[mjcarroll]

Description

Fixes the remaining flake in TestBusyWaiting seen in nightly_linux_repeated:

test_executors_busy_waiting.cpp:109
Expected: (waitable->get_is_ready_call_count()) < (10u * this->waitable->get_count()), actual: 2507 vs 20

This is the same symptom as #3154. #3155 fixed a race in the test, but this one is a real busy wait in the MultiThreadedExecutor.

Root cause

When thread A takes an entity from a MutuallyExclusive callback group, can_be_taken_from becomes false. If the entity (or another one in the group) becomes ready again while A is still executing, the idle thread B:

1. wakes from wait_for_work(),
2. skips the entity in get_next_ready_executable() because its group is busy,
3. calls wait_for_work() again with the same collection, since nothing set entities_need_rebuild_.

For level-triggered entities (a subscription with queued data, or TestWaitable, which re-triggers its guard condition in add_to_wait_set() while trigger_count_ > 0), the wait returns immediately and B spins until A finishes and the interrupt_guard_condition_ triggers a rebuild.

build_entities_collection() already excludes groups that can't be taken from, but that only helps if a rebuild happens while the group is busy. In test_spin that depends on whether the notify waitable fired in the same wait that picked up the first execution, which is a race. The busy wait then lasts as long as the first callback, which waits on the main thread and is long on a loaded CI machine.

Fix

In Executor::get_next_ready_executable(), set entities_need_rebuild_ whenever a ready entity is skipped because its callback group is gone or can't be taken from. The next wait then excludes the busy group, and the existing post-execution interrupt_guard_condition_ trigger in MultiThreadedExecutor::run() adds it back once it is released. The extra rebuild only happens in the case that would otherwise busy wait

[github-actions]

ABI Compliance Check

✅ Verdict: compatible

Library	Verdict	Summary
libcomponent_manager.so	✅ compatible	No ABI changes detected.
librclcpp.so	✅ compatible	No ABI changes detected.
librclcpp_action.so	✅ compatible	No ABI changes detected.
librclcpp_lifecycle.so	✅ compatible	No ABI changes detected.

✅ libcomponent_manager.so — full abidiff report

Compared:

• Base: lib-base/libcomponent_manager.so
• Head: lib-pr/libcomponent_manager.so @ 08558e7

(empty report — no differences printed by abidiff)

✅ librclcpp.so — full abidiff report

Compared:

• Base: lib-base/librclcpp.so
• Head: lib-pr/librclcpp.so @ 08558e7

(empty report — no differences printed by abidiff)

✅ librclcpp_action.so — full abidiff report

Compared:

• Base: lib-base/librclcpp_action.so
• Head: lib-pr/librclcpp_action.so @ 08558e7

(empty report — no differences printed by abidiff)

✅ librclcpp_lifecycle.so — full abidiff report

Compared:

• Base: lib-base/librclcpp_lifecycle.so
• Head: lib-pr/librclcpp_lifecycle.so @ 08558e7

(empty report — no differences printed by abidiff)

_{Updated for commit 08558e7 · check run · suppressions: /home/runner/work/_temp/ros2-abi-suppressions.txt}