[Zen2] Elect freshest master in upgrade #37122
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DaveCTurner
added
>bug
v7.0.0
:Distributed Coordination/Cluster Coordination
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Today when electing a master in Zen2 we use the cluster state version to
determine whether a node has a fresh-enough cluster state to become master.
However the cluster state version is not a reliable measure of freshness in the
Zen1 world; furthermore in 6.x the cluster state version is not persisted. This
means that when upgrading from 6.x via a full cluster restart a cluster state
update may be lost if a stale master wins the initial election.
This change fixes this by using the metadata version as a measure of freshness
when in term 0, since this is persisted in 6.x and does more reliably indicate
the freshness of nodes.
It also makes changes parallel to elastic/elasticsearch-formal-models#40 to
support situations in which nodes accept cluster state versions in term 0: this
does not happen in a pure Zen2 cluster, but can happen in mixed clusters and
during upgrades.