net: make holding the buffer in memory more robust

Set the `req.buffer` property, which serves as a way of keeping
a `Buffer` alive that is being written to a stream, on the C++
side instead of the JS side.

This closes a hole where buffers that were temporarily created
in order to write strings with uncommon encodings (e.g. `hex`)
were passed to the native side without being set as `req.buffer`.

Fixes: #8251
PR-URL: #8252
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

lib/net.js

@@ -685,7 +685,6 @@ Socket.prototype._writeGeneric = function(writev, data, encoding, cb) {
   } else {
     var enc;
     if (data instanceof Buffer) {
-      req.buffer = data;  // Keep reference alive.
       enc = 'buffer';
     } else {
       enc = encoding;

src/stream_base.cc

@@ -225,6 +225,7 @@ int StreamBase::WriteBuffer(const FunctionCallbackInfo<Value>& args) {
 
   err = DoWrite(req_wrap, bufs, count, nullptr);
   req_wrap_obj->Set(env->async(), True(env->isolate()));
+  req_wrap_obj->Set(env->buffer_string(), args[1]);
 
   if (err)
     req_wrap->Dispose();

test/parallel/test-net-write-fully-async-buffer.js

@@ -0,0 +1,34 @@
+'use strict';
+// Flags: --expose-gc
+
+// Note: This is a variant of test-net-write-fully-async-hex-string.js.
+// This always worked, but it seemed appropriate to add a test that checks the
+// behaviour for Buffers, too.
+const common = require('../common');
+const net = require('net');
+
+const data = Buffer.alloc(1000000);
+
+const server = net.createServer(common.mustCall(function(conn) {
+  conn.resume();
+})).listen(0, common.mustCall(function() {
+  const conn = net.createConnection(this.address().port, common.mustCall(() => {
+    let count = 0;
+
+    function writeLoop() {
+      if (count++ === 200) {
+        conn.destroy();
+        server.close();
+        return;
+      }
+
+      while (conn.write(Buffer.from(data)));
+      global.gc(true);
+      // The buffer allocated above should still be alive.
+    }
+
+    conn.on('drain', writeLoop);
+
+    writeLoop();
+  }));
+}));

test/parallel/test-net-write-fully-async-hex-string.js

@@ -0,0 +1,32 @@
+'use strict';
+// Flags: --expose-gc
+
+// Regression test for https://github.com/nodejs/node/issues/8251.
+const common = require('../common');
+const net = require('net');
+
+const data = Buffer.alloc(1000000).toString('hex');
+
+const server = net.createServer(common.mustCall(function(conn) {
+  conn.resume();
+})).listen(0, common.mustCall(function() {
+  const conn = net.createConnection(this.address().port, common.mustCall(() => {
+    let count = 0;
+
+    function writeLoop() {
+      if (count++ === 20) {
+        conn.destroy();
+        server.close();
+        return;
+      }
+
+      while (conn.write(data, 'hex'));
+      global.gc(true);
+      // The buffer allocated inside the .write() call should still be alive.
+    }
+
+    conn.on('drain', writeLoop);
+
+    writeLoop();
+  }));
+}));