release-2.1: sql,rpc/nodedialer: improve distsql node health checks #31014
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`Dialer.DialInternalClient` does not check the circuit breaker but blindly attempts a connection and can succeed, leaving the system in a state where there is a healthy connection to a node, but the circuit breaker used for dialing is open. DistSQL checks for connection health when scheduling processors, but the connection health check does not examine the breaker. So DistSQL will proceed to schedule a processor on a node but then be unable to use the connection to that node because `Dialer.Dial` will return with a `breaker open` error. The code contains a TODO to reconcile the handling of circuit breakers in the various `Dialer` methods, but changing the handling is risky in the short term. As a stop-gap, we reset the breaker after a connection is successfully opened. Fixes cockroachdb#29149 Release note: None
Change `DistSQLPlanner.checkNodeHealth` so that it uses `nodedialer.Dialer.ConnHealth` instead of `rpc.Context.ConnHealth`. The former is the right method to be calling to check a node's connection health. Refactor `DistSQLPlanner.checkNodeHealth` into a `distSQLNodeHealth` struct. This removed the need for `DistSQLPlannerTestingKnobs`. Enhance `nodedialer.Dialer.ConnHealth` to mark connections as unhealthy if the circuit breaker is open. This prevents DistSQL from planning processors on such nodes. Release note: None
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LGTM
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Backport 2/2 commits from #30987.
/cc @cockroachdb/release
Improve distsql node health checks so that the presence of an open
circuit breaker is consider. Previously it was possible for distsql to
plan a processor on a node with an open circuit breaker which ensured an
"unable to dial" error when the plan was run.
Fixes #29149
Fixes #28704
Release note: None