cs: avoid race with GC in (current-memory-use 'cumulative)

Thanks to Alex Harsanyi for reporting the problem.

pkgs/racket-test/tests/racket/stress/cumulative-memory.rkt

@@ -0,0 +1,17 @@
+#lang racket/base
+
+;; Check whether `(current-memory-use 'cumulative)` is always increasing
+
+(define (work n)
+  (length (let loop ([n n])
+            (if (zero? n)
+                '()
+                (cons (make-vector n) (loop (sub1 n)))))))
+
+(let loop ([n 20000] [u (current-memory-use 'cumulative)])
+  (unless (zero? n)
+    (work (random 1000))
+    (let ([u2 (current-memory-use 'cumulative)])
+      (if (u2 . < . u)
+          (error "oops")
+          (loop (sub1 n) u2)))))

racket/src/cs/rumble/memory.ss

@@ -190,7 +190,8 @@
    [(mode)
     (cond
      [(not mode) (bytes-allocated)]
-     [(eq? mode 'cumulative) (+ (bytes-deallocated) (bytes-allocated))]
+     [(eq? mode 'cumulative) (with-interrupts-disabled
+                              (+ (bytes-deallocated) (bytes-allocated)))]
      ;; must be a custodian; hook is reposnsible for complaining if not
      [else (custodian-memory-use mode (bytes-allocated))])]))