sqlite3_mutex_enter()

** The sqlite3_mutex_enter() and sqlite3_mutex_try() routines attempt
** to enter a mutex. {H17024} If another thread is already within the mutex,
** sqlite3_mutex_enter() will block and sqlite3_mutex_try() will return
** SQLITE_BUSY. {H17025}  The sqlite3_mutex_try() interface returns [SQLITE_OK]
** upon successful entry.  {H17026} Mutexes created using
** SQLITE_MUTEX_RECURSIVE can be entered multiple times by the same thread.
** {H17027} In such cases the,
** mutex must be exited an equal number of times before another thread
** can enter.  {A17028} If the same thread tries to enter any other
** kind of mutex more than once, the behavior is undefined.
** {H17029} SQLite will never exhibit
** such behavior in its own use of mutexes.
**
** Some systems (for example, Windows 95) do not support the operation
** implemented by sqlite3_mutex_try().  On those systems, sqlite3_mutex_try()
** will always return SQLITE_BUSY.  {H17030} The SQLite core only ever uses
** sqlite3_mutex_try() as an optimization so this is acceptable behavior.
**
** {H17031} The sqlite3_mutex_leave() routine exits a mutex that was
** previously entered by the same thread.  {A17032} The behavior
** is undefined if the mutex is not currently entered by the
** calling thread or is not currently allocated.  {H17033} SQLite will
** never do either. {END}
**
** If the argument to sqlite3_mutex_enter(), sqlite3_mutex_try(), or
** sqlite3_mutex_leave() is a NULL pointer, then all three routines
** behave as no-ops.