Module protocol 

Length-delimited frame codec and message payload types for the parent ↔ child worker process boundary.

Additive evolution

Every public enum here is #[non_exhaustive] so the wire format can gain a new request, response, or message kind without forcing a semver-major bump on consumers. Most structs are also #[non_exhaustive] and expose pub fn new(...) constructors so the shapes can grow fields without breaking external builders. The exception is DataRow, which is built so frequently with struct-literal syntax (tests, harnesses, fakes) that the ergonomic cost of #[non_exhaustive] outweighs the additive-evolution benefit; the wire schema for a data row is also already fixed by the stream contract.

Structs

DataRow

One row in a streaming response.

DataRowEmitter

Stateful emitter that assigns per-stream sequence numbers and tracks the running row count for the terminal StreamSummary.

Diagnostic

Intermediate diagnostic frame emitted by the child during a request.

FatalExit

Final frame the child writes before it tears down on an unrecoverable failure.

Frame

Versioned envelope around a single protocol Message.

ProgressTick

Intermediate progress frame emitted by the child during a long-running request.

StreamSummary

Terminal stream-completion summary returned alongside a streaming response.

Enums

HostSessionMode

How the worker child should load host-session capabilities.

Message

One protocol message exchanged over the worker boundary.

ProtocolError

Failure modes the codec can produce while reading or writing a frame.

Request

Parent-issued worker request body.

Response

Child-issued terminal response body for one Request.

Constants

MAX_FRAME_BYTES

Default per-frame size limit applied by the parent when no explicit cap is configured on the capability builder.

MAX_FRAME_BYTES_HARD_CAP

Ceiling on the configurable frame cap. Prevents callers from defeating the memory-safety policy by passing an absurdly large value.

MIN_FRAME_BYTES

Floor on the configurable frame cap. Trivial requests and the handshake itself must fit inside this; callers cannot configure smaller.

PROTOCOL_VERSION

Wire protocol version negotiated between parent and child during the handshake frame. Bump only on a breaking wire change.

Functions

read_frame

Read one length-delimited JSON frame from reader.

write_frame

Serialise message as a length-delimited JSON frame to writer.