bug#40775: 28.0.50; dns-query sometimes infloops

[Tassilo Horn]

I use Gnus to read mail and news.  When selecting a message in Gnus, my
configuration will call gnus-treat-from-gravatar and
gnus-treat-mail-gravatar which try to fetch gravatar icons of the mail's
sender and recipients using Emacs' gravatar.el and insert them in the
mail header.

Since the recent changes to gravatar.el, the standard gravatar service
is libravatar now.  gravatar--service-libravatar does some DNS queries
to check if the domain of the mail address hosts the gravatar images
itself.

Sometimes, emacs gets stuck while doing the dns-query.  The emacs
process uses 100% of one core of my system, and this situation persists
for at least a minute (before I C-g, i.e., I don't know if it would ever
recover from such a situation).

This is the Lisp backtrace in one of such situations:

"Automatic GC" (0x0)
"timer-event-handler" (0xffff9e28)
"accept-process-output" (0xffffa4b0)
"dns-query" (0xffffa8f0)
"let*" (0xffffaa98)
"let" (0xffffabd8)
"while" (0xffffacb8)
"let" (0xffffadf8)
"catch" (0xffffaef8)
"let" (0xffffb038)
"if" (0xffffb108)
"progn" (0xffffb1c8)
"unwind-protect" (0xffffb298)
"let" (0xffffb3d8)
"gravatar--service-libravatar" (0xffffb548)
"funcall" (0xffffb540)
"format" (0xffffb658)
"gravatar-build-url" (0xffffb770)
"let" (0xffffb938)
"gravatar-retrieve" (0xffffbae8)
"gnus-gravatar-transform-address" (0xffffbea8)
"gnus-treat-mail-gravatar" (0xffffc180)
"th/gnus-article-prepared" (0xffffc3f0)
"run-hooks" (0xffffc668)
"apply" (0xffffc660)
"gnus-run-hooks" (0xffffc970)
"gnus-article-prepare" (0xffffce30)
"gnus-summary-display-article" (0xffffd200)
"gnus-summary-select-article" (0xffffd5d8)
"gnus-summary-scroll-up" (0xffffda80)
"funcall-interactively" (0xffffda78)
"call-interactively" (0xffffdd10)
"command-execute" (0xffffe0c8)
(gdb)

The non-emacs function th/gnus-article-prepared is just

(defun th/gnus-article-prepared ()
  (gnus-treat-from-gravatar)
  (gnus-treat-mail-gravatar))

Looking at dns-query, there's just the while loop with
accept-process-output which could cause an infloop.  However, with the
values of times and step, I'd say this cannot loop longer than about 5
seconds in the worst case...

The attached backtrace.txt.gz contains the full C and Lisp backtraces of
such an infloop.

backtrace.txt.gz
Description: application/gzip

In GNU Emacs 28.0.50 (build 5, x86_64-pc-linux-gnu, GTK+ Version 3.24.17, cairo 
version 1.17.3)
 of 2020-04-21 built on thinkpad-t440p
Repository revision: f212daf0a96999b8c1e63217c22a86d6e1671687
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: Arch Linux

Recent messages:
Successfully sent message via fastmail account
20200422T190145.732> Opening nnml server on archive...
20200422T190145.733> Opening nnml server on archive...done
Wrote /home/horn/.gnus.d/News/archive/sent-mails/1914
20200422T190146.563> nnimap read 0k from imap.fastmail.com
Sending...done
command-execute: Buffer is read-only: #<buffer *Article 
nnimap+Fastmail:mailinglists/emacs-devel*>
20200422T190149.444> Exiting summary buffer and applying spam rules
20200422T190149.674> Expiring articles...
20200422T190149.907> Expiring articles...done

Configured using:
 'configure --with-modules 'CFLAGS=-O1 -g3 -ggdb''

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND GPM DBUS GSETTINGS GLIB NOTIFY
INOTIFY ACL GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS LIBSYSTEMD JSON
PDUMPER LCMS2 GMP

Important settings:
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  value of $LC_NUMERIC: de_DE.utf8
  value of $LC_TIME: de_DE.utf8
  value of $LANG: en_US.utf8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Group

Minor modes in effect:
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  cursor-sensor-mode: t
  gnus-topic-mode: t
  beacon-mode: t
  global-aggressive-indent-mode: t
  which-key-mode: t
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  electric-pair-mode: t
  recentf-mode: t
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  subword-mode: t
  save-place-mode: t
  savehist-mode: t
  show-paren-mode: t
  gnus-undo-mode: t
  shell-dirtrack-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-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
  buffer-read-only: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
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sort gnus-cite gnus-async gnus-bcklg gnus-ml hl-line cursor-sensor
nndraft nnmh rot13 utf-7 nnml nnnil gnus-agent gnus-srvr gnus-score
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ediff-hook vc-hooks lisp-float-type mwheel term/x-win x-win
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Memory information:
((conses 16 716983 96334)
 (symbols 48 48873 46)
 (strings 32 237412 6465)
 (string-bytes 1 7560922)
 (vectors 16 153404)
 (vector-slots 8 3583548 117924)
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 (buffers 992 49))