http: Proper KeepAlive behavior

Instead of destroying sockets when there are no pending requests, put
them in a freeSockets list, and unref() them so that they do not keep
the event loop open.

Also, set the default max sockets to Infinity, to prevent the awful
surprising deadlocks that happen when more connections are made.

lib/_http_agent.js

@@ -36,13 +36,20 @@ var EventEmitter = require('events').EventEmitter;
 // concerned with managing a connection pool.
 
 function Agent(options) {
+  if (!(this instanceof Agent))
+    return new Agent(options);
+
   EventEmitter.call(this);
 
   var self = this;
   self.options = options || {};
   self.requests = {};
   self.sockets = {};
+  self.freeSockets = {};
+  self.keepAliveMsecs = self.options.keepAliveMsecs || 1000;
+  self.keepAlive = self.options.keepAlive || false;
   self.maxSockets = self.options.maxSockets || Agent.defaultMaxSockets;
+
   self.on('free', function(socket, host, port, localAddress) {
     var name = host + ':' + port;
     if (localAddress) {
@@ -57,11 +64,31 @@ function Agent(options) {
         delete self.requests[name];
       }
     } else {
-      // If there are no pending requests just destroy the
-      // socket and it will get removed from the pool. This
-      // gets us out of timeout issues and allows us to
-      // default to Connection:keep-alive.
-      socket.destroy();
+      // If there are no pending requests, then put it in
+      // the freeSockets pool, but only if we're allowed to do so.
+      var req = socket._httpMessage;
+      if (req &&
+          req.shouldKeepAlive &&
+          !socket.destroyed &&
+          self.options.keepAlive) {
+        var freeSockets = self.freeSockets[name];
+        var count = freeSockets ? freeSockets.length : 0;
+        if (self.sockets[name])
+          count += self.sockets[name].length;
+
+        if (count > self.maxSockets) {
+          socket.destroy();
+        } else {
+          freeSockets = freeSockets || [];
+          self.freeSockets[name] = freeSockets;
+          socket.setKeepAlive(true, self.keepAliveMsecs);
+          socket.unref();
+          socket._httpMessage = null;
+          freeSockets.push(socket);
+        }
+      } else {
+        socket.destroy();
+      }
     }
   });
   self.createConnection = net.createConnection;
@@ -70,7 +97,7 @@ function Agent(options) {
 util.inherits(Agent, EventEmitter);
 exports.Agent = Agent;
 
-Agent.defaultMaxSockets = 5;
+Agent.defaultMaxSockets = Infinity;
 
 Agent.prototype.defaultPort = 80;
 Agent.prototype.addRequest = function(req, host, port, localAddress) {
@@ -81,7 +108,18 @@ Agent.prototype.addRequest = function(req, host, port, localAddress) {
   if (!this.sockets[name]) {
     this.sockets[name] = [];
   }
-  if (this.sockets[name].length < this.maxSockets) {
+
+  if (this.freeSockets[name] && this.freeSockets[name].length) {
+    // we have a free socket, so use that.
+    var socket = this.freeSockets[name].shift();
+
+    // don't leak
+    if (!this.freeSockets[name].length)
+      delete this.freeSockets[name];
+
+    socket.ref();
+    req.onSocket(socket);
+  } else if (this.sockets[name].length < this.maxSockets) {
     // If we are under maxSockets create a new one.
     req.onSocket(this.createSocket(name, host, port, localAddress, req));
   } else {
@@ -158,5 +196,16 @@ Agent.prototype.removeSocket = function(s, name, host, port, localAddress) {
   }
 };
 
+Agent.prototype.destroy = function() {
+  var sets = [this.freeSockets, this.sockets];
+  sets.forEach(function(set) {
+    Object.keys(set).forEach(function(name) {
+      set[name].forEach(function(socket) {
+        socket.destroy();
+      });
+    });
+  });
+};
+
 var globalAgent = new Agent();
 exports.globalAgent = globalAgent;

test/simple/test-http-client-abort.js

@@ -50,7 +50,7 @@ var server = http.Server(function(req, res) {
 });
 
 var responses = 0;
-var N = http.Agent.defaultMaxSockets - 1;
+var N = 16;
 var requests = [];
 
 server.listen(common.PORT, function() {

test/simple/test-http-keepalive-client.js

@@ -0,0 +1,92 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+var common = require('../common');
+var assert = require('assert');
+
+var http = require('http');
+
+
+var serverSocket = null;
+var server = http.createServer(function(req, res) {
+  // They should all come in on the same server socket.
+  if (serverSocket) {
+    assert.equal(req.socket, serverSocket);
+  } else {
+    serverSocket = req.socket;
+  }
+
+  res.end(req.url);
+});
+server.listen(common.PORT);
+
+var agent = http.Agent({ keepAlive: true });
+
+
+var clientSocket = null;
+var expectRequests = 10;
+var actualRequests = 0;
+
+
+makeRequest(expectRequests);
+function makeRequest(n) {
+  if (n === 0) {
+    server.close();
+    agent.destroy();
+    return;
+  }
+
+  var req = http.request({
+    port: common.PORT,
+    agent: agent,
+    path: '/' + n
+  });
+
+  req.end();
+
+  req.on('socket', function(sock) {
+    if (clientSocket) {
+      assert.equal(sock, clientSocket);
+    } else {
+      clientSocket = sock;
+    }
+  });
+
+  req.on('response', function(res) {
+    var data = '';
+    res.setEncoding('utf8');
+    res.on('data', function(c) {
+      data += c;
+    });
+    res.on('end', function() {
+      assert.equal(data, '/' + n);
+      setTimeout(function() {
+        actualRequests++;
+        makeRequest(n - 1);
+      }, 1);
+    });
+  });
+}
+
+process.on('exit', function() {
+  assert.equal(actualRequests, expectRequests)
+  console.log('ok');
+});