Record compiler hash in build fingerprints

[cpunion]

Summary

• compute a deterministic hash for development compiler builds based on binary metadata
• add LLGO compiler hash into the manifest env so cache entries invalidate when the compiler changes

Testing

• go test ./...

[gemini-code-assist]

Summary of Changes

Hello @cpunion, 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 enhances the build system by incorporating a unique hash of the llgo compiler into the build fingerprints. This change ensures that development builds can reliably detect when the compiler itself has been updated, leading to automatic cache invalidation and preventing the use of stale cached artifacts. The primary goal is to improve build consistency and reliability by tying cached outputs directly to the specific compiler version used.

Highlights

• Compiler Hash Integration: Introduced a mechanism to compute and record a deterministic hash for the llgo compiler in build fingerprints.
• Cache Invalidation: The LLGO_COMPILER_HASH is now added to the manifest environment, ensuring that build cache entries are automatically invalidated when the compiler binary changes.
• Deterministic Hashing: For development builds, the compiler hash is derived from the executable's metadata (modification time and file size), providing a consistent way to detect compiler changes.

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[gemini-code-assist]

Code Review

This pull request introduces a mechanism to generate a hash for development compiler builds, which is used to invalidate cache entries when the compiler is updated. The implementation is sound and primarily involves hashing the compiler executable's metadata (modification time and size). The changes are well-contained within the build fingerprinting and environment versioning logic. I have a couple of suggestions to enhance maintainability and error reporting in internal/env/version.go.

[xgopilot]

Code Review Summary

Overall Assessment: Well-implemented feature with minor improvements needed

This PR successfully adds compiler hash tracking to build fingerprints for proper cache invalidation during development. The implementation is clean, secure, and performant.

Key Strengths

• Performance: Negligible overhead with smart caching and early exit for release builds
• Security: No vulnerabilities; uses SHA-256 and system-controlled paths
• Integration: Cleanly integrates into existing manifest/fingerprint system

Required Improvements

1. Missing unit tests for hashMetadata() and compilerHashFromPath() - CLAUDE.md requires tests for all features
2. Incomplete documentation for internal functions and initialization logic

Recommendations

• Add unit tests covering normal cases, edge cases (size=0), and error conditions
• Document the internal functions to clarify metadata hashing vs content hashing
• Improve error message consistency in the init function

See inline comments for specific suggestions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.03%. Comparing base (c844c73) to head (046c0d9).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1516      +/-   ##
==========================================
+ Coverage   90.78%   91.03%   +0.25%     
==========================================
  Files          45       45              
  Lines       11970    11996      +26     
==========================================
+ Hits        10867    10921      +54     
+ Misses        929      899      -30     
- Partials      174      176       +2     

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