[libvirt] Cannot write data: Broken pipe when accesing libvirt from a forked process

[Daniel P. Berrange]

On Wed, Sep 30, 2015 at 01:06:48PM +0200, Josef Stribny wrote:
> Hi all,
> 
> I am trying to add vagrant-libvirt support for landrush plugin[0] and found
> out that libvirt fails for me with:
> 
> 
> Call to virConnectNumOfNetworks failed: Cannot write data: Broken pipe
> (Libvirt::RetrieveError)
> 
> I created a minimal reproducer that causes this:
> 
> ```
> #!/usr/bin/ruby
> require 'libvirt'
> 
> conn = Libvirt::open("qemu:///system")
> 
> fork do
>  puts conn.list_networks
> end
> 
> puts conn.list_networks
> ```
> 
> This works just fine on my host, but fails on my virtualized guest (when
> using nested KVM). The journal shows
> the following lines in logs:
> 
> Failed to acquire pid file '/run/user/1001/libvirt/libvirtd.pid': Resource
> temporarily unavailable
> 
> So the forked process here tries to access not-existing libvirtd pid file
> for root (1001 on the VM).
> 
> Is this a feature/bug? How can one avoid it?

It is a bug in your code. You open a connection to libvirt, then fork
and then use that single socket connection from two processes. The
data sent over the socket will be arbitrary interleaved from the
two processes causing corruption of the protocol, so libvirtd
kills your connection.

Regards,
Daniel
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