release-22.2: kv: campaign on rejected lease request when leader not live in node liveness

[nvanbenschoten]

Backport 2/2 commits from #87244.

/cc @cockroachdb/release

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Fixes #84655.
Related to #49220.

This commit extends the logic introduced in 8aa1c14 to simultaneously campaign for Raft leadership when rejecting a lease request on a Raft follower that observes that the current Raft leader is not live according to node liveness. These kinds of cases are often associated with asymmetric network partitions.

In such cases, the Raft leader will be unable to acquire an epoch-based lease until it heartbeats its liveness record. As a result, the range can only regain availability if a different replica acquires the lease. However, the protection added in 8aa1c14 prevents followers from acquiring leases to protect against a different form of unavailability.

After this commit, the followers will attempt to acquire Raft leadership when detecting such cases by campaigning. This allows these ranges to recover availability once Raft leadership moves off the partitioned Raft leader to one of the followers that can reach node liveness and can subsequently acquire the lease.

Campaigning for Raft leadership is safer than blindly allowing the lease request to be proposed (through a redirected proposal). This is because the follower may be arbitrarily far behind on its Raft log and acquiring the lease in such cases could cause unavailability (the kind we saw in #37906). By instead calling a Raft pre-vote election, the follower can determine whether it is behind on its log without risking disruption. If so, we don't want it to acquire the lease — one of the other followers that is caught up on its log can. If not, it will eventually become leader and can proceed with a future attempt to acquire the lease.

The commit adds a test that reproduces the failure mode described in #84655. It creates an asymmetric network partition scenario that looks like:

        [0]       raft leader / leaseholder
         ^
        / \
       /   \
      v     v
    [1]<--->[2]   raft followers
      ^     ^
       \   /
        \ /
         v
        [3]       liveness range

It then waits for the raft leader's lease to expire and demonstrates that one of the raft followers will now call a Raft election, which allows it to safely grab Raft leadership, acquire the lease, and recover availability. Without the change, the test failed.

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Release justification: None. Too risky for the stability period. Potential backport candidate after sufficient baking on master.

Release note (bug fix): A bug causing ranges to remain without a leaseholder in cases of asymmetric network partitions has been resolved.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[data-matt]

@erikgrinaker , is 22.1 possible? thanks

[erikgrinaker]

is 22.1 possible? thanks

See #95237.