bug#39235: 26.3; SHR: base handling errors on "."

[Kevin Brubeck Unhammer]

Hi,

    (require 'shr)
    (let ((shr-base (shr-parse-base ".")))
       (shr-expand-url "www.example.com"))

errors with

Debugger entered--Lisp error: (wrong-type-argument char-or-string-p nil)
  url-scheme-get-property(nil expand-file-name)
  url-expand-file-name("www.example.com" "./")
  shr-expand-url("www.example.com")
  (let ((shr-base (shr-parse-base "."))) (shr-expand-url "www.example.com"))
  (progn (require (quote shr)) (let ((shr-base (shr-parse-base "."))) 
(shr-expand-url "www.example.com")))
  eval((progn (require (quote shr)) (let ((shr-base (shr-parse-base "."))) 
(shr-expand-url "www.example.com"))) nil)
  eval-expression((progn (require (quote shr)) (let ((shr-base (shr-parse-base 
"."))) (shr-expand-url "www.example.com"))) nil nil 127)
  funcall-interactively(eval-expression (progn (require (quote shr)) (let 
((shr-base (shr-parse-base "."))) (shr-expand-url "www.example.com"))) nil nil 
127)
  call-interactively(eval-expression nil nil)
  command-execute(eval-expression)

I've seen this in html files containing

    <base href="." target="_blanc">

Regardless of whether it makes sense or not, it'd be nice if Emacs could
ignore it gracefully :)


In GNU Emacs 26.3 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.8)
 of 2019-09-16 built on lgw01-amd64-036
Windowing system distributor 'The X.Org Foundation', version 11.0.12005000
System Description:     Ubuntu 19.10

Recent messages:
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Quit [2 times]
Mark set
<VoidSymbol> is undefined
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Making completion list...

Configured using:
 'configure --build=x86_64-linux-gnu --prefix=/usr
 '--includedir=${prefix}/include' '--mandir=${prefix}/share/man'
 '--infodir=${prefix}/share/info' --sysconfdir=/etc --localstatedir=/var
 --disable-silent-rules '--libdir=${prefix}/lib/x86_64-linux-gnu'
 '--libexecdir=${prefix}/lib/x86_64-linux-gnu' --disable-maintainer-mode
 --disable-dependency-tracking --prefix=/usr --sharedstatedir=/var/lib
 --program-suffix=26 --with-modules --with-file-notification=inotify
 --with-mailutils --with-x=yes --with-x-toolkit=gtk3 --with-xwidgets
 --with-lcms2 'CFLAGS=-g -O2
 -fdebug-prefix-map=/build/emacs26-flSdgS/emacs26-26.3~1.git96dd019=. 
-fstack-protector-strong
 -Wformat -Werror=format-security -no-pie' 'CPPFLAGS=-Wdate-time
 -D_FORTIFY_SOURCE=2' 'LDFLAGS=-Wl,-Bsymbolic-functions -Wl,-z,relro
 -no-pie''

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS GLIB
NOTIFY LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS XWIDGETS
LIBSYSTEMD LCMS2

Important settings:
  value of $LANG: nn_NO.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
format-spec rfc822 mml mml-sec epa derived epg epg-config gnus-util
rmail rmail-loaddefs mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils url-expand url-util url-methods url-parse
auth-source cl-seq eieio eieio-core cl-macs eieio-loaddefs
password-cache url-vars shr svg xml seq dom browse-url help-mode
easymenu cl-print byte-opt gv bytecomp byte-compile cl-loaddefs cl-lib
cconv debug elec-pair time-date mule-util tooltip eldoc electric
uniquify ediff-hook vc-hooks lisp-float-type mwheel term/x-win x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow isearch timer select
scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame cl-generic cham georgian utf-8-lang misc-lang
vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms cp51932
hebrew greek romanian slovak czech european ethiopic indian cyrillic
chinese composite charscript charprop case-table epa-hook jka-cmpr-hook
help simple abbrev obarray minibuffer cl-preloaded nadvice loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote threads dbusbind inotify lcms2 dynamic-setting
system-font-setting font-render-setting xwidget-internal move-toolbar
gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 112733 5649)
 (symbols 48 23186 1)
 (miscs 40 48 126)
 (strings 32 34922 1100)
 (string-bytes 1 892560)
 (vectors 16 16810)
 (vector-slots 8 552698 7532)
 (floats 8 66 51)
 (intervals 56 289 0)
 (buffers 992 13))