[Bug] use fallback value to prevent divide zero error

[zyfy29]

Pull Request Description

Add unit test for leastExpectedLatencyRouter, with a fix of the bug which my unit test addresses

The code used AvgPromptToksPerReq as divisor without checking is it am empty value. This produces a math.MaxFloat64 causing the router failure and a random pod will be selected.

Related Issues

also a part of #1485

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[gemini-code-assist]

Summary of Changes

Hello @zyfy29, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical bug in the leastExpectedLatencyRouter by preventing division-by-zero errors that could lead to router failures and incorrect pod selection. It introduces robust handling for cases where average prompt or generation token metrics are zero, using sensible fallback values. Additionally, a comprehensive suite of unit tests has been added to validate the router's behavior under various conditions, including those that previously caused the bug.

Highlights

• Bug Fix for Division by Zero: Corrected an issue in leastExpectedLatencyRouter where division by zero occurred if AvgPromptToksPerReq or AvgGenerationToksPerReq metrics were zero, previously resulting in math.MaxFloat64 and router malfunction.
• Introduction of Fallback Values: Implemented a mechanism to use predefined guessPromptTokens and guessGenerationTokens as fallback values when the actual average token metrics are zero, ensuring stable latency calculations.
• Comprehensive Unit Testing: Added a new test file (least_latency_test.go) with extensive test cases for the leastExpectedLatencyRouter, covering scenarios like successful routing, no ready pods, multiple pods with similar latencies, and the newly fixed zero-metric conditions.

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[Copilot]

Pull Request Overview

This PR fixes a divide-by-zero bug in the least expected latency router by using fallback values when average token metrics are zero or empty. The fix prevents the router from returning math.MaxFloat64 values that cause routing failures.

• Adds comprehensive unit tests for the leastExpectedLatencyRouter covering various scenarios including zero token values
• Implements fallback logic to use guessPromptTokens and guessGenerationTokens when metric values are zero or unavailable
• Prevents divide-by-zero errors that previously caused router failures and random pod selection

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
pkg/plugins/gateway/algorithms/least_latency_test.go	Adds comprehensive unit tests covering normal operation, edge cases, and the divide-by-zero scenario
pkg/plugins/gateway/algorithms/least_latency.go	Implements fallback logic to prevent divide-by-zero errors when token metrics are zero

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[gemini-code-assist]

Code Review

This pull request effectively resolves a potential division-by-zero error within the leastExpectedLatencyRouter by implementing a fallback mechanism for token metrics. The change is well-implemented and correctly addresses the bug. Furthermore, the addition of a comprehensive unit test suite for this routing algorithm is a significant improvement, enhancing the robustness and reliability of the codebase. My review includes a minor suggestion aimed at improving code maintainability by reducing duplication.