Fix a bug where resources pools could empty with waiters

knots/resource-pool.scm

@@ -939,6 +939,10 @@
                               (eq? x resource))
                             resources)))
 
+           (when (and (not (q-empty? waiters))
+                      (< (- (length resources) 1)
+                         max-size))
+             (spawn-fiber-to-return-new-resource))
 
            (loop (if index
                      (remove-at-index! resources index)

tests/resource-pool.scm

@@ -211,4 +211,21 @@
 
      (destroy-resource-pool resource-pool))))
 
+(run-fibers-for-tests
+ (lambda ()
+   (let ((resource-pool (make-resource-pool
+                         (const 'foo)
+                         1
+                         #:lifetime 1
+                         #:destructor
+                         (const #t))))
+     (for-each
+      (lambda _
+        (with-resource-from-pool resource-pool
+            res
+          res))
+      (iota 20))
+
+     (destroy-resource-pool resource-pool))))
+
 (display "resource-pool test finished successfully\n")