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");