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.