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The time has come to move closer to a final spec for SRFI-21.  Michael
has put my latest version of the spec on the web.  Please take a look
and give me your comments.  I would like to finalize in a week or two.
Below is a summary of the changes.


- There is now an explicit description of the thread states (new,
  runnable, blocked, and terminated).  This makes the spec clearer.

- A "specific" field has been added to mutexes and condition
  variables, which is analogous to the "specific" field of threads.
  It can be used by the application to attach any data to a mutex or
  condition variable.  In particular this is useful to implement
  recursive mutexes (an example is provided in the spec).

- The specification of fairness, priority, and priority boosts has
  been improved.

- Applying the thread priority boost at thread creation is a bad idea
  because it can lead to unexpected thread creation delays.  For
  example, consider a monoprocessor system with round-robin scheduling
  of threads of the same priority, and a thread T0 that tries to
  create and start N threads T1 to TN that are CPU intensive (no I/O).
  If T0 has a nonzero thread boost, and it hasn't blocked recently,
  then as soon as T0 starts T1, T1 will start running and T0 won't get
  to start T2 until T1's quantum is up.  But as soon as T0 starts T2,
  T2 will start running until its quantum is over and then T1 will run
  until its quantum is over.  So it takes O(N*N) quantums before TN
  starts running.  For this reason the specification was changed so
  that the thread boost only applies when a thread blocks.

- The scheduler's "grace period" to take into account a change in
  priority (in thread-base-priority-set!, thread-priority-boost-set!,
  thread-quantum-set!, etc) has been removed.  It is just too
  difficult to specify a useful semantics correctly.

- I have removed the restriction on invoking continuations created by
  other threads, and clarified the interaction with dynamic-wind and
  the dynamic environment.

- The "started thread exception" has been removed.  It is an error to
  start a thread that is not new (i.e. runnable, blocked, or
  terminated) but there is no good reason for SRFI-21 to say how this
  error should be reported by the implementation.

- (thread-start! t) now returns the thread t.  This is convenient to
  immediately start the thread being created:

      (let ((t (thread-start! (make-thread (lambda () ...)))))
        ... do something with t ...)