Left (Error "Timeout") when using Aff.timeout in nested race conditions

Cancellation Behavior in Aff

In PureScript's Aff, cancellation is not an immediate asynchronous interrupt that can slice through synchronous JavaScript execution. Because Aff is built upon the JavaScript event loop, a fiber can only be interrupted at asynchronous boundaries (yield points). Synchronous, effect-free sections of code (pure computations) will execute to completion before the fiber checks for an interruption signal.

Finalizers and Race Conditions

Finalizers (registered via bracket or finally) are guaranteed to run when a fiber is cancelled, including when it is the losing branch of a race. When Aff.timeout triggers, it interrupts the underlying fiber; this interruption propagates through the fiber tree, triggering all registered finalizers in the order of their nesting (innermost to outermost).

Nested Timeout Guarantees

When nesting Aff.timeout calls, the behavior is determined by which timer expires first:

• Inner Timeout Fires First: The inner fiber is interrupted, returning Left (Error "Timeout"). The outer timeout continues to run, but it receives the result of the inner operation immediately.
• Outer Timeout Fires First: The outer fiber is interrupted. This sends a cancellation signal down to all child fibers, including the inner timeout. The inner fiber is cancelled, its finalizers run, and the outer operation returns Left (Error "Timeout").

Verification Steps

To verify resource safety in your specific nested structure, you can use a simple logging bracket:

-- Use a bracket to verify finalizer execution during race/timeout
bracket
  (log "Acquiring Resource")
  (\_ -> log "Releasing Resource")
  (\_ -> Aff.sleep (1000 * milliseconds))

If "Releasing Resource" appears regardless of whether the inner or outer timeout wins, your resource safety is intact.

Diagnostic Requirement

To provide a more precise answer regarding your specific resource leak or race, please specify if you are using any uninterruptible blocks or FFI calls that block the JavaScript main thread, as these can prevent Aff.timeout from triggering.