Re: [capnproto] Re: Correctly revoking capabilities holding onto resources

['Kenton Varda' via Cap'n Proto]

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(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  wrote:

> Hi Kenton,
>
> I've been digging around the last couple days and the approach of adding 
> *MembranePolicy::getCanceler()
> -> kj::Maybe* 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 has been revoked nor the exception to raise when attempting new requests.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* // Wraps
> requests made in hooks
> *MembranePolicy::onRevoked(kj::Function&& 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  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 

Re: [capnproto] Re: Correctly revoking capabilities holding onto resources

[Rowan Reeve]

Hi Kenton,

I've been digging around the last couple days and the approach of adding 
*MembranePolicy::getCanceler() 
-> kj::Maybe* 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* // Wraps 
requests made in hooks
*MembranePolicy::onRevoked(kj::Function&& 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  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] 
   

   [2] 
   

   2. New requests against a capability obtained before revocation are 
   rejected. [1] 
   

   3. New requests against a capability obtained after revocation (replaced 
   with a dummy) are rejected.[1] 
   

   
I think requests from (1) can be immediately cancelled given access to a 
*kj::Canceller* wrapping all membraned 

Re: [capnproto] Re: Correctly revoking capabilities holding onto resources

['Kenton Varda' via Cap'n Proto]

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  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]
>
> 
>[2]
>
> 
>2. New requests against a capability obtained before revocation are
>rejected. [1]
>
> 
>3. New requests against a capability obtained after revocation
>(replaced with a dummy) are rejected.[1]
>
> 
>
> 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 

Re: [capnproto] Re: Correctly revoking capabilities holding onto resources

[Rowan Reeve]

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] 
   

   [2] 
   

   2. New requests against a capability obtained before revocation are 
   rejected. [1] 
   

   3. New requests against a capability obtained after revocation (replaced 
   with a dummy) are rejected.[1] 
   

   
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`. 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  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 

Re: [capnproto] Re: Correctly revoking capabilities holding onto resources

['Kenton Varda' via Cap'n Proto]

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`. 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  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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[capnproto] Re: Correctly revoking capabilities holding onto resources

[Rowan Reeve]

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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