Address issue with failures when creating resource pool resources

Previously failures could lead to no resources in the pool, and
waiters which will never get a resource. Retrying here fixes that
issue, although maybe another approach is needed that keeps track of
new resources being created, as that'll allow keeping track of this
when destroying resource pools.

knots/resource-pool.scm

@@ -562,13 +562,15 @@
                  #f
                  (raise-exception exn)))
          (lambda ()
+           (let loop ()
+             (let ((success?
                     (with-parallelism-limiter
                      return-new-resource/parallelism-limiter
                      (let ((max-size
                             (assq-ref (resource-pool-configuration pool)
                                       'max-size))
                            (size (count-resources resources)))
-              (unless (>= size max-size)
+                       (or (>= size max-size)
                            (with-exception-handler
                                (lambda _ #f)
                              (lambda ()
@@ -585,8 +587,18 @@
                                    (let ((new-resource
                                           (start-stack #t (return-new-resource))))
                                      (put-message channel
-                                       (list 'add-resource new-resource))))))
+                                                  (list 'add-resource new-resource)))
-                  #:unwind? #t)))))
+                                   #t)))
+                             #:unwind? #t))))))
+               (unless success?
+                 ;; TODO Maybe this should be configurable?
+                 (sleep 1)
+
+                 ;; Important to retry here and eventually create
+                 ;; a new resource, as there might be waiters
+                 ;; stuck waiting for a resource, especially if
+                 ;; the pool is empty.
+                 (loop)))))
          #:unwind? #t))))
 
   (define (spawn-fiber-to-destroy-resource resource-id resource-value)