bug#40748: 28.0.50; Switching wifi networks hangs dns lookups

[Eric Abrahamsen <eric@ericabrahamsen.net>]

I'm on Emacs master, but this behavior has been evident for quite some
time, a year or two I think.

I often have trouble reconnecting Gnus' nntp servers after sleeping the
laptop in one location, and waking it up in another, connecting to a new
wifi network (I'm using Arch linux, if it matters).

I edebugged `nntp-open-connection', and realized that the error (which
is getting handled by `nnheader-report', that's why I didn't notice it)
is actually a dns error:

(error "news.gmane.io/nntp Temporary failure in name resol...")

Other Emacs network connections behave as normal. But this error for
this particular server persists until I restart Emacs.

Are DNS lookups cached? Cached per connection? Is there anything we can
do to flush whatever's "stuck"?

Here's the code that opens the connection; in my case it's
nntp-open-network-stream/network.

Happy to help with further debugging!

(let ((coding-system-for-read 'binary)
      (coding-system-for-write 'binary)
      (map '((nntp-open-network-stream network)
	     (network-only plain)	; compat
	     (nntp-open-plain-stream plain)
	     (nntp-open-ssl-stream tls)
	     (nntp-open-tls-stream tls))))
  (if (assoc nntp-open-connection-function map)
      (open-network-stream
       "nntpd" pbuffer nntp-address nntp-port-number
       :type (cadr (assoc nntp-open-connection-function map))
       :end-of-command "^\\([2345]\\|[.]\\).*\n"
       :capability-command "HELP\r\n"
       :success "^3"
       :starttls-function
       (lambda (capabilities)
	 (if (not (string-match "STARTTLS" capabilities))
	     nil
	   "STARTTLS\r\n")))
    (funcall nntp-open-connection-function pbuffer)))