bug#19555: shr-urlify: undecoded URIs make help-echo unhelpful

[Ivan Shmakov]

Package:  emacs
Severity: minor

        As of 88bc8332eb14 (2015-01-07 13:51:41 +0000), shr-urlify
        stores an undecoded URI as the link text’s help-echo property,
        while it should most probably use a decoded IRI string instead.

        The net effect is that, for instance, while TABbing over some
        Russian Wikipedia article, the user may stumble upon:

https://ru.wikipedia.org/wiki/%D0%A4%D0%BE%D0%BD%D0%B4_%D1%81%D0%B2%D0%BE%D0%B1%D0%BE%D0%B4%D0%BD%D0%BE%D0%B3%D0%BE_%D0%BF%D1%80%D0%BE%D0%B3%D1%80%D0%B0%D0%BC%D0%BC%D0%BD%D0%BE%D0%B3%D0%BE_%D0%BE%D0%B1%D0%B5%D1%81%D0%BF%D0%B5%D1%87%D0%B5%D0%BD%D0%B8%D1%8F

        instead of the (arguably) much more readable:

https://ru.wikipedia.org/wiki/Фонд_свободного_программного_обеспечения

        Since 2014-11-19, I use the code MIMEd to use the decoded string
        for help-echo instead, – yet I doubt the code’s correctness.
        For one thing, I’m pretty sure that I’ve seen non-UTF-8 codings
        being used for %-encoded URIs, and this change does by no means
        takes that into account.  Another concern is that in its current
        form, it /does/ decode %20s into blanks, invalidating the IRI.

        EWW should probably also use the decoded form while filing
        eww-data :url entry (subject to the concerns above.)

-- 
FSF associate member #7257  np. Brighter Than a Thousand Suns — Iron Maiden

--- a/lisp/net/shr.el
+++ b/lisp/net/shr.el
@@ -894,7 +918,13 @@ defun shr-urlify (start url &optional title)
   (add-text-properties
    start (point)
    (list 'shr-url url
-        'help-echo (if title (shr-fold-text (format "%s (%s)" url title)) url)
+        'help-echo (let ((iri (or (with-demoted-errors
+                                      "shr-urlify: %s"
+                                    (decode-coding-string
+                                     (url-unhex-string url)
+                                     'utf-8 t))
+                                  url)))
+                     (if title (format "%s (%s)" iri title) iri))
         'follow-link t
         'mouse-face 'highlight
         'keymap shr-map)))