Errors and cancellation

Every error is a FusionError with a stable code.

Code	Meaning
`REMOTE_ERROR`	The function in the worker raised (Python) or returned an error (Rust and Go). `err.remote` has `name`, `message` and, in Python, `traceback`, which is also appended to `err.stack`. A function can give its error a `code` of its own.
`TIMEOUT` / `ABORTED`	The call exceeded its timeout or its `AbortSignal` fired.
`STREAMING_FUNCTION`	A generator was called with `fusion.python()`: use `fusion.stream()`.
`CANCELLED`	The stream was cancelled by its consumer.
`WORKER_CRASHED`	The worker died during the call. The call is not retried, because it may have had side effects, unless it was declared idempotent. The worker is replaced.
`QUEUE_FULL`	The bounded queue is full: backpressure instead of unbounded memory growth.
`NOT_FOUND`	Unknown module or function (or not exposed).
`NOT_SERIALIZABLE` / `ENCODE_ERROR`	The result / the arguments are not valid JSON.
`SPAWN_FAILED` / `START_FAILED` / `START_TIMEOUT`	The interpreter or the worker was not found, cannot start, or did not become ready in `startTimeout`. `err.hint` says how to install a missing library.
`PROTOCOL_MISMATCH`	The worker speaks another version of the protocol (a Rust or Go worker built with another version of the crate or the module).
`WASM_TRAP` / `WASM_EXIT`	A WebAssembly module trapped (an out of bounds access, `unreachable`) or ended its own process while it ran the call. The call fails, and the module starts again with a new instance.
`INVALID_CONFIG` / `ALREADY_STARTED`	An option is wrong, or `configure()` was called after the pool started.
`CLOSED`	The pool was shut down while the call was pending.

Cancellation#

async def functions are cancelled for real (asyncio cancellation). Synchronous functions cannot be interrupted: after a timeout the caller is rejected immediately, and if the function does not finish within cancelGrace (2 s) the worker is killed and replaced.

Rust workers cancel cooperatively: the function looks at call.is_cancelled() from time to time and returns. One that never looks is killed after cancelGrace, like a synchronous function in Python.

Go workers do the same with call.Cancelled(), and with call.Context(), a context.Context that is cancelled at the same time and that can be passed to whatever the function calls.