child_process: fix handle delivery

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.

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.

This is a back-port of commit 21bd456 from the v0.10 branch. The test
has been dropped as it's not compatible with the v0.8 process model.

Fixes #5330.

Conflicts:
	lib/child_process.js

1 files changed, 7 insertions, 1 deletions

diff --git a/lib/child_process.js b/lib/child_process.js
index d4b58e770..0b2e9b37f 100644
--- a/lib/child_process.js
+++ b/lib/child_process.js
@@ -292,7 +292,13 @@ function setupChannel(target, channel) {
         var json = jsonBuffer.slice(start, i);
         var message = JSON.parse(json);
 
-        handleMessage(target, message, recvHandle);
+        // 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;
       }