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ICU-23352 Reset initOnce before clearing the collation root singleton - #4259
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uprv_collation_root_cleanup() cleared rootSingleton before resetting initOnce. A concurrent CollationRoot::getRoot() that had already passed umtx_initOnce() could then dereference the freed singleton. Reset initOnce first so concurrent callers are directed through initialization instead of the cleared pointer.
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Description
uprv_collation_root_cleanup() cleared rootSingleton before resetting initOnce. A concurrent CollationRoot::getRoot() that had already passed umtx_initOnce() could then dereference the freed singleton during library teardown. This change resets initOnce first, so concurrent callers are directed through initialization instead of reaching for the cleared pointer, as prescribed in the ticket.
The analysis and the fix direction come from ticket ICU-23352; an earlier pull request #3921 proposed the same change; the ticket was moved to Accepted while that PR was open, and after the discussion of the u_cleanup() threading contract there the PR went quiet and was closed without a technical rejection.
The regression test
TestCollationRootCleanupRace(ported from #3921) is included intsmthred.cpp/tsmthred.h: it runs ten rounds ofu_cleanup()followed by concurrentCollationRoot::getRoot()reader threads, failing under TSAN/ASAN on unfixed code.Testing
The race window is not deterministically observable in a regular test run, so this change ships without a new test. The ordering change is two lines, and the full rbbi and collation suites pass locally with it applied.