canceler` itself should have a
`cancelPermanently()` method which not only cancels current promises but
also all promises added in the future? Plus some way to query if it has
been permanently canceled...
-Kenton
On Thu, Apr 13, 2023 at 3:39 PM Rowan Reeve <rowanre...@gmail.com> wrote:
> Hi Kenton,
>
> I've been digging around the last couple days and the approach of adding
> *MembranePolicy::getCanceler()
> -> kj::Maybe<kj::Canceler&>* seems to be OK when it comes to getting rid
> of outstanding requests from within the *MembraneRequestHook *and
> *MembraneHook*, but I've run into some snags emulating the behaviour of
> *MembranePolicy::onRevoked()* but synchronously:
>
> Including just the above there is no way to determine whether a membrane
> has been revoked nor the exception to raise when attempting new requests. I
> have added two new functions to address this (which so happen to allow for
> backward compatibility):
>
> - *MembranePolicy::isRevoked() -> bool,* which not strictly necessary,
> though aids comprehension, as next function could be made to return a
> *kj::Maybe*.
> - *MembranePolicy::getRevocationReason() -> kj::Exception*, which is
> expected to throw if called when *isRevoked()* would return *false*.
>
> This allows us to reject not just outstanding requests but *future* requests
> (without relying on *onRevoked()*), where the new request promises are
> substituted with promises rejected given the exception provided by
> *MembranePolicy::getRevocationReason()*. This comes with the shortcoming
> that because we are no longer *notified* about revocation inside the
> hooks, we cannot permanently revoke existing capabilities (see *MembraneHook::
> MembraneHook* where we replace the wrapped capability with
> *newBrokenCap()*). This causes a knock-on effect where if the policy is
> unrevoked, then all the membraned capabilities which were previously
> rejecting new requests will suddenly become active again, which I don't
> think is desirable behaviour.
>
> The only thing I can think of to make this work like
> *MembranePolicy::onRevoked()* would be to register a callback, but then
> we'd need to return some sort of RAII subscription so that our hooks don't
> get called after being dropped by a client, for which I don't think KJ has
> any infrastructure. So *MembranePolicy* ends up with something like the
> following new virtual functions:
>
> *MembranePolicy::getCanceler() -> kj::Maybe<kj::Canceler&>* // Wraps
> requests made in hooks
> *MembranePolicy::onRevoked(kj::Function<void(kj::Exception)>&& callback)
> -> kj::Subscription*
>
> The following diff shows what I've got so far (as per first suggested
> implementation):
> https://github.com/capnproto/capnproto/compare/master...Zentren:capnproto:sync-membrane-policy
>
> Any thoughts?
>
>
> Thanks,
>
> Rowan Reeve
>
> On Wednesday, April 5, 2023 at 8:28:18 PM UTC+2 ken...@cloudflare.com
> wrote:
>
> Hi Rowan,
>
> Right, MembranePolicy today uses exclusiveJoin() to effect an asynchronous
> revocation. The problem with such asynchronous revocation is that it
> provides no explicit guarantee of when it's safe to assume revocation has
> occurred. To use this safely, you probably need to detect when your
> objects' destructors are eventually called, when their refcount reaches
> zero, before you can assume they are no longer used.
>
> What I'm saying is we should probably add a new feature to MembranePolicy
> that uses kj::Canceler instead of exclusiveJoin(). kj::Canceler allows
> synchronous revocation. I think this should probably be a feature of
> MembranePolicy; I agree it's annoying to force applications to do it
> manually. Perhaps we could deprecate the old exclusiveJoin() approach, too,
> as the synchronous approach seems strictly better.
>
> Note that doing something like `onRevoked.then([]() { canceler.cancel();
> })` doesn't really solve anything, since the `.then()` runs asynchronously,
> so once again you have no guarantee when it will take effect.
>
> -Kenton
>
> On Thu, Mar 30, 2023 at 4:25 PM Rowan Reeve <rowan...@gmail.com> wrote:
>
> Hi Kenton,
>
> No stress, your time is given freely and I appreciate it.
>
> Your suggestion makes sense to allow an immediate method of cancelling
> outstanding requests wrapped inside a membrane. After a look over
> *membrane.c++*, I do not see a *kj::Canceller* in use, so I presume this
> is done using *kj::Promise::exclusiveJoin.* I think I see three scenarios
> being dealt with when *kj::MembranePolicy::onRevoked* resolves:
>
> 1. Existing requests are eventually rejected, but the underlying call
> path might still run depending on exclusive join resolution order (i.e. it
> will run if made before *onRevoked *was resolved). [1]
>
> <https://github.com/capnproto/capnproto/blob/v0.10.3/c%2B%2B/src/capnp/membrane.c%2B%2B#L213>
> [2]
>
> <https://github.com/capnproto/capnproto/blob/v0.10.3/c%2B%2B/src/capnp/membrane.c%2B%2B#L225>
> 2. New requests against a capability obtained before revocation are
> rejected. [1]
>
> <https://github.com/capnproto/capnproto/blob/v0.10.3/c%2B%2B/src/capnp/membrane.c%2B%2B#L467>
> 3. New requests against a capability obtained after revocation
> (replaced with a dummy) are rejected.[1]
>
> <https://github.com/capnproto/capnproto/blob/v0.10.3/c%2B%2B/src/capnp/membrane.c%2B%2B#L331>
>
> I think requests from (1) can be immediately cancelled given access to a
> *kj::Canceller* wrapping all membraned requests. I think given the dummy
> capability injected in (3), those requests are safely rejected as-is. I
> however have a concern with (2); is it guaranteed that these new requests
> will be resolved *after* *onRevoked* is processed? I'd presume requests
> would land up on the event queue in-order, but I just wonder if there could
> be any race conditions involved. *If* it is all processed in-order, is it
> then also safe to assume that *kj* will eagerly evaluate a
> onRevoked*.then([this]()
> { canceller.cancel(); }* relying on the result of *onRevoked, *i.e. that *this
> *is still safe to use in the *MembraneHook* and/or *MembraneRequestHook*?
>
> It's a pity that the user would need to be responsible for both manually
> cancelling outstanding requests in addition to rejecting the promises
> exposed by *kj::MembranePolicy::**onRevoked* (unless I'm missing
> something). I wonder, it seems like *kj::MembranePolicy::onRevoked* seems
> to be intended to produce promises from a *kj::ForkedPromise* under the
> hood, which itself seems to have been done as a convenience as this
> provides a single-producer/multi-consumer interface to this revocation
> "event", and *kj::Promise::exclusiveJoin* already existed to reject
> calls. Could another single-producer/multi-consumer *protected* interface
> be exposed by *kj::MembranePolicy* which handles all this inline, i.e.
> without going to the event loop but leaving the public interface unchanged?
>
> Given your current and future feedback, could I raise an issue and look
> into creating a draft PR on GitHub to start exploring the change that
> you've suggested? I will probably only get to writing any code from the
> 10th of April, so further discussion can occur here and/or on the issue in
> the meantime (whichever is preferred).
>
> Look forward to hearing from you,
>
> Rowan Reeve
>
>
> On Monday, March 27, 2023 at 4:43:51 PM UTC+2 ken...@cloudflare.com wrote:
>
> Hi Rowan,
>
> Sorry for the slow reply, my inbox is overloaded as always.
>
> Indeed, since the `onRevoked` mechanism is triggered by a promise, the
> actual revocation and cancellation occurs asynchronously. It's possible
> that some other promise will be queued in between the point where you
> resolve the revoker promise and when the revocation actually takes effect.
>
> kj::Canceler has better behavior, in that all cancellation happens
> synchronously. But, capnp::Membrane does not currently use that. I have
> myself hit this a couple times and ended up using hacks like you suggest.
>
> Perhaps we should extend MembranePolicy with `getCanceler()` that returns
> `kj::Maybe<kj::Canceler&>`. If non-null, the canceler wraps all promises
> and capabilities passed through the membrane.
>
> -Kenton
>
> On Mon, Mar 27, 2023 at 7:35 AM Rowan Reeve <rowan...@gmail.com> wrote:
>
> I've added an ugly unit test to a branch on my GitHub to illustrate:
>
>
> https://github.com/capnproto/capnproto/compare/master...Zentren:capnproto:membrane_issue?expand=1#diff-49ad79a4fffcbe88fcd8681ec67d49f5f6e5fc9010961c1b10ef1b462f0e957eR477
>
> Note line 477 in *c++/src/capnp/membrane-test.c++* where I'd expect the
> request to have been cancelled as per membrane policy *onRevoked()* docs
> ("all outstanding calls cancelled"). Looking at the behavior, it seems like
> chained promises in the request are not cancelled as part of this (only the
> initial *call(CallContext)* IF we have not yet entered its body).
>
>
> Thanks,
>
> Rowan Reeve
> On Wednesday, March 15, 2023 at 3:42:39 PM UTC+2 Rowan Reeve wrote:
>
> Hi Kenton,
>
> I am encountering a problem where capabilities acting as views over some
> resources are intermittently causing segfaults. The capability is wrapped
> using *capnp::membrane* given a membrane policy where the promise
> returned by *onRevoked* can be rejected on-demand via a synchronous
> reject function (a kj::PromiseFulfillerPair is used to do this).
>
> The resources may be destroyed together at any time, whereby the membrane
> managing the capabilities accessing the resource states is revoked.
> However, this does not seem to be an instantaneous operation (presumably
> due to revocation being managed by a promise), and I have encountered the
> following issue as a result:
>
> Unresolved requests made before the membrane policy has been revoked and
> where the resource has since been destroyed are not cancelled but will
> rather resolve, accessing invalid memory.
>
> The workaround I have found to address this issue is to add a flag and a
> *kj::Canceller* to the capability implementations whereby new requests
> are rejected if the flag is set, and in addition when the flag is first
> set, the canceler cancels all returned promises in cases where a chained
> promise was returned rather than *kj::READY_NOW*. However, this is very
> ugly and necessitates keeping around references to the capability
> implementations before they are converted to *::Client* objects (so that
> we can set that flag). I'm thinking that surely there has to be a better
> way I have not considered.
>
> Do you have any thoughts on a better solution to this problem? If needed,
> I can try create a minimal reproducible example to illustrate.
>
> In case it matters, OS is Ubuntu 20.04 and capnp version is 8.0.0, both
> currently contained by my production environment.
>
> Thank you for your time,
>
> Rowan Reeve
>
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