lazily init futures worker pool

* module/ice-9/futures.scm (%workers, %create-workers!)
  (create-workers!): Define a mechanism to spawn off the future threads
  only when the first future is created.
  (make-future): Call create-workers! here.

module/ice-9/futures.scm

@@ -1,6 +1,6 @@
 ;;; -*- mode: scheme; coding: utf-8; -*-
 ;;;
-;;; Copyright (C) 2010 Free Software Foundation, Inc.
+;;; Copyright (C) 2010, 2011 Free Software Foundation, Inc.
 ;;;
 ;;; This library is free software; you can redistribute it and/or
 ;;; modify it under the terms of the GNU Lesser General Public
@@ -54,6 +54,7 @@
   "Return a new future for THUNK.  Execution may start at any point
 concurrently, or it can start at the time when the returned future is
 touched."
+  (create-workers!)
   (let ((future (%make-future thunk #f (make-mutex))))
     (register-future! future)
     future))
@@ -145,19 +146,27 @@ touched."
       (- (current-processor-count) 1)
       0))
 
-(define %workers
 ;; A dock of workers that stay here forever.
 
 ;; TODO
-  ;; 1. Allocate lazily.
+;; 1. Allow the pool to be shrunk, as in libgomp (though that we'd
-  ;; 2. Allow the pool to be shrunk, as in libgomp (though that we'd
 ;;    need semaphores, which aren't yet in libguile!).
-  ;; 3. Provide a `worker-count' fluid.
+;; 2. Provide a `worker-count' fluid.
+(define %workers '())
+
+(define (%create-workers!)
+  (lock-mutex %futures-mutex)
+  (set! %workers
         (unfold (lambda (i) (>= i %worker-count))
                 (lambda (i)
                   (call-with-new-thread process-futures))
                 1+
                 0))
+  (set! create-workers! (lambda () #t))
+  (unlock-mutex %futures-mutex))
+
+(define create-workers!
+  (lambda () (%create-workers!)))
 
 
 ;;;