fix(thread_aware): dedup factory call on first relocate when source =…

[Vaiz]

…= destination

Arc<T, S>::with_closure eagerly builds self.value for the calling thread's affinity but does not record which affinity it was. The first relocated(source, destination) call observes source from the caller — by contract the affinity of the thread that ran with_closure.

When that first relocate has source == destination, the eager self.value is already the correct per-affinity value, so reuse it instead of running the factory again.

Without this shortcut, scheduler.spawn_multiple(Instantiation::All, ...) called from a worker thread races on the storage write lock: the source==destination task can win first and factory-build (with no source-seed step, since the existing source != destination guard elides it) before any peer task populates the source slot via the end-of-function fallback. The result is one extra factory invocation non-deterministically, observable as counts > len in fetch::isolated_on_oxidizer (e.g. 9 > 8).

This complements PR #310 (which fixed storage corruption from the source == destination path) by also eliminating the extra factory call on that same path.

Replaces the previous test_relocated_source_equals_destination_does_not_corrupt_storage regression test, which was asserting the now-fixed behavior (factory re-run on first relocate(s, s)). Adds a deterministic Arc::ptr_eq test covering the new invariant.

AB#7312281

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (cacf0bf) to head (fe826d1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #399   +/-   ##
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  Coverage   100.0%   100.0%           
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  Files         225      225           
  Lines       16350    16355    +5     
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+ Hits        16350    16355    +5     

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