[NUAAwqy]

When multiple clients automatically start sccache with the same private Unix socket, each child currently removes the socket before binding. Concurrent starters can unlink a live listener or race into File exists (os error 17).

This change serializes Unix socket probing, stale-socket cleanup, and bind with a persistent 0600 sibling lock. It reports a live listener as AddrInUse, reclaims only a current-user-owned stale socket, and leaves regular or uncertain paths intact. TCP and abstract Unix sockets are unchanged.

Closes #2906.

Validation on Linux against upstream main at d72ff0bb07ca3be71000e0922a3ef80b21d51403:

• Added a regression test for preserving a live listener, reclaiming its stale path, and creating a 0600 lock.
• cargo test --locked --lib --bins --tests: 583 passed, 0 failed, 1 ignored.
• The issue reproduction with 20 rounds of 4 concurrent clients: 0 failed clients after the fix.
• cargo clippy --locked --all-targets passes with the repository's existing removed/renamed-lint and async-trait compatibility exceptions; git diff --check passes.
• The local cargo fmt -- --check reports only the pre-existing start_server closure formatting difference from this checkout's rustfmt; the changed lines are formatted. Release build validation is unavailable here because the server's GCC 9.4 fails the aws-lc-sys compiler safety check.

Prepared with Codex assistance; no human review is claimed.

Follow-up: the repository rustfmt failure was reproduced locally in start_server and fixed by applying rustfmt's closure formatting; cargo fmt -- --check now passes.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 72.22222% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.14%. Comparing base (d72ff0b) to head (3c2c0f9).

Files with missing lines	Patch %	Lines
src/server.rs	72.22%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2920      +/-   ##
==========================================
+ Coverage   78.13%   78.14%   +0.01%     
==========================================
  Files          74       74              
  Lines       43570    43628      +58     
==========================================
+ Hits        34043    34093      +50     
- Misses       9527     9535       +8     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.