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. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to capnproto+...@googlegroups.com. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/capnproto/2d126940-b82e-4ef8-9f41-304d8a23c97cn%40googlegroups.com >> >> <https://groups.google.com/d/msgid/capnproto/2d126940-b82e-4ef8-9f41-304d8a23c97cn%40googlegroups.com?utm_medium=email&utm_source=footer> >> . >> > -- You received this message because you are subscribed to the Google Groups "Cap'n Proto" group. 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