bug#46641: process-tests assume network connection

[Robert Pluim <rpluim@gmail.com>]

>>>>> On Sun, 21 Feb 2021 14:41:11 +0100, Lars Ingebrigtsen <larsi@gnus.org> said:

    >> I guess we could wrap them all in
    >> 
    >> (skip-unless (dns-query "google.com"))

    Lars> It'd be nice if Emacs did have a predicate for "is there any network
    Lars> here?"  But I don't know what that would look like.  That is, there's a
    Lars> difference between having a local network (i.e., 127.0.0.1), and being
    Lars> on the Internet.  

    Lars> But if any of our tests require Emacs to be on a functioning internet
    Lars> connection, they should indeed be guarded by something like the
    Lars> `skip-unless' you propose.

Well, the tests in question are asking 'does emacs have a correctly
functioning internet connection', so making that a prerequisite for
the test seems kind of redundant, but we can do it. But first:
disabling my network connection causes dns-query to hang, so something
like this is needed, I think (we can skip the first hunk if you want):

diff --git a/lisp/net/dns.el b/lisp/net/dns.el
index 2045d4dfca..598ceebab8 100644
--- a/lisp/net/dns.el
+++ b/lisp/net/dns.el
@@ -332,7 +332,7 @@ dns-set-servers
 	  (setq dns-servers (nreverse dns-servers))))
       (when (executable-find "nslookup")
 	(with-temp-buffer
-	  (call-process "nslookup" nil t nil "localhost")
+	  (call-process "nslookup" nil t nil "-retry=0" "-timeout=2" "localhost")
 	  (goto-char (point-min))
           (when (re-search-forward
 	   "^Address:[ \t]*\\([0-9]+\\.[0-9]+\\.[0-9]+\\.[0-9]+\\|[[:xdigit:]:]*\\)" nil t)
@@ -496,15 +496,17 @@ dns-query
   "Query a DNS server for NAME of TYPE.
 If FULL, return the entire record returned.
 If REVERSE, look up an IP address."
-  (let ((result nil))
-    (dns-query-asynchronous
-     name
-     (lambda (response)
-       (setq result (list response)))
-     type full reverse)
-    ;; Loop until we get the callback.
-    (while (not result)
-      (sleep-for 0.01))
+  (let ((result nil)
+        (query-started
+         (dns-query-asynchronous
+          name
+          (lambda (response)
+            (setq result (list response)))
+          type full reverse)))
+    (if query-started
+        ;; Loop until we get the callback.
+        (while (not result)
+          (sleep-for 0.01)))
     (car result)))
 
 (provide 'dns)