Expose remote cache read errors and fail on server disconnects

[seungpyoson]

Summary

• in the direct single-cache daemon path, surface non-NotFound remote read failures through the existing cache-error statistics path instead of counting them as ordinary misses
• make post-CompileStarted response I/O failures follow the documented default-fatal policy while preserving SCCACHE_IGNORE_SERVER_IO_ERROR=1

Deliberately unchanged

• startup rate-limit tolerance
• automatic read-only downgrade when a configured backend cannot write
• explicit read-only and public-cache configurations
• client-side IPC and multi-level cache fallback policies

Scope

One commit changes src/cache/cache.rs and src/commands.rs only. There are no documentation, protocol, cache-format, dependency, disk-cache, multi-level-cache, or client-side-storage changes. Existing documentation already describes the server-I/O policy.

Validation

Focused regression tests cover unexpected direct remote read errors, default-fatal post-start disconnects, and the existing opt-in local fallback. Upstream CI and maintainer review remain required before merge.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.87234% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 70.94%. Comparing base (7cd4f2c) to head (a35fe90).

Files with missing lines	Patch %	Lines
src/commands.rs	96.15%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2779      +/-   ##
==========================================
+ Coverage   70.90%   70.94%   +0.03%     
==========================================
  Files          71       71              
  Lines       40393    40414      +21     
==========================================
+ Hits        28642    28673      +31     
+ Misses      11751    11741      -10     

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[codspeed-hq]

Merging this PR will degrade performance by 45.99%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 10 regressed benchmarks
✅ 48 untouched benchmarks
⏩ 6 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Memory	lru_sequential_scan_pattern	128 B	260 B	-50.77%
❌	Memory	lru_hotcold_access_pattern	128 B	260 B	-50.77%
❌	Memory	hash_multiple_files	128 B	260 B	-50.77%
❌	Memory	time_macro_finder_chunked	128 B	260 B	-50.77%
❌	Memory	lru_cache_get_hit	128 B	260 B	-50.77%
❌	Memory	hash_large_data	192 B	324 B	-40.74%
❌	Memory	hash_header_file	192 B	324 B	-40.74%
❌	Memory	cache_key_generation	192 B	324 B	-40.74%
❌	Memory	time_macro_finder_with_macros	192 B	324 B	-40.74%
❌	Memory	time_macro_finder_no_macros	192 B	324 B	-40.74%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing seungpyoson:agent/fail-closed-cache-errors (a35fe90) with main (7cd4f2c)}

Footnotes

1. 6 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩