fix(engine): Prevent drain starvation

[clabby]

Overview

Alternative version of #1851, which keeps the BinaryHeap priority queue and achieves the same goal of not starving the engine actor control loop.

[Copilot]

Pull Request Overview

This PR introduces a mechanism to prevent drain from starving the engine’s control loop by waiting for new tasks before invoking drain. Key changes include:

• New TaskWaiter future and Engine::wait_for_tasks() to await incoming tasks.
• Updated NodeActor loop in engine.rs to use wait_for_tasks() instead of unconditional drain.
• Exposed TaskWaiter in public APIs and adjusted re-exports.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
crates/node/service/src/actors/engine.rs	Replaced the final unconditional drain() branch with a wait_for_tasks() select branch, moving drain into that branch.
crates/node/engine/src/task_queue/mod.rs	Re-exported TaskWaiter alongside existing types.
crates/node/engine/src/task_queue/core.rs	Added TaskWaiter type, Engine::wait_for_tasks() method, and its Future implementation.
crates/node/engine/src/lib.rs	Updated top-level exports to include TaskWaiter.

Comments suppressed due to low confidence (2)

crates/node/engine/src/task_queue/core.rs:155

• The doc comment states it resolves after “more than one task” arrives, but the implementation actually fires when any task exists. Update the comment to accurately reflect that it resolves when the queue becomes non-empty.

/// A [`Future`] that resolves once more than one task has arrived in the inner [`Engine`] task queue.

crates/node/engine/src/task_queue/core.rs:128

• There are no tests for TaskWaiter to verify its wake behavior. Consider adding unit tests that enqueue tasks and assert TaskWaiter completes only when expected.

pub const fn wait_for_tasks(&self) -> TaskWaiter<'_> {

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.7%. Comparing base (39eb2b5) to head (6297d6c).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

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