Using a custodian shutdown to shutdown ao instead of a plumber.

This seems much more reliable.

libao.rkt

@@ -4,6 +4,7 @@
          (prefix-in fin: finalizer)
          (prefix-in ffi: "libao-async-ffi.rkt")
          ffi/unsafe
+         ffi/unsafe/custodian
          data/queue
          "private/utils.rkt"
          )
@@ -37,7 +38,23 @@
 
 (ao_initialize)
 
-(define libao-plumber-flus-handle
+(define (ao-finalizer devices)
+  (hash-for-each devices
+                 (lambda (handle-num device)
+                   (ao-close handle-num)))
+  (set! devices (make-hash))
+  (ao_shutdown)
+  )
+
+(define custodian-finalizer
+  (register-custodian-shutdown devices
+                               (λ (devices)
+                                 (ao-finalizer devices))
+                               #:at-exit? #t)
+  )
+
+#|
+(define libao-plumber-flush-handle
   (plumber-add-flush! (current-plumber)
                       (lambda (my-handle)
                         (hash-for-each devices
@@ -49,6 +66,9 @@
                         (ao_shutdown)
                         (plumber-flush-handle-remove! my-handle)
                         )))
+|#
+
+