server: don't panic when a finished compile has neither code nor signal

[jetm]

Part of #2775 (splitting the OpenEmbedded/Yocto sccache-dist work into focused
PRs; replaces the closed #2750).

Problem

get_signal did status.signal().expect("must have signal"), assuming the Unix
invariant that an ExitStatus with no exit code was terminated by a signal. That
does not always hold: an ExitStatus reconstructed for a distributed compile (or
an abnormal wait status such as WIFSTOPPED) can report neither a code nor a
signal. When that happened the expect() panicked the compile task, which the
server surfaced as a misleading "Failed to bind socket" and, under load,
repeatedly fell back to local compilation. Found under a Yocto/BitBake host-mode
distributed build.

Approach

Return Option<i32> from get_signal and assign it straight into res.signal,
so a compile that reports neither code nor signal leaves res.signal unset
instead of crashing the in-flight task. The Windows arm returns None rather
than panicking; ExitStatus::code() is always Some there, so the signal branch
is never reached anyway.

Test

Adds a unit test covering a real terminating signal (SIGKILL) and the
neither-code-nor-signal case (WIFSTOPPED via from_raw), which previously
panicked.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 52.63158% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.44%. Comparing base (7cd4f2c) to head (32fc12f).

Files with missing lines	Patch %	Lines
src/server.rs	52.63%	9 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (7cd4f2c) and HEAD (32fc12f). Click for more details.

HEAD has 5 uploads less than BASE

Flag	BASE (7cd4f2c)	HEAD (32fc12f)
	13	8

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2776      +/-   ##
==========================================
- Coverage   70.90%   62.44%   -8.47%     
==========================================
  Files          71       71              
  Lines       40393    35911    -4482     
==========================================
- Hits        28642    22425    -6217     
- Misses      11751    13486    +1735     

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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.

---

_{Comparing jetm:dist-server-signalless-panic (32fc12f) 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. ↩