ident.c: add support for IPv6

Add IPv6 support by implementing name resolution with the
protocol agnostic getaddrinfo(3) API. The old gethostbyname(3)
code is still available when git is compiled with NO_IPV6.

Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Helped-by: Jeff King <peff@peff.net>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Jeff King <peff@peff.net>

ident.c

@@ -70,10 +70,35 @@ static int add_mailname_host(struct strbuf *buf)
 	return 0;
 }
 
+static int canonical_name(const char *host, struct strbuf *out)
+{
+	int status = -1;
+
+#ifndef NO_IPV6
+	struct addrinfo hints, *ai;
+	memset (&hints, '\0', sizeof (hints));
+	hints.ai_flags = AI_CANONNAME;
+	if (!getaddrinfo(host, NULL, &hints, &ai)) {
+		if (ai && strchr(ai->ai_canonname, '.')) {
+			strbuf_addstr(out, ai->ai_canonname);
+			status = 0;
+		}
+		freeaddrinfo(ai);
+	}
+#else
+	struct hostent *he = gethostbyname(buf);
+	if (he && strchr(he->h_name, '.')) {
+		strbuf_addstr(out, he->h_name);
+		status = 0;
+	}
+#endif /* NO_IPV6 */
+
+	return status;
+}
+
 static void add_domainname(struct strbuf *out)
 {
 	char buf[1024];
-	struct hostent *he;
 
 	if (gethostname(buf, sizeof(buf))) {
 		warning("cannot get host name: %s", strerror(errno));
@@ -82,9 +107,7 @@ static void add_domainname(struct strbuf *out)
 	}
 	if (strchr(buf, '.'))
 		strbuf_addstr(out, buf);
-	else if ((he = gethostbyname(buf)) && strchr(he->h_name, '.'))
+	else if (canonical_name(buf, out) < 0)
-		strbuf_addstr(out, he->h_name);
-	else
 		strbuf_addf(out, "%s.(none)", buf);
 }