On Fri, 29 Sep 2023 14:13:08 -0500 Kenton Varda <ken...@cloudflare.com> wrote:
> On Fri, Sep 29, 2023 at 1:01 PM Alex <capnpr...@centromere.net> wrote: > > > Currently when an EOF occurs, there is no way to discern between an > > exceptional circumstance and a normal/expected circumstance. > > > > That may be, but all we really want to decide here is whether to send > an abort message back to the peer. In the case of an EOF that was the > result of an exceptional situation, it's almost certainly the case > that the peer can no longer receive messages anyway, and therefore > sending a message back to them is pointless. It's only when the peer > carefully shut down the socket only in a single direction that it'll > be able to receive a reply at all -- and in that case, it's not an > error situation. > > So I think we can safely say: If we receive an EOF, we might as well > send an EOF. > I agree: EOFs ought to be reciprocal irrespective of whether or not they were expected. Note that I am using "EOF" synonymously with "receipt of a TCP FIN packet". Based on our discussion, my current understanding is as follows: 1. EOFs may be either expected or unexpected, 2. Unexpected EOFs are exceptional and ought to trigger immediate/ungraceful shutdown of the connection, and 3. Expected EOFs ought not ever invoke exception machinery in either the local process or the remote process. Invoking the exception machinery for every-day normal disconnects is contrary to the KJ style guide[0]. There were two questions: A. How shall a peer discern between an exceptional connection closure and a normal one? My proposal is to add a new message, "GOODBYE", which allows the system to signal to its remote peer an intent to close the connection. Note that this message is communicated within the context of any encryption/authentication layer which may be in place (such as TLS), thus allowing application designers to discern between proper operation and outside interference. B. How should an application signal this intent to the RPC system? What would the API look like? Based on your recommendation, I will study the design of rpc-twoparty.h and submit a PR for your consideration. Is this a good summary, Kenton? [0] https://github.com/capnproto/capnproto/blob/f7e8d58ac67635d7e09997bca3254ff376a568a0/style-guide.md#exceptions > (I kind of wish that if you closed a socket without doing > shutdown(SHUT_WR) first, the recv() on the other end would fail > instead of signaling EOF, but alas...) > > > > > To extend the protocol in this way we would have to think about > > > backwards compatibility. If a peer running an older version of > > > capnp receives the "goodbye" message, it will respond with an > > > "unimplemented" message, which seems like it could make things > > > worse? > > > > It's unclear to me how it would make things worse, since the > > connection is in the process of being shut down anyway. I am not > > saying it can't/wouldn't make things worse, I am only saying that > > it is not clear to me how that could be so. > > > > Just in that it's sending *another* unwanted message back to a peer > that has already disconnected. But I suppose that's not that much > worse compared to the status quo. > Yes. > > > In the CapTP/E/Vat > > paradigm, is it valid for a single RPC system to form multiple > > independent connections to the same VatId? > > > That's an interesting question. > > Ideally, no more than one connection is formed between any two vats, > and this is especially helpful if the application needs to be able to > compare capabilities for equality. But in practice I think this gets > difficult to ensure if vats cannot all directly address each other or > cannot use asymmetric cryptography to authenticate each other. I > think it'll be hard to answer this question definitively without a > specific real-world system to talk about. > I am only inquiring about the ideal case, so your answer is helpful. > VatNetwork::connect(vatId) is currently designed to return the > existing connection if there is one but I know of at least one > real-world implementation where it doesn't actually work that way. > Good to know, thank you. > > > In other words, if I call: > > > > connA = VatNetwork::connect(vatIdA); > > > > followed by: > > > > connB = VatNetwork::connect(vatIdB); > > > > where vatIdA == vatIdB, should connA and connB refer to the same > > object in memory -- thus only ever creating a single > > RpcConnectionState? Or, should connA and connB instead be two > > independent objects in memory, each with their own independent > > underlying connection and thus, independently evolving > > RpcConnectionState? > > > I think, then, it has to be up to the RPC system to shut down > > > connections that are idle. Probably RpcSystem could signal to the > > > underlying VatNetwork whenever a connection has reached an idle > > > state, meaning it has no outstanding RPCs nor capabilities. The > > > VatNetwork could choose to close such a connection if it feels > > > like it -- some transports may want to do this on a timeout, > > > others may decide it's better to keep the connection open. > > > > > > > I have no strong opinion on this. > > > > > But I'd suggest not worrying about that for now and focusing just > > > on rpc-twoparty, since that's what most people are using today. > > > > > > > Indeed, a PR is forthcoming. > > > > > -Kenton > > > > > > > Alex > > > > > > > > > I considered modifying the > > > > signature of the BootstrapFactory's createFor method in this > > > > manner: > > > > > > > > Before: > > > > > > > > capnp::Capability::Client createFor(VatId::Reader clientId) > > > > > > > > After: > > > > > > > > capnp::Capability::Client createFor(VatId::Reader clientId, > > > > kj::Own<kj::PromiseFulfiller<void>> shutdown) > > > > > > > > The PromiseFulfiller can then be passed to the constructor of > > > > the Server: > > > > > > > > class AdderImpl final: public Adder::Server { > > > > public: > > > > AdderImpl(kj::Own<kj::PromiseFulfiller<void>> shutdown) : > > > > shutdown(kj::mv(shutdown)) {} > > > > > > > > kj::Promise<void> add(AddContext context) override { > > > > auto params = context.getParams(); > > > > context.getResults().setValue(params.getLeft() + > > > > params.getRight()); return kj::READY_NOW; > > > > } > > > > > > > > kj::Promise<void> cleanupGracefully(CleanupGracefullyContext > > > > context) override { > > > > this->shutdown->fulfill(); > > > > } > > > > > > > > private: > > > > kj::Own<kj::PromiseFulfiller<void>> shutdown; > > > > }; > > > > > > > > Another approach could be to add a shutdown() method to > > > > FooContext. > > > > > > > > On the client side, perhaps it is best to simply allow the > > > > rpcSystem to fall out of scope, at which point the destructors > > > > can invoke the necessary machinery to send the GOODBYE and FIN > > > > the TCP stream. > > > > > > > > What do you think? Are these approaches going to lead to a leaky > > > > abstraction? Do you know of an elegant way to design this? > > > > > > > > Regards, > > > > Alex > > > > > > > > On Wed, 27 Sep 2023 14:07:07 -0500 > > > > "'Kenton Varda' via Cap'n Proto" <capnproto@googlegroups.com> > > > > wrote: > > > > > (Happy to accept a PR. The relevant code is in > > > > > `messageLoop()` and `RpcConnectionState::disconnect()` in > > > > > `rpc.c++`.) > > > > > > > > > > On Wed, Sep 27, 2023 at 2:05 PM Kenton Varda > > > > > <ken...@cloudflare.com> wrote: > > > > > > > > > > > Indeed, there isn't really a clean shutdown mechanism right > > > > > > now. I guess it hasn't come up as a priority because in most > > > > > > use cases we just haven't really cared if there's a TCP RST > > > > > > triggered under the hood... since we're already killing the > > > > > > connection, we ignore that error anyway. > > > > > > > > > > > > I suppose what we should do is, in the case that we receive > > > > > > a clean EOF, inhibit the sending of an abort message back, > > > > > > just send EOF back. > > > > > > > > > > > > -Kenton > > > > > > > > > > > > On Thu, Sep 21, 2023 at 6:09 PM 'Alex' via Cap'n Proto < > > > > > > capnproto@googlegroups.com> wrote: > > > > > > > > > > > >> Hi all, > > > > > >> > > > > > >> I am designing an application (in C++) where, upon > > > > > >> invocation of a particular RPC call, both the server and > > > > > >> the client agree to cleanly disconnect from one another. > > > > > >> By "cleanly", I mean that both the server and the client > > > > > >> send a TCP FIN/ACK and nothing more (e.g. no RSTs). > > > > > >> Unfortunately, in the current design the receipt of a FIN > > > > > >> will cause AsyncIoMessageStream::tryReadMessage() to > > > > > >> abort, whereupon it will throw KJ_EXCEPTION(DISCONNECTED, > > > > > >> "Peer disconnected.")[0]. This exception is eventually > > > > > >> written to the client socket, and if the client is already > > > > > >> gone, there will be one or more RSTs in response: > > > > > >> > > > > > >> C -> S: "Goodbye" (RPC call) > > > > > >> C -> S: "I have nothing more to say" (TCP FIN) > > > > > >> > > > > > >> (the client does not expect the server to say anything > > > > > >> more and closes the socket) > > > > > >> > > > > > >> S -> C: "Exception! You disconnected from me" (RPC message) > > > > > >> C -> S: "Error: Connection reset by peer" (TCP RST) > > > > > >> > > > > > >> Given that both the server and client have agreed to shut > > > > > >> down the connection, this is not an exceptional > > > > > >> circumstance. Therefore, an exception should not be thrown. > > > > > >> > > > > > >> Unfortunately, there does not seem to be a way to indicate > > > > > >> to the RpcSystem that the DISCONNECTED exception ought to > > > > > >> be suppressed. Is there something I am missing? I > > > > > >> appreciate any assistance. > > > > > >> > > > > > >> Regards, > > > > > >> Alex > > > > > >> > > > > > >> [0] > > > > > >> > > > > > > https://github.com/capnproto/capnproto/blob/761aeb17563a59f43b3fe9bae93df83c6bd57d06/c%2B%2B/src/capnp/rpc.c%2B%2B#L2775 > > > > > > > > > > > >> > > > > > >> -- > > > > > >> 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+unsubscr...@googlegroups.com. To view this > > > > > >> discussion on the web visit > > > > > > https://groups.google.com/d/msgid/capnproto/20230921190853.115b911d%40centromere.net > > > > > > > > > > > >> . > > > > > >> > > > > > > > > > > > > > > > > > > > -- > > > > 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+unsubscr...@googlegroups.com. > > > > To view this discussion on the web visit > > > > > > https://groups.google.com/d/msgid/capnproto/20230927173552.7ad4b38e%40centromere.net > > > > > > . > > > > > > > > -- 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+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/capnproto/20230929163640.61fa549c%40centromere.net.
