net: Track state of setKeepAlive and prevent unnecessary system calls

The state of .setKeepAlive() is now tracked and code will prevent repeated
system calls to setsockopt() when the value has already been set to the
desired value for the socket.

lib/net.js

@@ -257,7 +257,7 @@ function initSocketHandle(self) {
 
 const kBytesRead = Symbol('kBytesRead');
 const kBytesWritten = Symbol('kBytesWritten');
-
+const kSetKeepAlive = Symbol('kSetKeepAlive');
 
 function Socket(options) {
   if (!(this instanceof Socket)) return new Socket(options);
@@ -272,6 +272,7 @@ function Socket(options) {
   this._parent = null;
   this._host = null;
   this[kLastWriteQueueSize] = 0;
+  this[kSetKeepAlive] = null;
   this[kTimeout] = null;
   this[kBuffer] = null;
   this[kBufferCb] = null;
@@ -501,8 +502,15 @@ Socket.prototype.setKeepAlive = function(setting, msecs) {
     return this;
   }
 
-  if (this._handle.setKeepAlive)
-    this._handle.setKeepAlive(setting, ~~(msecs / 1000));
+  const newValue = [setting, ~~(msecs / 1000)];
+  if (this._handle.setKeepAlive &&
+   (this[kSetKeepAlive] == null ||
+    this[kSetKeepAlive][0] !== newValue[0] ||
+    this[kSetKeepAlive][1] !== newValue[1])
+  ) {
+    this[kSetKeepAlive] = newValue;
+    this._handle.setKeepAlive(...newValue);
+  }
 
   return this;
 };