child_process: fix handle delivery

Commit 9352c19 ("child_process: don't emit same handle twice") trades
one bug for another.

Before said commit, a handle was sometimes delivered with messages it
didn't belong to.

The bug fix introduced another bug that needs some explaining. On UNIX
systems, handles are basically file descriptors that are passed around
with the sendmsg() and recvmsg() system calls, using auxiliary data
(SCM_RIGHTS) as the transport.

node.js and libuv depend on the fact that none of the supported systems
ever emit more than one SCM_RIGHTS message from a recvmsg() syscall.
That assumption is something we should probably address someday for the
sake of portability but that's a separate discussion.

So, SCM_RIGHTS messages are never coalesced. SCM_RIGHTS and normal
messages however _are_ coalesced. That is, recvmsg() might return this:

  recvmsg();  // { "message-with-fd", "message", "message" }

The operating system implicitly breaks pending messages along
SCM_RIGHTS boundaries. Most Unices break before such messages but Linux
also breaks _after_ them.  When the sender looks like this:

  sendmsg("message");
  sendmsg("message-with-fd");
  sendmsg("message");

Then on most Unices the receiver sees messages arriving like this:

  recvmsg();  // { "message" }
  recvmsg();  // { "message-with-fd", "message" }

The bug fix in commit 9352c19 assumes this behavior. On Linux however,
those messages can also come in like this:

  recvmsg();  // { "message", "message-with-fd" }
  recvmsg();  // { "message" }

In other words, it's incorrect to assume that the file descriptor is
always attached to the first message. This commit makes node wise up.

Fixes #5330.

lib/child_process.js

@@ -336,8 +336,13 @@ function setupChannel(target, channel) {
         var json = jsonBuffer.slice(start, i);
         var message = JSON.parse(json);
 
-        handleMessage(target, message, recvHandle);
-        recvHandle = undefined;
+        // There will be at most one NODE_HANDLE message in every chunk we
+        // read because SCM_RIGHTS messages don't get coalesced. Make sure
+        // that we deliver the handle with the right message however.
+        if (message && message.cmd === 'NODE_HANDLE')
+          handleMessage(target, message, recvHandle);
+        else
+          handleMessage(target, message, undefined);
 
         start = i + 1;
       }

test/simple/test-child-process-recv-handle.js

@@ -0,0 +1,85 @@
+// 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.
+
+// Test that a Linux specific quirk in the handle passing protocol is handled
+// correctly. See https://github.com/joyent/node/issues/5330 for details.
+
+var common = require('../common');
+var assert = require('assert');
+var net = require('net');
+var spawn = require('child_process').spawn;
+
+if (process.argv[2] === 'worker')
+  worker();
+else
+  master();
+
+function master() {
+  // spawn() can only create one IPC channel so we use stdin/stdout as an
+  // ad-hoc command channel.
+  var proc = spawn(process.execPath, [__filename, 'worker'], {
+    stdio: ['pipe', 'pipe', 'pipe', 'ipc']
+  });
+  var handle = null;
+  proc.on('exit', function() {
+    handle.close();
+  });
+  proc.stdout.on('data', function(data) {
+    assert.equal(data, 'ok\r\n');
+    net.createServer(assert.fail).listen(common.PORT, function() {
+      handle = this._handle;
+      proc.send('one');
+      proc.send('two', handle);
+      proc.send('three');
+      proc.stdin.write('ok\r\n');
+    });
+  });
+  proc.stderr.pipe(process.stderr);
+}
+
+function worker() {
+  process._channel.readStop();  // Make messages batch up.
+  process.stdout.ref();
+  process.stdout.write('ok\r\n');
+  process.stdin.once('data', function(data) {
+    assert.equal(data, 'ok\r\n');
+    process._channel.readStart();
+  });
+  var n = 0;
+  process.on('message', function(msg, handle) {
+    n += 1;
+    if (n === 1) {
+      assert.equal(msg, 'one');
+      assert.equal(handle, undefined);
+    }
+    else if (n === 2) {
+      assert.equal(msg, 'two');
+      assert.equal(typeof handle, 'object');  // Also matches null, therefore...
+      assert.ok(handle);                      // also check that it's truthy.
+      handle.close();
+    }
+    else if (n === 3) {
+      assert.equal(msg, 'three');
+      assert.equal(handle, undefined);
+      process.exit();
+    }
+  });
+}