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state: preserve previous primary when all HA advertisers unhealthy

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electPrimaryRoutes' all-unhealthy fallback picked candidates[0]
(lowest NodeID) regardless of who was prev. Under cable-pull
semantics IsOnline lags reality (long-poll TCP half-open), so
both routers stay in candidates and both go Unhealthy via the
prober — the fallback then churned primary to a node that was
itself unreachable.

Prefer prev when still in candidates; fall through to
candidates[0] only when prev is gone. Anti-blackhole holds.

Update the property test reference model and split the unit
test into existence (KeepsAPrimary) and identity
(PreservesPrevious) cases.

Fixes #3203
This commit is contained in:
Kristoffer Dalby 2026-04-29 13:34:51 +00:00
parent 27c9113af8
commit 3d5c0af4e7
3 changed files with 49 additions and 13 deletions
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16
hscontrol/state/node_store.go
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@ -620,9 +620,12 @@ func snapshotFromNodes(
// electPrimaryRoutes picks the primary advertiser for each non-exit
// prefix. The previous primary is preserved when it is still online
// and healthy (anti-flap); otherwise the lowest-NodeID healthy
// advertiser wins, falling back to the lowest-NodeID candidate when
// every advertiser is unhealthy so peers see *some* primary instead
// of none.
// advertiser wins. When every advertiser is unhealthy the previous
// primary is preserved if still a candidate, falling back to the
// lowest-NodeID candidate so peers see *some* primary instead of
// none. Anti-flap in the all-unhealthy case matters under cable-pull
// where IsOnline lags reality and a naive lowest-ID fallback churns
// primaries to a node that is itself unreachable (issue #3203).
func electPrimaryRoutes(
nodes map[types.NodeID]types.Node,
prev map[netip.Prefix]types.NodeID,
@ -675,7 +678,12 @@ func electPrimaryRoutes(
}
if !found && len(candidates) >= 1 {
selected = candidates[0]
if cur, ok := prev[prefix]; ok && slices.Contains(candidates, cur) {
selected = cur
} else {
selected = candidates[0]
}
found = true
}