Fix the destroy behaviour for fixed size thread pools

destroy-thread-pool should block until the thread pool has been
destroyed.

tests/thread-pool.scm

@@ -1,5 +1,6 @@
 (use-modules (tests)
              (ice-9 atomic)
+             (ice-9 threads)
              (srfi srfi-71)
              (fibers)
              (unit-test)
@@ -142,4 +143,33 @@
                 ref-and-decrement))
     (error)))
 
+;; Test that the destructor is called when a size 1 fixed-size thread
+;; pool is destroyed, and that destroy-thread-pool blocks until it has
+;; completed.
+(let* ((destructor-called? #f)
+       (thread-pool
+        (make-fixed-size-thread-pool
+         1
+         #:thread-destructor
+         (lambda ()
+           (set! destructor-called? #t)))))
+  (destroy-thread-pool thread-pool)
+  (assert-equal #t destructor-called?))
+
+;; Test that the destructor is called for every thread when a
+;; multi-thread fixed-size thread pool is destroyed, and that
+;; destroy-thread-pool blocks until all destructors have completed.
+(let* ((destructor-count 0)
+       (mutex (make-mutex))
+       (pool-size 3)
+       (thread-pool
+        (make-fixed-size-thread-pool
+         pool-size
+         #:thread-destructor
+         (lambda ()
+           (with-mutex mutex
+             (set! destructor-count (+ destructor-count 1)))))))
+  (destroy-thread-pool thread-pool)
+  (assert-equal pool-size destructor-count))
+
 (display "thread-pool test finished successfully\n")