fix: replace panicking expect() with map_or() for edge-case file paths

[srpatcha]

Fix panic on edge-case file paths in LRU disk cache init

Problem

LruDiskCache::init() in src/lru_disk_cache/mod.rs calls .expect("Bad path?") on file.file_name(), which panics if the path ends in .. or is otherwise unusual. A build cache tool should handle unexpected file system states gracefully rather than crashing.

Root Cause

Path::file_name() returns None when the path terminates in .. or consists solely of a root or prefix. The .expect() call converts this into a panic. While uncommon, symlinks or filesystem corruption could produce such paths in the cache directory.

Fix

Replaced .expect("Bad path?").starts_with(TEMPFILE_PREFIX) with .map_or(false, |name| name.starts_with(TEMPFILE_PREFIX)). Paths without a valid file name component are now treated as non-temporary files (skipping the cleanup branch) rather than crashing.

Testing

• Create a cache directory containing a path component ending in ...
• Previously: sccache panics during cache init. Now: the entry is handled gracefully.
• Normal cache operation should be unaffected.

Impact

Affects sccache users whose cache directories may contain unusual file paths. A panic in cache init prevents the entire build cache from working.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 2.77778% with 350 lines in your changes missing coverage. Please review.
✅ Project coverage is 25.77%. Comparing base (d11e2e0) to head (4b5da5d).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/stats.rs	0.00%	348 Missing ⚠️
src/lru_disk_cache/mod.rs	83.33%	1 Missing ⚠️
src/server.rs	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (d11e2e0) and HEAD (4b5da5d). Click for more details.

HEAD has 13 uploads less than BASE

Flag	BASE (d11e2e0)	HEAD (4b5da5d)
	14	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2679       +/-   ##
===========================================
- Coverage   74.17%   25.77%   -48.41%     
===========================================
  Files          70       60       -10     
  Lines       39207    37206     -2001     
===========================================
- Hits        29083     9588    -19495     
- Misses      10124    27618    +17494     

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[sylvestre]

you pr is doing too many different things (like fix the panic and change files permissions)
please split it into several prs

[srpatcha]

Hi @sylvestre, thank you for the feedback! You're right — I'll split this into separate PRs:

1. Fix the panicking expect() with map_or()
2. File permission changes (separate concern)

I'll close this PR and submit them individually. Thanks for the review!

[srpatcha]

Closing this PR as requested by @sylvestre. Split into focused PRs:

1. fix: replace panicking expect() with map_or() for file_name() check #2688 - fix: replace panicking expect() with map_or() for file_name() check
2. fix: rollback pending state on tempfile failure in prepare_add #2689 - fix: rollback pending state on tempfile failure in prepare_add

The cache statistics module will be submitted as a separate PR. Thank you for the feedback!