A follow up after simplifying the API to the core issue:
The data flow is now:
1. A client connects to a server, passing a Schema.Node as a parameter.
2. A server loads the Schema.Node and creates a new message of the given
schema, which is returned as an AnyStruct
Problem 1 above still exists.
We have figured out how to handle point 2, but not without a malloc. How
can I init the result as a DynamicStruct, instead of copying as in the
below snippet:
auto message = context.getParams();
auto schema_ = message.getSchema();
auto schema_loader = capnp::SchemaLoader();
auto loaded_schema = schema_loader.load(schema_);
// how to do below without malloc?
// cannot init result as a DynamicStruct
auto mb = capnp::MallocMessageBuilder();
auto msg = mb.initRoot(loaded_schema.asStruct());
msg.set("text", "abc");
auto results = context.initResults();
results.setValue(msg.asReader());
Would this work as above if we had nested structs?
Aside: I am also trying to replicate this in pycapnp, which is proving
deeply problematic since it skips out on a lot of ported functionality from
capnp.
- Dane
On Monday, 27 February 2023 at 11:34:50 UTC+2 Dane wrote:
> Hi
>
> I am trying to implement a dynamic API. I have followed the suggestions in
> https://groups.google.com/g/capnproto/c/Ivl3u4JJoEQ/m/MIH2a2nPBQAJ and
> https://groups.google.com/g/capnproto/c/fNnr3Om9haM/m/1TdBqwsNBwAJ
>
> My implementation attempt lies at
> https://github.com/DaneSlattery/capnp_generic_poc .
>
> The desired data flow is as follows:
>
> 1. A client connects to a server, and receives a capability that allows
> the client to load a schema (the InterfaceLoader). When connecting, the
> client also passes a ClientInterface to the server.
> 2. The client registers a schema using the InterfaceLoader. The client
> must send a Schema.Node parameter to the server. The server must load the
> schema.
> 3. The server uses the given loaded schema to communicate with the client
> interface, which takes a parameter of AnyStruct.
>
> I have a few problems:
>
> Firstly, when the client attempts to set the schema using
> capnp::Schema::from().getProto() , line 683 of
> capnproto-c++-0.10.3/src/capnp/dynamic.c++ blows up. I have changed this
> line to, instead assessing the ID of the schema for equality :
> KJ_REQUIRE(structValue.getSchema().getProto().getId() ==
> structType.getProto().getId(), "Value type mismatch.")
> {
> return;
> }
> I think this change should perhaps go in a PR, unless there is another,
> better way.
>
> Secondly, I can't figure out how to send messages of the loaded schema
> back from the server to the client. Can anyone see if the POC linked is
> viable, and how to get to the final step?
>
> - Dane
>
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