[PATCHv2] ident.c: add support for IPv6

[Elia Pinto <gitter.spiros@gmail.com>]

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>
---
This is the second version of the patch ($gmane/280488)
Changes from previous:

- Simplified the implementation, avoiding the duplication of 
the function add_domainname (Jeff King) ($gmane/280512)

- Fixed a possible memory leak with getaddrinfo (Eric Sunshine)
  ($gmane/280507)


 ident.c | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/ident.c b/ident.c
index 5ff1aad..283e83f 100644
--- a/ident.c
+++ b/ident.c
@@ -73,7 +73,12 @@ static int add_mailname_host(struct strbuf *buf)
 static void add_domainname(struct strbuf *out)
 {
 	char buf[1024];
+#ifndef NO_IPV6
+	struct addrinfo hints, *ai;
+	int gai;
+#else
 	struct hostent *he;
+#endif /* NO_IPV6 */
 
 	if (gethostname(buf, sizeof(buf))) {
 		warning("cannot get host name: %s", strerror(errno));
@@ -82,10 +87,23 @@ static void add_domainname(struct strbuf *out)
 	}
 	if (strchr(buf, '.'))
 		strbuf_addstr(out, buf);
-	else if ((he = gethostbyname(buf)) && strchr(he->h_name, '.'))
-		strbuf_addstr(out, he->h_name);
-	else
-		strbuf_addf(out, "%s.(none)", buf);
+	else	{
+#ifndef NO_IPV6
+		memset (&hints, '\0', sizeof (hints));
+		hints.ai_flags = AI_CANONNAME;
+		if (!(gai = getaddrinfo(buf, NULL, &hints, &ai)) && ai && strchr(ai->ai_canonname, '.')) 
+			strbuf_addstr(out, ai->ai_canonname);
+#else
+		if ((he = gethostbyname(buf)) && strchr(he->h_name, '.'))
+			strbuf_addstr(out, he->h_name);
+#endif /* NO_IPV6 */
+		else
+			strbuf_addf(out, "%s.(none)", buf);
+
+#ifndef NO_IPV6
+		if (gai) freeaddrinfo(ai);
+#endif /* NO_IPV6 */
+	}
 }
 
 static void copy_email(const struct passwd *pw, struct strbuf *email)
-- 
2.5.0