dns: implement light dns caching

lib/dns.js

@@ -98,6 +98,10 @@ const {
   stopPerf,
 } = require('internal/perf/observe');
 
+const {
+  setInterval,
+} = require('timers');
+
 const dnsException = errors.dnsException;
 
 let promises = null; // Lazy loaded
@@ -133,10 +137,24 @@ function onlookupall(err, addresses) {
   }
 }
 
+function refreshDNSCache() {
+  const { DateNow } = primordials;
+  setInterval(() => {
+    for (const hostname in dnsCache) {
+      if (dnsCache[hostname].expiresAt <= DateNow) {
+        delete dnsCache[hostname];
+      }
+    }
+  }, 1000);
+}
+
+refreshDNSCache();
+
 
 // Easy DNS A/AAAA look up
 // lookup(hostname, [options,] callback)
 const validFamilies = [0, 4, 6];
+const dnsCache = {};
 function lookup(hostname, options, callback) {
   let hints = 0;
   let family = 0;
@@ -212,11 +230,34 @@ function lookup(hostname, options, callback) {
     return {};
   }
 
+  if (dnsCache[hostname]) {
+    const cachedResult = dnsCache[hostname].result;
+    if (all) {
+      process.nextTick(callback, null, cachedResult);
+    } else {
+      process.nextTick(callback, null, hostname, cachedResult[0].family);
+    }
+    return {};
+  }
+
   const req = new GetAddrInfoReqWrap();
   req.callback = callback;
   req.family = family;
   req.hostname = hostname;
-  req.oncomplete = all ? onlookupall : onlookup;
+  req.oncomplete = function(err, result) {
+    if (!err) {
+      const { DateNow } = primordials;
+      dnsCache[hostname] = {
+        result,
+        expiresAt: DateNow + 1000,
+      };
+    }
+    if (all) {
+      onlookupall(err, result);
+    } else {
+      onlookup(err, result);
+    }
+  };
 
   const err = cares.getaddrinfo(
     req, hostname, family, hints, verbatim,