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Misc/NEWS.d/next/Tests/2026-10-19-15-06-48.gh-issue-103758.zLjRZV.rst

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+Fixes gh-103758.
+Supersedes and closes gh-103759.
+
+### Summary
+Adds a regression test in `Lib/test/test_os/test_posix.py` to verify that registering callbacks multiple times via `os.register_at_fork()` correctly preserves both old and newly registered handlers across successive forks.
+
+Specifically, it verifies:
+- `before` handlers execute in LIFO (reverse) order.
+- `after_in_parent` handlers execute in FIFO order.
+- `after_in_child` handlers execute in FIFO order.
+- Handlers registered after an earlier fork accumulate and run alongside previous generations in subsequent forks.
+
+### Note on Previous Work
+This issue was originally reported by @itsankitkp with an initial implementation in #103759. 
+
+This PR supersedes #103759 by addressing reliability issues in the earlier attempt:
+- Eliminates multi-process race conditions on a shared pipe by validating events per-process in memory.
+- Eliminates pipe token-starvation deadlocks on child exit.
+- Uses `test.support.wait_process` to reap child processes cleanly and verify exit status.
+
+Credit is attributed to the original author via the commit trailer (`Co-authored-by`).
+
+### Checklist
+- [x] Tested locally with `./python -m test -v test_os.test_posix`
+- [x] All required CI checks passed cleanly
+- [x] No NEWS entry required (test-only change as per Devguide)