Recover a built-in function that I accidentally unbound

Question

I accidentally unbound a built-in function (a subr) from its usual symbol. Is there a generic way to recover it?

Of course I could save my buffers and my session, exit Emacs and start another instance. That is, if the function I lost wasn't needed to save and exit. What can I do if I lose some really fundamental function such as write-region or get-buffer?

Answer 1

After accidentally losing write-region (there's some weird interaction between flet and advice that I don't understand), I added this code early in my .emacs. I save all the subroutines in a separate obarray, so as long as I don't lose both fset and defalias I should be able to recover.

;; Before anything else, keep a copy of all built-in functions. This
;; is not used during normal operation, only when I accidentally
;; unbind a primitive, e.g. due to ill-advised advice.
;; https://emacs.stackexchange.com/questions/16806/recover-a-built-in-function-that-i-accidentally-unbound
(defvar subr-backup-obarray
  (let ((backup-obarray (make-vector (length obarray) 0)))
    (mapatoms (lambda (symbol)
                (if (and (fboundp symbol) (subrp (symbol-function symbol)))
                    (fset (intern (symbol-name symbol) backup-obarray)
                          (symbol-function symbol)))))
    backup-obarray)
  "Backup of all built-in functions.")
(defalias 'subr-backup
  ;; Why this strange definition? This way we get a byte-compiled
  ;; object that doesn't contain any call to a function symbol, only
  ;; bytecode opcodes and subr objects.
  (byte-compile
   (let ((intern-soft (list 'funcall (symbol-function 'intern-soft)))
         (symbol-function (list 'funcall (symbol-function 'symbol-function)))
         (symbol-name (list 'funcall (symbol-function 'symbol-name)))
         (symbolp (list 'funcall (symbol-function 'symbolp))))
     `(lambda (name)
        (,@symbol-function
         (,@intern-soft (if (,@symbolp name) (,@symbol-name name) name)
                        subr-backup-obarray)))))
  "Return the backup of the built-in function (subr) originally bound to NAME.
NAME may be a string or symbol.
If a symbol that is normally bound to a built-in function has become unbound
or bound to another function, the original subr binding can be restored with
  (fset 'SYMBOL (subr-backup 'SYMBOL))
or (if SYMBOL has no `defalias-fset-function' property)
  (defalias 'SYMBOL (subr-backup 'SYMBOL))")