[msan] Make test to fall-back to IPv6 if IPv4 is not available.

test/msan/Linux/sendmsg.cc

 const int kBufSize = 10;
 int sockfd;
 
 int main() {
   int ret;
   char buf[kBufSize] = {0};
   pthread_t client_thread;
   struct sockaddr_in serveraddr;
-
-  sockfd = socket(AF_INET, SOCK_DGRAM, 0);
+  struct sockaddr_in6 serveraddr6;
 
   memset(&serveraddr, 0, sizeof(serveraddr));
   serveraddr.sin_family = AF_INET;
   serveraddr.sin_addr.s_addr = htonl(INADDR_ANY);
   serveraddr.sin_port = 0;
-
-  bind(sockfd, (struct sockaddr *)&serveraddr, sizeof(serveraddr));
+  struct sockaddr *addr = (struct sockaddr *)&serveraddr;
   socklen_t addrlen = sizeof(serveraddr);
-  getsockname(sockfd, (struct sockaddr *)&serveraddr, &addrlen);
+
+  sockfd = socket(addr->sa_family, SOCK_DGRAM, 0);
+  if (sockfd <= 0) {
+    // Try to fall-back to IPv6
+    memset(&serveraddr6, 0, sizeof(serveraddr6));
+    serveraddr6.sin6_family = AF_INET6;
+    serveraddr6.sin6_addr = in6addr_any;
+    serveraddr6.sin6_port = 0;
+    addr = (struct sockaddr *)&serveraddr6;
+    addrlen = sizeof(serveraddr6);
+
+    sockfd = socket(addr->sa_family, SOCK_DGRAM, 0);
+  }
+  assert(sockfd > 0);
+
+  bind(sockfd, addr, addrlen);
+  getsockname(sockfd, addr, &addrlen);
 
 #if defined(POISON)
   __msan_poison(buf + 7, 1);
 #endif
 
 #if defined(SENDMSG)
   struct iovec iov[2] = {{buf, 5}, {buf + 5, 5}};
   struct msghdr msg;
-  msg.msg_name = &serveraddr;
+  msg.msg_name = addr;
   msg.msg_namelen = addrlen;
   msg.msg_iov = iov;
   msg.msg_iovlen = 2;
   msg.msg_control = 0;
   msg.msg_controllen = 0;
   msg.msg_flags = 0;
 #endif
 
 #if defined(SEND)
-  ret = connect(sockfd, (struct sockaddr *)&serveraddr, addrlen);
+  ret = connect(sockfd, addr, addrlen);
   assert(ret == 0);
   ret = send(sockfd, buf, kBufSize, 0);
   // SEND: Uninitialized bytes in __interceptor_send at offset 7 inside [{{.*}}, 10)
   assert(ret > 0);
 #elif defined(SENDTO)
-  ret =
-      sendto(sockfd, buf, kBufSize, 0, (struct sockaddr *)&serveraddr, addrlen);
+  ret = sendto(sockfd, buf, kBufSize, 0, addr, addrlen);
   // SENDTO: Uninitialized bytes in __interceptor_sendto at offset 7 inside [{{.*}}, 10)
   assert(ret > 0);
 #elif defined(SENDMSG)
   ret = sendmsg(sockfd, &msg, 0);
   // SENDMSG: Uninitialized bytes in {{.*}} at offset 2 inside [{{.*}}, 5)
   assert(ret > 0);
 #endif
   fprintf(stderr, "== done\n");
   // NEGATIVE: == done
   return 0;
 }