pimd: fix elected BSR not updating when priority changes

[enkechen-panw]

When the elected BSR changes its own priority, the change was silently
ignored. This happened because pim_cand_bsr_trigger() returned early
when current_bsr matched our own address, without checking if the
priority had changed.

Fix by updating current_bsr_prio and regenerating BSM immediately when
the elected BSR's priority changes.

[greptile-apps]

Greptile Summary

This PR fixes a bug where an elected BSR silently ignored its own priority increases. The root cause was pim_cand_bsr_trigger() returning early when current_bsr == run_addr without checking if the priority had diverged from cand_bsr_prio.

• Core fix (pim_bsm.c): When the elected BSR's address matches our own and state is BSR_ELECTED, the code now detects a changed cand_bsr_prio, syncs current_bsr_prio, and calls pim_bsm_generate() to rebuild and resend fragments with the updated priority — using the correct regeneration path that frees stale fragments rather than replaying them.
• Test (test_pim_cand_rp_bsr.py): A new test step raises r2's priority from 250 → 255 and validates the change on both r2's own state and r1's received BSM, confirming the on-wire packet is properly regenerated; test_pim_bsr_election_fallback_r2 is updated to reflect the new baseline priority of 255.

Confidence Score: 5/5

The change is safe to merge: it adds a narrowly scoped guard inside pim_cand_bsr_trigger that only fires when the router is BSR_ELECTED and the two priority fields have diverged, then delegates to the existing pim_bsm_generate() path which has its own assertf(state == BSR_ELECTED) guard.

The fix is minimal and correct. It updates current_bsr_prio before calling pim_bsm_generate(), so the rebuilt fragment carries the new priority. The priority-decrease path is still handled by the early-return in pim_cand_bsr_apply and is unaffected by this change. The test validates the on-wire behaviour by checking that a downstream neighbour (r1) also reflects the updated priority.

No files require special attention.

Important Files Changed

Filename	Overview
pimd/pim_bsm.c	Adds a priority-change guard inside pim_cand_bsr_trigger; correctly updates current_bsr_prio and calls pim_bsm_generate() (which rebuilds fragments with the new priority and then calls pim_bsm_changed()) only when state == BSR_ELECTED and the priority has actually diverged.
tests/topotests/pim_cand_rp_bsr/test_pim_cand_rp_bsr.py	Adds test_pim_elected_bsr_priority_change which raises r2's priority to 255 and verifies both r2's state and r1's received BSM reflect the new priority; updates the priority baseline in test_pim_bsr_election_fallback_r2.

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
    participant Op as Operator
    participant BSR as r2 (BSR_ELECTED)
    participant r1 as r1 (Neighbour)

    Note over BSR: current_bsr_prio = 250, cand_bsr_prio = 250

    Op->>BSR: configure priority 255
    BSR->>BSR: pim_cand_bsr_apply()
    Note over BSR: current_bsr_prio(250) NOT > cand_bsr_prio(255), does NOT early-return

    BSR->>BSR: pim_cand_bsr_trigger()
    Note over BSR: current_bsr == run_addr, state == BSR_ELECTED, priorities differ

    BSR->>BSR: "current_bsr_prio = 255"
    BSR->>BSR: "pim_bsm_generate() - free stale frags, rebuild bsr_prio=255, pim_bsm_changed()"

    BSR-->>r1: "BSM (bsr_prio=255)"
    Note over r1: Updates current_bsr_prio = 255

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sequenceDiagram
    participant Op as Operator
    participant BSR as r2 (BSR_ELECTED)
    participant r1 as r1 (Neighbour)

    Note over BSR: current_bsr_prio = 250, cand_bsr_prio = 250

    Op->>BSR: configure priority 255
    BSR->>BSR: pim_cand_bsr_apply()
    Note over BSR: current_bsr_prio(250) NOT > cand_bsr_prio(255), does NOT early-return

    BSR->>BSR: pim_cand_bsr_trigger()
    Note over BSR: current_bsr == run_addr, state == BSR_ELECTED, priorities differ

    BSR->>BSR: "current_bsr_prio = 255"
    BSR->>BSR: "pim_bsm_generate() - free stale frags, rebuild bsr_prio=255, pim_bsm_changed()"

    BSR-->>r1: "BSM (bsr_prio=255)"
    Note over r1: Updates current_bsr_prio = 255

Loading

_{Reviews (4): Last reviewed commit: "tests: pim_cand_rp_bsr: add elected BSR ..." | Re-trigger Greptile}

[enkechen-panw]

@greptileai review

[enkechen-panw]

@greptileai review

[Jafaral]

@merifyio backport stable/10.7 stable/10.6

[enkechen-panw]

Hi, @Jafaral: Did you try to trigger backport? There was a typo in the command: "@merifyio" vs "@Mergifyio"?

[Jafaral]

@Mergifyio backport stable/10.7 stable/10.6

[mergify]

backport stable/10.7 stable/10.6

✅ Backports have been created

Details

• #22477 pimd: fix elected BSR not updating when priority changes (backport #22465) has been created for branch stable/10.7
• #22478 pimd: fix elected BSR not updating when priority changes (backport #22465) has been created for branch stable/10.6

[Jafaral]

Hi, @Jafaral: Did you try to trigger backport? There was a typo in the command: "@merifyio" vs "@Mergifyio"?

Thanks! I fixed it.