Fix scale-to-zero bug when spec.replicas is nil

[otrosien]

This fixes a critical regression introduced in PR #511 (commit c53f570) where an EDS could scale to 0 replicas despite having minReplicas set.

Bug Scenario:

1. kubectl patch clears spec.replicas to nil
2. edsReplicas() returns 0 (per c53f570 change)
3. scaleEDS() writes eds.Spec.Replicas = &0 at line 935
4. Autoscaler returns no-op (e.g., excludeSystemIndices filters all indices)
5. No-op means scalingOperation.NodeReplicas = nil
6. Line 945 condition fails, so line 959 doesn't execute
7. Result: spec.replicas stays at 0, violating minReplicas

Root Cause:
The c53f570 change made edsReplicas() return 0 for autoscaling-enabled EDS with nil replicas, intending to let the autoscaler calculate the initial value. However, when the autoscaler returns a no-op (nil NodeReplicas), there's no fallback to prevent spec.replicas from being set to 0.

Fix:
Add defensive check in scaleEDS() before writing to spec.replicas:

• When currentReplicas == 0 and minReplicas > 0:
  • First try status.replicas (reflects actual StatefulSet state)
  • Fall back to minReplicas for new/uninitialized EDS
• This prevents writing 0 when it would violate bounds

Test Coverage:

• Added TestScaleToZeroPrevention with 3 test cases
• Validates the defensive logic for all scenarios
• All existing tests continue to pass

[mchernyavskaya]

1. Does this mean that intentional scaling to 0 (like we do with the deployments directly sometimes before we want to drop them) would not be possible?
2. Does this mean that es-operator scaling would not apply at all to system EDSes (those that only hold system indices)? So they would never scale?

[otrosien]

We don't support scaling down to 0 pods, and shouldn't support it without additional safeguarding considerations. See this ticket for a some old discussion: #68

Regarding the second question, system indices can/should be treated special in most cases, see spec.excludeSystemIndices. Beware that hiding system indices for an EDS that is meant for holding these exclusively will make ES-Operator assume the EDS is empty, so here the situation is different.

[mikkeloscar]

👍

[adrobisch]

👍