Add explicit support for localhost DNS lookup

This change also includes a few micro-optimizations for the DNS helpers.

Fixes: #22633

src/library.js

@@ -913,27 +913,24 @@ addToLibrary({
   $DNS: {
     address_map: {
       id: 1,
-      addrs: {},
-      names: {}
+      addrs: {'localhost': '127.0.0.1'},
+      names: {'127.0.0.1': 'localhost'}
     },
 
     lookup_name(name) {
       // If the name is already a valid ipv4 / ipv6 address, don't generate a fake one.
-      var res = inetPton4(name);
-      if (res !== null) {
+      if (inetPton4(name) != null) {
         return name;
       }
-      res = inetPton6(name);
-      if (res !== null) {
+      // Unlike the inetPton4 above we don't need and explict null comparison
+      // here since there are no valie v6 addresses that are falsey.
+      if (inetPton6(name)) {
         return name;
       }
 
       // See if this name is already mapped.
-      var addr;
-
-      if (DNS.address_map.addrs[name]) {
-        addr = DNS.address_map.addrs[name];
-      } else {
+      var addr = DNS.address_map.addrs[name];
+      if (!addr) {
         var id = DNS.address_map.id++;
         assert(id < 65535, 'exceeded max address mappings of 65535');
 
@@ -947,11 +944,8 @@ addToLibrary({
     },
 
     lookup_addr(addr) {
-      if (DNS.address_map.names[addr]) {
-        return DNS.address_map.names[addr];
-      }
-
-      return null;
+      // Returns `undefined` if that address is not in the map.
+      return DNS.address_map.names[addr];
     }
   },
 

test/sockets/test_getaddrinfo.c

@@ -293,6 +293,20 @@ int main() {
   assert(sa4->sin_port == ntohs(80));
   freeaddrinfo(servinfo);
 
+  // test loopback address
+  err = getaddrinfo("localhost", "89", &hints, &servinfo);
+  assert(!err);
+  print_addrinfo(servinfo);
+  sa4 = ((struct sockaddr_in*)servinfo->ai_addr);
+  assert(servinfo->ai_family == AF_INET);
+  assert(servinfo->ai_socktype == SOCK_STREAM);
+  assert(servinfo->ai_protocol == IPPROTO_TCP);
+  assert(sa4->sin_port == ntohs(89));
+  struct in_addr addr;
+  inet_aton("127.0.0.1", &addr);
+  assert(sa4->sin_addr.s_addr == addr.s_addr);
+  freeaddrinfo(servinfo);
+
 #ifdef __EMSCRIPTEN__
   // test gai_strerror
   CHECK_ERR(0, "Unknown error");