On Windows, malloc hooks can be called after the final call to
xmlCleanupParser in various tests. This means that xmlMemMutex can still
be accessed if memory debugging is enabled, so the mutex should not be
cleaned.
This also means that tests may report spurious memory leaks on Windows.
The old implementation avoided the issue by keeping track of all
global state objects in a doubly linked list, so they could be cleaned
during xmlCleanupParser.
But as far as I can tell all memory will be freed eventually, so this is
mostly an issue with our test suite.
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