+
+;;; Signalling an error when trying to print an error condition is
+;;; generally a PITA, so whatever the failure encountered when
+;;; wondering about FILE-POSITION within a condition printer, 'tis
+;;; better silently to give up than to try to complain.
+(defun file-position-or-nil-for-error (stream &optional (pos nil posp))
+ ;; Arguably FILE-POSITION shouldn't be signalling errors at all; but
+ ;; "NIL if this cannot be determined" in the ANSI spec doesn't seem
+ ;; absolutely unambiguously to prohibit errors when, e.g., STREAM
+ ;; has been closed so that FILE-POSITION is a nonsense question. So
+ ;; my (WHN) impression is that the conservative approach is to
+ ;; IGNORE-ERRORS. (I encountered this failure from within a homebrew
+ ;; defsystemish operation where the ERROR-STREAM had been CL:CLOSEd,
+ ;; I think by nonlocally exiting through a WITH-OPEN-FILE, by the
+ ;; time an error was reported.)
+ (if posp
+ (ignore-errors (file-position stream pos))
+ (ignore-errors (file-position stream))))
+
+(defun stream-error-position-info (stream &optional position)
+ (unless (interactive-stream-p stream)
+ (let ((now (file-position-or-nil-for-error stream))
+ (pos position))
+ (when (and (not pos) now (plusp now))
+ ;; FILE-POSITION is the next character -- error is at the previous one.
+ (setf pos (1- now)))
+ (let (lineno colno)
+ (when (and pos
+ (< pos sb!xc:array-dimension-limit)
+ (file-position stream :start))
+ (let ((string
+ (make-string pos :element-type (stream-element-type stream))))
+ (when (= pos (read-sequence string stream))
+ ;; Lines count from 1, columns from 0. It's stupid and traditional.
+ (setq lineno (1+ (count #\Newline string))
+ colno (- pos (or (position #\Newline string :from-end t) 0)))))
+ (file-position-or-nil-for-error stream now))
+ (remove-if-not #'second
+ (list (list :line lineno)
+ (list :column colno)
+ (list :file-position pos)))))))