fix: rollout controller cannot shrink bindings to zero when none are ready

[Copilot]

When a placement's target is scaled to 0, stale bindings are never removed if no bindings have reached a ready state. calculateMaxToRemove computes maxNumberToRemove = len(readyBindings) - len(canBeUnavailableBindings) - minAvailableNumber, which is ≤ 0 when no bindings are ready — blocking all removals unconditionally.

Changes

• pkg/controllers/rollout/controller.go: Add a scale-to-zero fast path at the top of determineBindingsToUpdate. When targetNumber == 0, all removeCandidates, updateCandidates, and applyFailedUpdateCandidates are returned immediately, bypassing rolling update constraints (there is no minimum availability to preserve when targeting zero).

if targetNumber == 0 {
    toBeUpdatedBindingList := append(removeCandidates, updateCandidates...)
    toBeUpdatedBindingList = append(toBeUpdatedBindingList, applyFailedUpdateCandidates...)
    return toBeUpdatedBindingList, nil
}

• pkg/controllers/rollout/controller_test.go: Add TestDetermineBindingsToUpdate with three cases:
  • targetNumber=0, no ready bindings → all candidates removed
  • targetNumber=0, some ready bindings + apply-failed candidates → all candidates removed
  • targetNumber>0, no ready bindings → rolling update constraints preserved (existing behavior)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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