I was trying to answer another SO question when I came across some very strange behavior. Here is my little test case:
(make-variable-buffer-local (defvar my-override-mode-on-save nil "Can be set to automatically ignore read-only mode of a file when saving.")) (defadvice file-writable-p (around my-overide-file-writeable-p act) "override file-writable-p if `my-override-mode-on-save' is set." (or my-override-mode-on-save ad-do-it)) (defun my-override-toggle-read-only () "Toggle buffer read-only status, keeping `my-override-mode-on-save' in sync." (interactive) (setq my-override-mode-on-save (not my-override-mode-on-save)) (toggle-read-only)) (defun tester-fn () (interactive) (let ((xxx (file-writable-p "/tmp/foofoo")) (yyy (file-writable-p "/tmp/fooxxfoo"))) (message (concat "XXX: " (if xxx "yes" "no") " - YYY: " (if yyy "yes" "no")))))
Where:
/tmp/foofoo is the my-override-toggle-read-only file I visited and run my-override-toggle-read-only ./tmp/fooxxfoo does not exist./tmp is writable by the user I logged in with.
If I run tester-fn in a buffer where my-override-mode-on-save set to t , I get an unexpected result: XXX: no - YYY: no . If I run tester-fn , and in some other buffer (for example, scratch), I get the expected answer in the minibuffer: XXX: no - YYY: yes . Following the advice through the debugger shows that it is doing exactly what I think it should do, doing the parts that I expect, skipping the parts that I expect, returning the expected value. However, tracing tester-fn through the debugger shows very different values returned ( nil and t if the variable evaluates to nil, nil and nil if the variable evaluates to non-nil). The return of nil and nil really what I find strange.
I do not know what is going on here. Does anyone know why I am not getting the expected results?
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