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Subject: bug#4175: 23.1;
 nxml-mode: Internal error in rng-validate-mode triggered
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Cc: Eli Zaretskii <eliz@gnu.org>, Stephen Berman <stephen.berman@gmx.net>,
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The bug is still very much there: I can reproduce it by reducing =
emacs_re_max_failures from 40000 to 4000. It's just a matter of file =
size. The failing regexp (used at xmltok.el:735) is, after rx =
conversion,

(rx (group
     (| (group "xmlns")
        (: (in "_" alpha)
           (* (in "._-" alnum))))
     (? (group ":"
               (in "_" alpha)
               (* (in "._-" alnum)))))
    (* (in "\t\n\r "))
    "=3D"
    (? (* (in "\t\n\r "))
       (group
        (| (: "'"
              (* (not (in "\t\n\r&'<")))
              (? (group
                  (in "\t\n\r&")
                  (* (not (in "'<")))))
              "'")
           (: "\""
              (* (not (in "\t\n\r\"&<")))    ;;
              (? (group                      ;;
                  (in "\t\n\r&")             ;;
                  (* (not (in "\"<")))))     ;;
              "\"")))
       (| (group
           (* (in "\t\n\r "))
           ">")
          (: (group
              (* (in "\t\n\r "))
              "/")
             (? (group ">")))
          (group
           (+ (in "\t\n\r "))))))

and the overflow likely occurs somewhere in the ;;-marked section above, =
while parsing the big d=3D"..." attribute value. That value isn't huge =
(55 KiB) and in any case our parser clearly shouldn't need stack space =
in proportional to an XML attribute value. (The default stack limit =
fails with attributes around 300 KiB in size, which is not big for an =
SVG file.) Isolated test case:

(let ((s (concat "'" (make-string 300000 ?a) "'")))
  (string-match
   (rx "'"
       (* (not (in "\t\n\r&'<")))
       (? (group
           (in "\t\n\r&")
           (* (not (in "'<")))))
       "'")
   s))

I suggest you rewrite the attribute parser so that it doesn't eat regexp =
stack. For instance,

(rx "'" (* (not (in "'<"))) "'")

doesn't consume stack (thanks to the on_failure_keep_string_jump =
optimisation). The parser needs to be a little more complex than that =
and validate entities (the &xyz; things) and detect (and recover from) =
common errors such as missing end quotes, so a single regexp isn't =
sufficient.