🐛 manager: suppress expected leader-election-lost error on graceful shutdown

[liketosweep]

Fixes #3535

What the Issue Was

On graceful shutdown with LeaderElection enabled, cancelling the
leader election context triggers OnStoppedLeading, which pushes
errLeaderElectionLost onto errChan. The drain goroutine in
engageStopProcedure only filtered context.Canceled, so this
expected error was logged as unexpected:

level=ERROR msg="error received after stop sequence was engaged" err="leader election lost"

When combined with t.Output() in tests, this also causes panics or
data races as the manager writes to the test log after the test exits.

Solution

• Added a package-level errLeaderElectionLost sentinel for reliable
  errors.Is matching.
• Used the sentinel in OnStoppedLeading instead of an anonymous
  errors.New.
• Added the sentinel to the engageStopProcedure drain filter
  alongside context.Canceled.
  Genuine mid-run leader election loss (before stop is engaged) still
  propagates to Start's caller unchanged.

Changes

• pkg/manager/internal.go: sentinel + filter fix.
• pkg/manager/manager_test.go: regression test for graceful shutdown.

Testing

$ go test sigs.k8s.io/controller-runtime/pkg/manager -v \
  -run "should not log an error when leader election is lost during graceful shutdown"
--- PASS
PASS

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[alvaroaleman]

This means we will not emit an error anymore even if we lost leader election for a reason other than having stopped the binary which would be wrong?

[liketosweep]

The two errChan readers are sequential, not concurrent. engageStopProcedure is called via defer in Start() (line 386), so this drain goroutine only starts after Start()'s select loop has already exited consuming and returning any mid-run errLeaderElectionLost to the caller. Any instance seen here was caused by our own internalCancel(). I've added a comment in the latest commit to make this explicit.

[sbueringer]

I think it was a valid point on the issue that this Go routine should be completed before we return from engageStopProcedure.

Otherwise if we get any other error here and then log to cm.logger we would have the same problem again.

[sbueringer]

/ok-to-test