[Haskell-cafe] Discussion: The CLOEXEC problem

[Niklas Hambüchen]

On 30/08/15 15:19, Alexander Kjeldaas wrote:
> The directory is irrelevant.  fork() + exec() is not an atomic operation:
> 
> * Thread 1 writes its file completely (opens and closes an Fd 1, using
> O_CLOEXEC)
> * Thread 2 starts writing its file (Fd 2 open for writing, using O_CLOEXEC)
> * Thread 1 starts executing "myBinary" by calling *fork()*. Fd 2 is
> inherited by the child process
> * Thread 2 finishes writing (closes its Fd 2)
> * Thread 2 executes "myBinary", which fails with `Text file busy`
> because an Fd is still open to it in the child of Process 1
> * Thread 1 executes "myBinary" (calling exec()). Fd 2 is automatically
> closed during exec(), but it's too late.
> 
> You need the file descriptor to not be inherited by a child process,
> which is != from O_CLOEXEC.

You are right. This makes solving my original problem impossible, and

* writing the file
* then renaming it
* then executing it

seems to be the only way to do it safely.

Let us then move the discussion back to whether CLOEXEC by default or not.