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Including just the above there is no way to determine whether a membrane has been revoked nor the exception to raise when attempting new requests.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 -- You received this message because you are subscribed to the Google Groups "Cap'n Proto" group. 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