DEFSTRUCT's :PRINT-OBJECT and (nested) printer errors

[Tobias C. Rittweiler]

Specifying an explicit print-function for a structure results in cryptic
errors if that print function signals errors.

Minimal test case:

  (defun print-bar (obj stream)
    (error "PRINT-BAR: ~S~%" obj))

  (defstruct (bar (:print-object print-bar))
    a b c)

then type (MAKE-BAR) at the REPL. The resulting error message is

  Unable to display error condition

In the actual case I was bitten by this the backtrace didn't look as
good as in this simple case, and pressing `C' in SLDB wasn't useful at
all.

Perhaps the PRINT-OBJECT method that is generated by DEFSTRUCT should
look more like the following

  (defmethod print-object ((obj bar) stream)
    (if (boundp '%error-occured-during-printing%)
        (print-unreadable-object (obj stream :type t :identity t)
          (format stream "[PRINT ERROR]"))
        (handler-case (funcall #'PRINT-BAR obj stream)
          (error (e)
            (let ((%error-occured-during-printing% t))
              (declare (special %error-occured-during-printing%))
              (error e))))))

[Nikodemus Siivola (nikodemus)]

We should generally speaking review treatment of errors during PRINT-OBJECT and document some guidelines on how to write robust PRINT-OBJECT methods, and how to debug problems with hairy ones.