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Fix for atomic-invoke
3 points by CatDancer 4235 days ago | discuss
atomic-invoke doesn't reset ar-sema-cell if its thunk throws an error:

  arc> (errsafe (atomic (/ 1 0)))
  nil
  arc> ^C

  > (thread-cell-ref ar-sema-cell)
  #t
After an error has been thrown from the thunk in one non-nested call to atomic, further calls to atomic execute its thunk immediately without being guarded by the semaphore.

One fix is to use dynamic-wind:

  (xdef 'atomic-invoke
    (lambda (f)
      (if (thread-cell-ref ar-sema-cell)
          (ar-apply f '())
          (dynamic-wind
            (lambda () (thread-cell-set! ar-sema-cell #t))
            (lambda ()
              (call-with-semaphore ar-the-sema
                                   (lambda () (ar-apply f '()))))
            (lambda () (thread-cell-set! ar-sema-cell #f))))))
...though full continuation jumps are blocked across call-with-semaphore, so dynamic-wind might be heavier than you need.