A reentrant lock is a synchronization primitive that may be
acquired multiple times by the same thread. Internally, it uses
the concepts of ``owning thread'' and ``recursion level'' in
addition to the locked/unlocked state used by primitive locks. In
the locked state, some thread owns the lock; in the unlocked
state, no thread owns it.
To lock the lock, a thread calls its acquire() method; this
returns once the thread owns the lock. To unlock the lock, a
thread calls its release() method.
acquire()/release() call pairs may be nested; only
the final release() (the release() of the outermost
pair) resets the lock to unlocked and allows another thread blocked in
acquire() to proceed.
When invoked without arguments: if this thread already owns
the lock, increment the recursion level by one, and return
immediately. Otherwise, if another thread owns the lock,
block until the lock is unlocked. Once the lock is unlocked
(not owned by any thread), then grab ownership, set the
recursion level to one, and return. If more than one thread
is blocked waiting until the lock is unlocked, only one at a
time will be able to grab ownership of the lock. There is no
return value in this case.
When invoked with the blocking argument set to true, do the
same thing as when called without arguments, and return true.
When invoked with the blocking argument set to false, do not
block. If a call without an argument would block, return false
immediately; otherwise, do the same thing as when called
without arguments, and return true.
Release a lock, decrementing the recursion level. If after the
decrement it is zero, reset the lock to unlocked (not owned by any
thread), and if any other threads are blocked waiting for the lock to
become unlocked, allow exactly one of them to proceed. If after the
decrement the recursion level is still nonzero, the lock remains
locked and owned by the calling thread.
Only call this method when the calling thread owns the lock.
Do not call this method when the lock is unlocked.
