fix: apply fallback in polling loop to enable scaling from zero

[rickbrouwer]

When a ScaledObject is scaled to zero and the scaler encounters an error, the fallback mechanism is only triggered during HPA metric requests via GetScaledObjectMetrics, but not during the polling loop in getScaledObjectState. This caused the ScaledObject to remain at zero replicas indefinitely when errors occurred, as the polling loop would continuously fail without activating the fallback, preventing HPA from ever being consulted.

Fallback now works in both, HPA requests and polling interval checks.

The only question I have is whether it's by design that it doesn't switch to Fallback with 0 replicas, but I can't think of that myself. Please consider this when considering whether this was a deliberate choice. If not, I believe this PR resolves the issue.

Some extra new info;

It turned out to be a more change than I anticipated. I initially thought I'd add a simple fallback to getScalerState, but it turned out that scalingModifiers started having issues.
Therefore, I had to make some further adjustments to ensure the fallback worked properly from all positions.

Checklist

• I have verified that my change is according to the deprecations & breaking changes policy
• Tests have been added
• Changelog has been updated and is aligned with our changelog requirements
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes #7239

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[rickbrouwer]

/run-e2e internals
Update: You can check the progress here

[rickbrouwer]

/run-e2e internals
Update: You can check the progress here

[rickbrouwer]

/run-e2e internals
Update: You can check the progress here

[rickbrouwer]

/run-e2e internals
Update: You can check the progress here

[rickbrouwer]

/run-e2e internals
Update: You can check the progress here

[rickbrouwer]

/run-e2e internals
Update: You can check the progress here

[rickbrouwer]

/run-e2e internals
Update: You can check the progress here

[rickbrouwer]

/run-e2e internals
Update: You can check the progress here

[rickbrouwer]

/run-e2e internals
Update: You can check the progress here

[Copilot]

Pull request overview

This PR addresses a critical bug where ScaledObjects remained at zero replicas indefinitely when scalers encountered errors, because fallback was only triggered during HPA metric requests but not during the polling loop. The fix ensures fallback works consistently in both paths, enabling proper scaling from zero even when errors occur.

• Introduced processMetricsWithFallback helper function to consolidate fallback logic across HPA requests and polling interval checks
• Extended scalerState struct to track fallback activation state and fallback metrics
• Modified getScalerState to treat fallback-active scalers as active, enabling scale-up from zero replicas
• Added comprehensive test coverage for scaling from zero with fallback

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
pkg/scaling/scale_handler.go	Adds processMetricsWithFallback helper function and integrates fallback logic into both GetScaledObjectMetrics (HPA path) and getScalerState (polling path); moves scalerState struct definition earlier and extends it with fallback fields; ensures fallback-active scalers are considered active to enable scaling from zero
tests/internals/fallback/fallback.go	Adds TestFallbackFromZero test case to verify that fallback correctly triggers and scales from 0 to fallback replica count when metrics server is unavailable; integrates new test into main test suite
CHANGELOG.md	Documents the fix for applying fallback in polling loop to enable scaling from zero, referencing issue #7239

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[rickbrouwer]

/run-e2e internals
Update: You can check the progress here

[rickbrouwer]

/run-e2e internals
Update: You can check the progress here

[rickbrouwer]

/run-e2e internals
Update: You can check the progress here

[rickbrouwer]

/run-e2e internals
Update: You can check the progress here

[rickbrouwer]

/run-e2e internals
Update: You can check the progress here

[rickbrouwer]

/run-e2e internals
Update: You can check the progress here

[rickbrouwer]

/run-e2e internals
Update: You can check the progress here

[JorTurFer]

/run-e2e internals
Update: You can check the progress here

[rickbrouwer]

/run-e2e internals
Update: You can check the progress here

[rickbrouwer]

/run-e2e internals
Update: You can check the progress here