KITFORMA · EDITORIAL ANSWER
Why can locking the same std::sync::Mutex twice hang one thread?
KitForma editorial guide. A standard mutex is not a recursive lock. Re-entering code that attempts another lock while its guard remains alive can deadlock or otherwise violate the intended usage.
Step-by-step answer
A standard mutex is not a recursive lock. Re-entering code that attempts another lock while its guard remains alive can deadlock or otherwise violate the intended usage.
Pass a mutable reference to already-protected data into internal helpers instead of having every helper lock again. Keep the public lock-taking boundary distinct from internal operations. Avoid callbacks into unknown code while holding the guard.
Build a bounded test around the call graph and verify progress after refactoring. Do not rely on the exact observed failure mode of recursive locking. Replacing it with a different primitive may hide a confused ownership boundary and still leave lock-order cycles across multiple mutexes.
Sources and verification
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