This is not 100% reproducible, since I started another build and
that works.

This suggests it's another race in the start-up of the session
libvirtd.

Another build, same error:
http://download.devel.redhat.com/brewroot/work/tasks/6393/9476393/build.log

Adding Regression keyword, since this is a regression over
previous behaviour.

Rich, this looks like a daemon crasher to me. Let me see if my investigation will lead to anything. BTW do you know if it is possible to get the daemon logs from the build? Because the logs you've linked show client logs (I might be able to reproduce even so - the domain XML is there).

Created attachment 1049893 [details]
libvirtd.log

It turns out this is reproducible on baremetal, although it's
much rarer.  To try reproducing it, do the following as NON-root:

killall libvirtd
libguestfs-test-tool

Attached is libvirtd.log from a failing run, and in the next
comment I will attach libvirt-client.log which is the LIBVIRT_DEBUG=1
output from the same run.

Created attachment 1049894 [details]
libvirt-client.log

LIBVIRT_DEBUG=1 output from the same run as the comment above.

(In reply to Richard W.M. Jones from comment #6)
> Created attachment 1049893 [details]
> libvirtd.log
> 
> It turns out this is reproducible on baremetal, although it's
> much rarer.  To try reproducing it, do the following as NON-root:
> 
> killall libvirtd
> libguestfs-test-tool

Actually I see why this is intermittent/difficult to reproduce.

When libguestfs launch() is called, it basically does:

 - conn = virConnectOpenAuth
 - call virConnectGetCapabilities (conn)
 - rebuild the libguestfs appliance, if it needs to be rebuilt
 - call virDomainCreateXML (conn)

The rebuild step is either instantaneous (if it doesn't need to
be rebuilt) or on RHEL takes about 2 minutes.

I think this is only failing when the rebuild step happens.  Could
it be that libvirtd --timeout is broken, so that it's timing out
the daemon even though there is a client connection?

I can reliably reproduce this bug when I trigger an appliance
rebuild (do this command: sudo touch /usr/lib64/guestfs/supermin.d/ )

Created attachment 1049908 [details]
bz1240283.py

Based on the comment above, this is a simple reproducer
which doesn't involve anything except python + libvirt.

$ ./bz1240283.py 
libvirt: XML-RPC error : Cannot write data: Broken pipe
Traceback (most recent call last):
  File "./bz1240283.py", line 25, in <module>
    caps = conn.getCapabilities()
  File "/usr/lib64/python2.7/site-packages/libvirt.py", line 3505, in getCapabilities
    if ret is None: raise libvirtError ('virConnectGetCapabilities() failed', conn=self)
libvirt.libvirtError: Cannot write data: Broken pipe

Fixed upstream by commit v1.2.17-77-gb7ea58c26219:

commit b7ea58c262194037042284a14fb1608c9cf31884
Author: Martin Kletzander <mkletzan>
Date:   Fri Jul 10 10:35:31 2015 +0200

    rpc: Rework timerActive logic in daemon

Cound reproduce this bug with libvirt-1.2.17-1.el7.x86_64 with Richard's reproducer

$ ./reproduce.py 
libvirt: XML-RPC error : Cannot write data: Broken pipe
Traceback (most recent call last):
  File "./reproduce.py", line 25, in <module>
    caps = conn.getCapabilities()
  File "/usr/lib64/python2.7/site-packages/libvirt.py", line 3714, in getCapabilities
    if ret is None: raise libvirtError ('virConnectGetCapabilities() failed', conn=self)
libvirt.libvirtError: Cannot write data: Broken pipe

Verify the bug with libvirt-1.2.17-9.el7, could get the expect info
1.Run the reproducer, could get the expect info
$ ./reproduce.py 
NOT reproduced RHBZ#1240283

2.Get the host capabilities info with no-root user in read-only mode, could list the info correctly
$virsh -r -c qemu:///system capabilities
<capabilities>

  <host>
    <uuid>0028bd0f-d97b-e111-0000-e839354bfeea</uuid>
    <cpu>
      <arch>x86_64</arch>
      <model>SandyBridge</model>
      <vendor>Intel</vendor>
      <topology sockets='1' cores='4' threads='2'/>
---

According to upper steps, mark this bug verifed

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHBA-2015-2202.html