http: Do not let Agent hand out destroyed sockets

Fix #4373

lib/http.js

@@ -1131,7 +1131,8 @@ function Agent(options) {
       name += ':' + localAddress;
     }
 
-    if (self.requests[name] && self.requests[name].length) {
+    if (!socket.destroyed &&
+        self.requests[name] && self.requests[name].length) {
       self.requests[name].shift().onSocket(socket);
       if (self.requests[name].length === 0) {
         // don't leak

test/simple/test-http-agent-destroyed-socket.js

@@ -0,0 +1,106 @@
+// 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 server = http.createServer(function(req, res) {
+  res.writeHead(200, {'Content-Type': 'text/plain'});
+  res.end('Hello World\n');
+}).listen(common.PORT);
+
+var agent = new http.Agent({maxSockets: 1});
+
+agent.on('free', function(socket, host, port) {
+  console.log('freeing socket. destroyed? ', socket.destroyed);
+});
+
+var requestOptions = {
+  agent: agent,
+  host: 'localhost',
+  port: common.PORT,
+  path: '/'
+};
+
+var request1 = http.get(requestOptions, function(response) {
+  // assert request2 is queued in the agent
+  var key = 'localhost:' + common.PORT;
+  assert(agent.requests[key].length === 1);
+  console.log('got response1');
+  request1.socket.on('close', function() {
+    console.log('request1 socket closed');
+  });
+  response.pipe(process.stdout);
+  response.on('end', function() {
+    console.log('response1 done');
+    /////////////////////////////////
+    //
+    // THE IMPORTANT PART
+    //
+    // It is possible for the socket to get destroyed and other work
+    // to run before the 'close' event fires because it happens on
+    // nextTick. This example is contrived because it destroys the
+    // socket manually at just the right time, but at Voxer we have
+    // seen cases where the socket is destroyed by non-user code
+    // then handed out again by an agent *before* the 'close' event
+    // is triggered.
+    request1.socket.destroy();
+
+    process.nextTick(function() {
+      // assert request2 was removed from the queue
+      assert(!agent.requests[key]);
+      console.log("waiting for request2.onSocket's nextTick");
+      process.nextTick(function() {
+        // assert that the same socket was not assigned to request2,
+        // since it was destroyed.
+        assert(request1.socket !== request2.socket);
+        assert(!request2.socket.destroyed, 'the socket is destroyed');
+      });
+    });
+  });
+});
+
+var request2 = http.get(requestOptions, function(response) {
+  assert(!request2.socket.destroyed);
+  assert(request1.socket.destroyed);
+  // assert not reusing the same socket, since it was destroyed.
+  assert(request1.socket !== request2.socket);
+  console.log('got response2');
+  var gotClose = false;
+  var gotResponseEnd = false;
+  request2.socket.on('close', function() {
+    console.log('request2 socket closed');
+    gotClose = true;
+    done();
+  });
+  response.pipe(process.stdout);
+  response.on('end', function() {
+    console.log('response2 done');
+    gotResponseEnd = true;
+    done();
+  });
+
+  function done() {
+    if (gotResponseEnd && gotClose)
+      server.close();
+  }
+});