Cooperative Async Task Group

Maintain a bounded submission queue and active child set under one event loop. A first failure requests cancellation and skips unstarted children, while join closes submissions and waits for all started children to settle before returning collected outcomes.

1. Validate concurrency and queue bounds. submit accepts a task factory only while OPEN and returns an outcome handle; overload and closed submission are explicit errors.
2. Drain up to the concurrency limit, wrapping synchronous factory throws and immediately observing each returned promise. Store success or failure outcomes and remove the child from the active set in a finalizer.
3. On first failure or explicit cancellation, set the cancellation flag before invoking abort callbacks, skip queued factories and request cooperative cancellation of active tasks. Capture later failures as additional outcomes.
4. join is idempotent and changes OPEN to CLOSING. Continue normal queued work if not cancelled, and settle the shared join promise only when no active or queued work remains. A child ignoring cancellation can keep it pending; diagnostics must not misreport termination.

• Running child count never exceeds the concurrency bound.
• A synchronous factory throw cancels siblings and skips queued tasks.
• join remains pending until an abort-ignoring child actually settles.
• Concurrent repeated join calls share one result and all task failures remain observed.

Trade-offs to explain

• Cooperative cancellation preserves resource ownership but cannot stop uncooperative work.
• Waiting for all children can delay error reporting; intermediate failure notifications can supplement join.
• Bounded queues prevent runaway memory at the cost of explicit overload handling.