pimd: fix BSR_PENDING timer being overwritten by BS liveness timer

[enkechen-panw]

This addresses the likely root cause of the flaky test pim_cand_rp_bsr.test_pim_bsr_priority_modify which had a 3.4 - 11% failure rate in the weekly topotest report.

The bs_timer is used for two different purposes:

1. During BSR_PENDING state: a ~5 second timer before becoming BSR_ELECTED (callback: pim_cand_bsr_pending_expire)
2. During other states: a 130 second BS liveness timer (callback: pim_on_bs_timer)

When a candidate BSR is in BSR_PENDING state and receives a BSM from another BSR, the pim_bs_timer_restart() call would overwrite the pending timer with the liveness timer. This prevented the BSR_PENDING timer from ever expiring, causing the candidate BSR to never become elected even when it had higher priority. Fix by skipping pim_bs_timer_restart() when in BSR_PENDING state.

Additionally, when pim_bsm_update() drops a router out of BSR_PENDING state (due to receiving a BSM from a different BSR), the bs_timer with the pim_cand_bsr_pending_expire callback was not cancelled. This could cause an assertion failure when the timer fired with state != BSR_PENDING. Fix by cancelling the bs_timer in pim_bsm_update() before leaving BSR_PENDING state.

This also fixes a related issue where pim_cand_bsr_apply() would return early if the address selection hadn't changed, preventing priority-only changes from triggering the BSR state machine re-evaluation. Fix by always calling pim_cand_bsr_trigger() regardless of address change.

[greptile-apps]

Greptile Summary

This PR fixes a set of interrelated bugs in the PIM Bootstrap Router (BSR) election state machine in pimd/pim_bsm.c that caused a flaky topology test. The core problem is that bs_timer serves three different roles depending on state — a pending-to-elected countdown, a periodic BSM generation trigger (elected), and a BS liveness timer (all other states) — and several code paths were mixing them up.

• BSR_PENDING timer protection (pim_bsm_process): The BS liveness timer is no longer restarted when in BSR_PENDING state, preventing it from overwriting the pending-expiry timer and blocking the candidate from ever becoming elected.
• Timer cleanup on state exit (pim_bsm_update): When a better BSR's message drops the router out of BSR_PENDING, the pending-expiry callback is now cancelled before the transition, and the liveness timer is explicitly started — closing both an assertion crash path and the timer gap flagged in earlier review threads.
• Priority-change coverage (pim_cand_bsr_apply): The early return on cand_addrsel_update returning false is removed so that priority-only changes (no address change) now correctly trigger the BSR state machine re-evaluation, with a new guard that safely handles the cases where the current BSR is already stronger than our candidacy.

Confidence Score: 5/5

Safe to merge. All three state-machine bugs are correctly addressed, including the previously flagged assertion crash on priority changes and the timer gap when transitioning out of BSR_PENDING.

The three independent fixes are each self-contained and correctly handle their respective edge cases. The guard added to pim_cand_bsr_apply prevents pim_cand_bsr_trigger assertions from being reachable in BSR_PENDING and BSR_ELECTED states. The pim_bsm_update change correctly cancels the pending-expiry callback and starts the liveness timer when exiting BSR_PENDING, so no timer gap remains.

No files require special attention.

Important Files Changed

Filename	Overview
pimd/pim_bsm.c	Three coordinated fixes to the BSR state machine: timer guard in pim_bsm_process, timer cancel+restart in pim_bsm_update for BSR_PENDING exit, and assertion-crash guard in pim_cand_bsr_apply for priority/address changes in BSR_PENDING and BSR_ELECTED states. All previously flagged assertion-crash and timer-gap concerns appear resolved.

_{Reviews (4): Last reviewed commit: "pimd: fix BSR_PENDING timer being overwr..." | Re-trigger Greptile}

[enkechen-panw]

@greptileai review

[enkechen-panw]

@greptileai review

[Jafaral]

@Mergifyio backport stable/10.7 stable/10.6

[mergify]

backport stable/10.7 stable/10.6

✅ Backports have been created

Details

• #22463 pimd: fix BSR_PENDING timer being overwritten by BS liveness timer (backport #22460) has been created for branch stable/10.7
• #22464 pimd: fix BSR_PENDING timer being overwritten by BS liveness timer (backport #22460) has been created for branch stable/10.6