On Wed, Sep 27, 2023 at 4:37 PM 'Alex' via Cap'n Proto <
capnproto@googlegroups.com> wrote:
> 1. I would like to add a new RPC Message in rpc.capnp:
>
> goodbye @14 :Void;
>
> This message indicates to the recipient that the sender has nothing
> more to say, and that it should stop read()ing the socket. In other
> words, upon receipt of rpc::Message::GOODBYE, messageLoop() ends
> gracefully (no exceptions are thrown). The sender then shuts down its
> write() side of the connection, causing a TCP FIN to be delivered to
> the recipient. Because there is no read() in progress, an exception shall
> not be thrown.
>
> The recipient performs whatever cleanup is necessary and sends a
> reciprocal GOODBYE, causing the same logic described above to be
> invoked on the other end.
>
> Do you think this is a good solution?
>
Hmm, what's the benefit of this, vs. simply sending EOF?
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?
> 2. In your opinion, what is the best way to expose this graceful
> disconnect functionality to applications?
This is a bit tricky.
For rpc-twoparty.h I think it's straightforward, it could simply be a
method on `TwoPartyClient` and `TwoPartyServer` to signal graceful
disconnect. (This would have to be a method returning a promise which
resolves when all buffers are flushed and such, so I don't think it can
just be destructor behavior.)
But in the full many-party vision of Cap'n Proto, the application is not
really intended to know what connections exist. The application could
receive two capabilities from two different parties which both happened to
point to the same third party, and those two capabilities end up sharing a
connection, even though they came from different places. So it seems like
the application has no reasonable way to express that it wants a connection
to shut down, if it doesn't even know a connection exists.
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.
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.
-Kenton
> 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
> > >>
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> > >>
> > >
> >
>
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