only perfom when the output is missing

[Attila Lendvai]

FTR, i have managed to solve it with this:

(defclass generate-lisp-op (selfward-operation)
  ((selfward-operation :initform '()))) ; we will specify it in our own
COMPONENT-DEPENDS-ON

(defmethod component-depends-on ((op generate-lisp-op) (c c2ffi-file))
  `((load-op ,(find-system "cffi/c2ffi-generator"))
    ;; Regenerating the spec file is a lot of headache, so we ignore
    ;; the file modification times, and only communicate the
    ;; dependency to ASDF if the spec file is missing.
    ,@(let ((spec-file (input-file op c))) ; TODO is it legal to call
ASDF:INPUT-FILES here?
        (when (or (not (probe-file spec-file))
                  (zerop (with-input-from-file (s spec-file)
                           (file-length s))))
          `((generate-spec-op ,c))))
    ,@(call-next-method)))

the commit:
https://github.com/cffi/cffi/commit/46975c644aeb0a832cd170e953cc8ec3cbdc78df

- attila

PS: sorry for the double send, i have cleaned up my mail settings.
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