Code Generation

The CLN project has a multitude of interfaces, most of which are generated from an abstract schema:

• Wire format for peer-to-peer communication: this is the binary format that is specific by the LN spec. It uses the generate-wire.py script to parse the (faux) CSV files that are automatically extracted from the specification and writes C source code files that are then used internally to encode and decode messages, as well as provide print functions for the messages.

• Wire format for inter-daemon communication: CLN follows a multi-daemon architecture, making communication explicit across daemons. For this inter-daemon communication we use a slightly altered message format from the LN spec. The changes are

  1. addition of FD passing semantics to allow establishing a new connection between daemons (communication uses socketpair, so no connect)
  2. change the message length prefix from u16 to u32, allowing for messages larger than 65Kb. The CSV files are with the respective sub-daemon and also use generate-wire.py to generate encoding, decoding and printing functions

• We describe the JSON-RPC using JSON Schema in the doc/schemas directory. Each method has a lightning-*.json for request and response. During tests the pytest target will verify responses, however the JSON-RPC methods are not generated (yet?). We do generate various client stubs for languages, using the msggen[msggen] tool. More on the generated stubs and utilities below.

Man pages

The manpages are generated from the JSON schemas using the fromschema tool. It reads the request and response schema from lightning-*.json and generates markdown contents and manpages:

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Some of this functionality overlaps with msggen (parsing the Schemas) and blockreplace.py (filling in the template). It is likely that this will eventually be merged.

msggen

msggen is used to generate JSON-RPC client stubs, and converters between in-memory formats and the JSON format. In addition, by chaining some of these we can expose a grpc interface that matches the JSON-RPC interface. This conversion chain is implemented in the grpc-plugin.

cln-rpc

We use msggen to generate the Rust bindings crate cln-rpc. These bindings contain the stubs for the JSON-RPC methods, as well as types for the request and response structs. The generator code maps each abstract JSON-RPC type to a Rust type, minimizing size (e.g., binary data is hex-decoded).

The calling pattern follows the call(req_obj) -> resp_obj format, and the individual arguments are not expanded. For more ergonomic handling of generic requests and responses we also define the Request and Response enumerations, so you can hand them to a generic function without having to resort to dynamic dispatch.

The remainder of the crate implements an async/await JSON-RPC client, that can deal with the Unix Domain Socket transport used by CLN.

cln-grpc

The cln-grpc crate is mostly used to provide the primitives to build the grpc-plugin. As mentioned above, the grpc functionality relies on a chain of generated parts:

• First msggen is used to generate the protobuf file, containing the service definition with the method stubs, and the types referenced by those stubs.
• Next it generates the convert.rs file which is used to convert the structs for in-memory representation from cln-rpc into the corresponding protobuf structs.
• Finally msggen generates the server.rs file which can be bound to a grpc endpoint listening for incoming grpc requests, and it will convert the request and forward it to the JSON-RPC. Upon receiving the response it gets converted back into a grpc response and sent back.