electPrimaryRoutes' all-unhealthy fallback picked candidates[0] when
the previous primary was no longer a candidate. The Phase-5
simultaneous dual-disconnect path in TestHASubnetRouterFailoverDocker
Disconnect hits this asymmetrically: a batched probe cycle marks both
routers unhealthy with prev=r2 preserved, then the grace-period
Disconnect for r2 drops it from candidates. With prev gone and the
remaining r1 still carrying its Unhealthy bit, the fallback pointed
peers at the cable-pulled r1 — flapping primary to an unreachable
node and tripping requirePrimaryStable.
Leave the prefix unmapped when prev is gone and every candidate is
unhealthy. Peers see no advertiser instead of an unreachable one,
which is honest: the next probe cycle re-evaluates and picks
whichever node responds. The property-test model that mirrored the
old behaviour is updated to match.
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