inet/getaddrinfo: fix AF_V4MAPPED behavior for non IPv6 host resolution

When trying to resolve a hostname by getaddrinfo() using some specific
settings, it will always return -EAI_NONAME (Name or service not known).

To reproduce this behavior, you need to request an IPv6 address with the
additional AF_V4MAPPED flag set from an non IPv6 capable hostname. If
you choose a IPv4/IPv6 capable hostname like google.com, everything
works fine.

This patch is more or less a port [1][2] from the glibc and their behavior
for the AF_V4MAPPED flag. To test the bug you can use the following snippet.

---- 8< ----

int ret;
struct addrinfo* result;
struct addrinfo hints;

memset(&hints, 0, sizeof(struct addrinfo));
hints.ai_family = AF_INET6;
hints.ai_flags = AI_V4MAPPED;

ret = getaddrinfo("test.com", NULL, &hints, &result);
printf("getaddrinfo(): %i", ret);

---- 8< ----

[1] https://sourceware.org/git/?p=glibc.git;a=commit;f=sysdeps/posix/getaddrinfo.c;h=925c3c5c71596c02f7e58a0ffcdcaae44eb065c1
[2] https://sourceware.org/git/?p=glibc.git;a=commit;f=sysdeps/posix/getaddrinfo.c;h=28977c2c1acb789660ad47e0d88e42486059c916

Signed-off-by: Alexander Wenzel <alexander.wenzel@qsc.de>

libc/inet/getaddrinfo.c

@@ -391,6 +391,9 @@ static uint8_t __gai_precedence = 0;	/* =1 - IPv6, IPv4
 			memcpy((*pat)->addr, h->h_addr_list[i], sizeof(_type));	\
 			pat = &((*pat)->next);					\
 		}								\
+		if (_family == AF_INET6 && i > 0) {				\
+			got_ipv6 = true;					\
+		}								\
 	}									\
 }
 
@@ -404,6 +407,7 @@ gaih_inet(const char *name, const struct gaih_service *service,
 	struct gaih_servtuple *st;
 	struct gaih_addrtuple *at;
 	int rc;
+	bool got_ipv6 = false;
 	int v4mapped = req->ai_family == PF_INET6 && (req->ai_flags & AI_V4MAPPED);
 	unsigned seen = 0;
 	if (req->ai_flags & AI_ADDRCONFIG) {
@@ -586,7 +590,7 @@ gaih_inet(const char *name, const struct gaih_service *service,
 #endif
 			if (req->ai_family == AF_INET
 			 || (!v4mapped && req->ai_family == AF_UNSPEC)
-			 || (v4mapped && (no_inet6_data != 0 || (req->ai_flags & AI_ALL)))
+			 || (v4mapped && (!got_ipv6 || (req->ai_flags & AI_ALL)))
 			) {
 				if (!(req->ai_flags & AI_ADDRCONFIG) || (seen & SEEN_IPV4))
 					gethosts(AF_INET, struct in_addr);
@@ -705,6 +709,14 @@ gaih_inet(const char *name, const struct gaih_service *service,
 			if (at2->family == AF_INET6 || v4mapped) {
 				family = AF_INET6;
 				socklen = sizeof(struct sockaddr_in6);
+
+				/* If we looked up IPv4 mapped address discard them here if
+				   the caller isn't interested in all address and we have
+				   found at least one IPv6 address.  */
+				if (got_ipv6
+				  && (req->ai_flags & (AI_V4MAPPED|AI_ALL)) == AI_V4MAPPED
+				  && IN6_IS_ADDR_V4MAPPED (at2->addr))
+				goto ignore;
 			}
 #endif
 #if defined __UCLIBC_HAS_IPV4__ && defined __UCLIBC_HAS_IPV6__
@@ -781,7 +793,7 @@ gaih_inet(const char *name, const struct gaih_service *service,
 				(*pai)->ai_next = NULL;
 				pai = &((*pai)->ai_next);
 			}
-
+ignore:
 			at2 = at2->next;
 		}
 	}