Re: [Qemu-devel] [PATCH for-2.12] os: truncate pidfile on creation

[Florian Larysch <fl@n621.de>]

On Tue, Mar 20, 2018 at 04:02:44PM +0000, Daniel P. Berrangé wrote:
> No, it is unsafe - we rely on lockf() to get the mutual exclusion.
> If a QEMU is running with pidfile locked, and its pid written into
> it, then a 2nd QEMU comes along it will truncate the pidfile owned
> by the original QEMU because the truncation happens before it has
> tried to acquire the lock. The 2nd QEMU will still exit, but the
> original QEMU's pid has now been lost.

That's correct, thanks for pointing it out.

> We must call ftruncate() after lockf(), but before writing the new
> pid into the file. That ensures there is no window in which it is
> possible to see the new & old pids mixed together.

I'll send a revised version doing exactly that.

>From my reading of the Windows API documentation, this might not be a
problem there: The file is opened with FILE_SHARE_READ, which prohibits
opening the file in a writable mode and CREATE_ALWAYS will only recreate
the file if it is writable.

Florian