[atpneonex]

Fixes #89.

Problem

DiskDict exposes partly written files when multiple processes share the same cache directory.

Changes

DiskDict (cotengra/utils.py)

• Writes are now atomic replacements. This ensures no reader see a half-written file. On Windows the replacements may fail, so the OSError is caught and turned into a warning.

ReusableOptimizer (cotengra/reusable.py)

• _fname helper
• The original check-then-read behaviors (_maybe_run_optimizer, update_from_tree) now use a single .get.

Tests

• New tests/test_diskcache.py: regular dict behavior checks and a concurrent read/write stress test.
• New tests in tests/test_optimizers.py: cache_only with a missing entry should fail; corrupt entries are recomputed.

[jcmgray]

Thanks for this, I've pushed a few small followup changes:

• fixed the windows CI (test only)
• use threads to stress test so KeyError can be caught directly.
• just removed max_retries and retry_delay since these are internal and no longer used.
• some other stylistic changes to do with comments etc.