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From: Ryan
Subject: bug#3984:
Date: Wed, 18 Sep 2013 17:47:58 -0700
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:17.0) Gecko/20130801 Thunderbird/17.0.8

After reading and finally comprehending the definition of "advice--called-interactively-skip", I think I have a possible solution that doesn't require a top-down search, but it would require some minor rearchitecting of nadvice. Basically, once we know that a particular stack frame is a call to the innermost unadvised form of an advised function, it is relatively easy to walk up the stack looking for the outermost advice. This is what "advice--called-interactively-skip" does. (Although reading through it I don't see where the bug is that prevents it recognizing the before advice in my example.) The problem, then, is knowing when a function is advised, given only the unadvised form and a position in the stack. If we always unconditionally wrap an unadvised function with a function that we can easily identify, then we can easily check whether it has been advised. To that end, I propose the following:

;; Generate a private symbol
(defvar nadvice--indicator-symbol (make-symbol "nadvice--indicator-symbol"))
(defun wrap-function-in-indicator-lambda (func)
  `(lambda (&rest args)
     (apply ,func args)))
(defun indicator-lambda-p (func)
  (eq nadvice--indicator-symbol
    (nth 2 (wrap-function-in-indicator-lambda (indirect-function func)))))

If all advised functions are wrapped by a call to the above function "wrap-function-in-indicator-lambda", then when they are called, the call to the "indicator lambda" would always be 2 frames up from the call to the original unadvised function. This allows us to easily recognize an advised function on the stack by testing the function 2 frames up with "indicator-lambda-p". Once we know the function is advised, we can then initiate the search for the outermost advice's stack position.

In order to implement this, I think "advice-add" and "advice-remove" need to be modified to automatically wrap/unwrap the original function in/out of the indicator. What do you think of this? Obviously my "indicator-lambda" could be replaced by e.g. a no-op before/after advice or something similar, which would have the same effect of making it easy to recognize the innermost call of an advised function based on the contents of specific stack frame positions above it.

What do you think of this?


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