Skip to main content

Blocking

hclient_rt

Trait Blocking 

Source 
pub trait Blocking {
    // Required method
    fn run<T, F>(&self, f: F) -> impl Future<Output = Result<T, Cancelled>>
       where T: Send + 'static,
             F: FnOnce() -> T + Send + 'static;
}
Expand description

A separate trait, not a method: getaddrinfo blocks, and wasm and embedded have no blocking pool at all. The absence of the capability must be a compile error, not an unimplemented!() in the runtime.

The one place in the whole project where we declare Send ourselves, and here it’s honest: both tokio::task::spawn_blocking and blocking::unblock require Send + 'static, and the Blocking capability doesn’t exist on wasm at all — there’s nothing for it to infect. The justification is amendment-C5 (docs/exceptions.md), an amendment separate from C1/C2: those two are about erasing auto-traits in dyn Trait on the Client -> Transport path, whereas here the bound is declared directly in the signature of a capability trait that simply doesn’t exist on wasm.

The bounds live in where, not in the generic parameter list fn run<T: Send + …>, so each one can carry its own send-bound-exception marker on its own line: the CI no-declared-send job matches bound declarations line by line, and a single shared comment after the generic list wouldn’t cover it.

Two distinct failure modes of f are not conflated into one channel:

  • A panic in f is a bug in the calling code. It must be re-raised as a panic (std::panic::resume_unwind, with the original payload), not quietly turned into a value that can be ?-propagated — otherwise the implementation hides a defect in the caller’s code behind a Result.
  • The background thread pool going away (for example, the runtime shutting down while a task is still queued and hasn’t started running) is not a bug in the calling code, but an ordinary runtime lifecycle event. The implementation must return Cancelled, not panic: a library panicking on a normal (if rare) runtime-shutdown scenario would contradict the rest of the project (“no silent no-ops… typed error, never a discarded value” — the same principle applied here, just to failure instead of success).

Required Methods§

Source

fn run<T, F>(&self, f: F) -> impl Future<Output = Result<T, Cancelled>>
where T: Send + 'static, F: FnOnce() -> T + Send + 'static,

Dyn Compatibility§

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety".

Implementors§