Vibecode: reducer return values.

[gefjon]

Description of Changes

Initial Codex-assissted prototype of reducer return values. Implemented only for Rust modules and the Rust client SDK, plus the HTTP API.

This is part of our weekend hackathon to implement distributed TPC-C.

Note that this PR is based on phoebe/tpcc-distributed-naive-http-requests. Reviewers (or cherry-pickers) should start from 4781c22 .

I asked Codex for some examples, and it spit out:

Reducer return value examples

1) cURL HTTP client calling a reducer

# Call reducer `return_greeting` which returns a String
curl -sS \
  -X POST \
  -H 'Authorization: Bearer <TOKEN>' \
  -H 'Content-Type: application/json' \
  'http://127.0.0.1:3000/v1/database/<DATABASE_ID_OR_NAME>/call/return_greeting' \
  -d '[]'

# Example response body:
# "Hello"

If the reducer returns (), the response body is empty on success.

2) Rust module defining a reducer

use spacetimedb::ReducerContext;

#[spacetimedb::reducer]
fn return_sum(_ctx: &ReducerContext, a: i32, b: i32) -> Result<i32, String> {
    Ok(a + b)
}

#[spacetimedb::reducer]
fn return_greeting(_ctx: &ReducerContext, name: String) -> Result<String, String> {
    Ok(format!("Hello, {name}!"))
}

Reducers may return () or Result<T, E>, where T: SpacetimeType and E: Display.

3) Rust client SDK calling a reducer

use spacetimedb_sdk::{DbConnectionBuilder, ReducerEvent, Status};
use my_module::RemoteModule;

let conn = DbConnectionBuilder::<RemoteModule>::new()
    .with_url("http://127.0.0.1:3000")
    .build()
    .unwrap();
conn.run_threaded();

conn.reducers
    .return_sum_then(2, 3, |ctx, status| {
        match status {
            Ok(Ok(value)) => {
                assert_eq!(value, 5);
                assert!(matches!(ctx.event.status, Status::Committed));
            }
            Ok(Err(msg)) => panic!("Reducer error: {msg}"),
            Err(err) => panic!("Reducer internal error: {err:?}"),
        }
    })
    .unwrap();

API and ABI breaking changes

Should be additive-only, but I don't trust that Codex did it write. This is marked draft for a reason.

Expected complexity level and risk

4: pretty deep changes, and it's all generated, so this will need thorough review.

Testing

• New smoketest for the HTTP /v1/database/:name/call/:reducer POST behavior.
• New SDK test for the Rust client SDK behavior.
• We'll integrate this into our TPC-C experiment.