option always removes all mutex code. For application-defined mutexes only,
use SQLITE_THREADSAFE=1 with SQLITE_MUTEX_NOOP=1. Ticket #3421. (CVS 5779)
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When an unlock fails, do not leak file descriptors (ticket #1611).
But we really ought to report SQLITE_MISUSE or some other error instead
of just returning SQLITE_OK. (CVS 2945)
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