ffi/unsafe/custodian: fix weak-reference problem

Also fix a related problem in the `math' MPFR binding (which was
masked by the `ffi/unsafe/custodian' bug).

collects/ffi/unsafe/custodian.rkt

@@ -20,21 +20,23 @@
   (get-ffi-obj 'scheme_remove_managed #f
                (_fun _Scheme_Custodian_Reference-pointer _racket -> _void)))
 
-(define (shutdown-callback impl proc+self) 
-  ((car proc+self) impl))
+(define (shutdown-callback impl proc+box) 
+  ((car proc+box) impl))
+(define shutdown-callback-box (box #f))
 (define shutdown_callback
-  (cast shutdown-callback (_fun #:atomic? #t _racket _racket -> _void) _fpointer))
+  (cast shutdown-callback (_fun #:atomic? #t #:keep shutdown-callback-box
+                                _racket _racket -> _void) _fpointer))
 
 (define (register-custodian-shutdown obj proc [custodian (current-custodian)]
                                      #:at-exit? [at-exit? #f]
                                      #:weak? [weak? #f])
-  (define proc+self (cons proc
-                          shutdown-callback)) ; proc as data -> ffi callback retained
+  (define proc+box (cons proc
+                          shutdown-callback-box)) ; proc as data -> ffi callback retained
   (if at-exit?
       (scheme_add_managed_close_on_exit custodian
-                                        obj shutdown_callback proc+self)
+                                        obj shutdown_callback proc+box)
       (scheme_add_managed custodian
-                          obj shutdown_callback proc+self
+                          obj shutdown_callback proc+box
                           (if weak? 0 1))))
 
 (define (unregister-custodian-shutdown obj mref)

collects/math/private/bigfloat/mpfr.rkt

@@ -76,9 +76,14 @@
     [(_ name type fail-thunk) (get-ffi-obj name mpfr-lib type fail-thunk)]))
 
 (define mpfr-free-cache (get-mpfr-fun 'mpfr_free_cache (_fun -> _void)))
-#;
+
 (define mpfr-shutdown
-  (register-custodian-shutdown mpfr-free-cache (λ (free) (free))))
+  (register-custodian-shutdown 
+   mpfr-free-cache ; acts as a "random" object for a shutdown handle
+   (λ (free) 
+      ;; The direct reference here is important, since custodian holds only
+      ;; a weak reference to shutdown handle:
+      (mpfr-free-cache))))
 
 ;; ===================================================================================================
 ;; MPFR types

collects/math/tests/bigfloat-custodian.rkt

@@ -0,0 +1,19 @@
+#lang racket/base
+
+;; Not crashing means success for this stress test:
+
+(let loop ([i 0] [l null])
+  (unless (= i 100)
+    (displayln "next")
+    (let ([l (if (zero? (random 5))
+                 (begin
+                   (printf "shutdown\n")
+                   (for-each custodian-shutdown-all l)
+                   null)
+                 l)])
+      (let ([c (make-custodian)])
+        (parameterize ([current-custodian c])
+          (for ([i (random 10)]) (thread (lambda () (sleep 10))))
+          (parameterize ([current-namespace (make-base-namespace)])
+            (dynamic-require `math/bigfloat #f)))
+        (loop (add1 i) (cons c l))))))