http: reuse socket only when it is drained

Ensuring every request is assigned to a drained socket or nothing.
Because is has no benifit for a request to be attached to a non
drained socket and it prevents the request from being assigned to
a drained one, which might occur soon or already in the free pool
We achieve this by claiming a socket as free only when the socket
is drained.

PR-URL: #43902
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
Reviewed-By: Robert Nagy <ronagy@icloud.com>

lib/_http_client.js

@@ -665,7 +665,7 @@ function parserOnIncomingClient(res, shouldKeepAlive) {
 
   // Add our listener first, so that we guarantee socket cleanup
   res.on('end', responseOnEnd);
-  req.on('prefinish', requestOnPrefinish);
+  req.on('finish', requestOnFinish);
   socket.on('timeout', responseOnTimeout);
 
   // If the user did not listen for the 'response' event, then they
@@ -737,12 +737,16 @@ function responseOnEnd() {
         socket.end();
     }
     assert(!socket.writable);
-  } else if (req.finished && !this.aborted) {
+  } else if (req.writableFinished && !this.aborted) {
+    assert(req.finished);
     // We can assume `req.finished` means all data has been written since:
     // - `'responseOnEnd'` means we have been assigned a socket.
     // - when we have a socket we write directly to it without buffering.
     // - `req.finished` means `end()` has been called and no further data.
     //   can be written
+    // In addition, `req.writableFinished` means all data written has been
+    // accepted by the kernel. (i.e. the `req.socket` is drained).Without
+    // this constraint, we may assign a non drained socket to a request.
     responseKeepAlive(req);
   }
 }
@@ -755,7 +759,9 @@ function responseOnTimeout() {
   res.emit('timeout');
 }
 
-function requestOnPrefinish() {
+// This function is necessary in the case where we receive the entire reponse
+// from server before we finish sending out the request
+function requestOnFinish() {
   const req = this;
 
   if (req.shouldKeepAlive && req._ended)

lib/_http_outgoing.js

@@ -985,6 +985,8 @@ OutgoingMessage.prototype.end = function end(chunk, encoding, callback) {
 };
 
 
+// This function is called once all user data are flushed to the socket.
+// Note that it has a chance that the socket is not drained.
 OutgoingMessage.prototype._finish = function _finish() {
   assert(this.socket);
   this.emit('prefinish');
@@ -1008,7 +1010,7 @@ OutgoingMessage.prototype._finish = function _finish() {
 // the socket yet. Thus the outgoing messages need to be prepared to queue
 // up data internally before sending it on further to the socket's queue.
 //
-// This function, outgoingFlush(), is called by both the Server and Client
+// This function, _flush(), is called by both the Server and Client
 // to attempt to flush any pending messages out to the socket.
 OutgoingMessage.prototype._flush = function _flush() {
   const socket = this.socket;

test/parallel/test-http-agent-reuse-drained-socket-only.js

@@ -0,0 +1,122 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const http = require('http');
+const net = require('net');
+
+const agent = new http.Agent({
+  keepAlive: true,
+  maxFreeSockets: Infinity,
+  maxSockets: Infinity,
+  maxTotalSockets: Infinity,
+});
+
+const server = net.createServer({
+  pauseOnConnect: true,
+}, (sock) => {
+  // Do not read anything from `sock`
+  sock.pause();
+  sock.write('HTTP/1.1 200 OK\r\nContent-Length: 0\r\nConnection: Keep-Alive\r\n\r\n');
+});
+
+server.listen(0, common.mustCall(() => {
+  sendFstReq(server.address().port);
+}));
+
+function sendFstReq(serverPort) {
+  const req = http.request({
+    agent,
+    host: '127.0.0.1',
+    port: serverPort,
+  }, (res) => {
+    res.on('data', noop);
+    res.on('end', common.mustCall(() => {
+      // Agent's socket reusing code is registered to process.nextTick(),
+      // and will be run after this function, make sure it take effect.
+      setImmediate(sendSecReq, serverPort, req.socket.localPort);
+    }));
+  });
+
+  // Make the `req.socket` non drained, i.e. has some data queued to write to
+  // and accept by the kernel. In Linux and Mac, we only need to call `req.end(aLargeBuffer)`.
+  // However, in Windows, the mechanism of acceptance is loose, the following code is a workaround
+  // for Windows.
+
+  /**
+   * https://docs.microsoft.com/en-US/troubleshoot/windows/win32/data-segment-tcp-winsock says
+   *
+   * Winsock uses the following rules to indicate a send completion to the application
+   * (depending on how the send is invoked, the completion notification could be the
+   * function returning from a blocking call, signaling an event, or calling a notification
+   * function, and so forth):
+   * - If the socket is still within SO_SNDBUF quota, Winsock copies the data from the application
+   * send and indicates the send completion to the application.
+   * - If the socket is beyond SO_SNDBUF quota and there's only one previously buffered send still
+   * in the stack kernel buffer, Winsock copies the data from the application send and indicates
+   * the send completion to the application.
+   * - If the socket is beyond SO_SNDBUF quota and there's more than one previously buffered send
+   * in the stack kernel buffer, Winsock copies the data from the application send. Winsock doesn't
+   * indicate the send completion to the application until the stack completes enough sends to put
+   * back the socket within SO_SNDBUF quota or only one outstanding send condition.
+   */
+
+  req.on('socket', () => {
+    req.socket.on('connect', () => {
+      // Print tcp send buffer information
+      console.log(process.report.getReport().libuv.filter((handle) => handle.type === 'tcp'));
+
+      const dataLargerThanTCPSendBuf = Buffer.alloc(1024 * 1024 * 64, 0);
+
+      req.write(dataLargerThanTCPSendBuf);
+      req.uncork();
+      if (process.platform === 'win32') {
+        assert.ok(req.socket.writableLength === 0);
+      }
+
+      req.write(dataLargerThanTCPSendBuf);
+      req.uncork();
+      if (process.platform === 'win32') {
+        assert.ok(req.socket.writableLength === 0);
+      }
+
+      req.end(dataLargerThanTCPSendBuf);
+      assert.ok(req.socket.writableLength > 0);
+    });
+  });
+}
+
+function sendSecReq(serverPort, fstReqCliPort) {
+  // Make the second request, which should be sent on a new socket
+  // because the first socket is not drained and hence can not be reused
+  const req = http.request({
+    agent,
+    host: '127.0.0.1',
+    port: serverPort,
+  }, (res) => {
+    res.on('data', noop);
+    res.on('end', common.mustCall(() => {
+      setImmediate(sendThrReq, serverPort, req.socket.localPort);
+    }));
+  });
+
+  req.on('socket', common.mustCall((sock) => {
+    assert.notStrictEqual(sock.localPort, fstReqCliPort);
+  }));
+  req.end();
+}
+
+function sendThrReq(serverPort, secReqCliPort) {
+  // Make the third request, the agent should reuse the second socket we just made
+  const req = http.request({
+    agent,
+    host: '127.0.0.1',
+    port: serverPort,
+  }, noop);
+
+  req.on('socket', common.mustCall((sock) => {
+    assert.strictEqual(sock.localPort, secReqCliPort);
+    process.exit(0);
+  }));
+}
+
+function noop() { }