master cannot deploy charms

The errors looks like this

services:
  dummy-sink:
    charm: local:trusty/dummy-sink-0
    exposed: true
    service-status:
      current: error
      message: 'cannot assign unit "dummy-sink/0" to machine: cannot assign unit "dummy-sink/0"
        to new machine or container: cannot assign unit "dummy-sink/0" to new machine:
        unit is already assigned to a machine'
      since: 05 Nov 2015 06:43:14Z
    relations:
      source:
      - dummy-source
    units:
      dummy-sink/0:
        workload-status:
          current: error
          message: 'cannot assign unit "dummy-sink/0" to machine: cannot assign unit
            "dummy-sink/0" to new machine or container: cannot assign unit "dummy-sink/0"
            to new machine: unit is already assigned to a machine'
          since: 05 Nov 2015 06:43:14Z
        agent-status:
          current: lost
          message: agent is not communicating with the server
          since: 05 Nov 2015 06:43:14Z
        agent-state: error
        agent-state-info: 'cannot assign unit "dummy-sink/0" to machine: cannot assign
          unit "dummy-sink/0" to new machine or container: cannot assign unit "dummy-sink/0"
          to new machine: unit is already assigned to a machine'
        machine: "2"
        public-address: 54.254.252.81
  dummy-source:
    charm: local:trusty/dummy-source-0
    exposed: false
    service-status:
      current: error
      message: 'cannot assign unit "dummy-source/0" to machine: cannot assign unit
        "dummy-source/0" to new machine or container: cannot assign unit "dummy-source/0"
        to new machine: unit is already assigned to a machine'
      since: 05 Nov 2015 06:43:14Z
    relations:
      sink:
      - dummy-sink
    units:
      dummy-source/0:
        workload-status:
          current: error
          message: 'cannot assign unit "dummy-source/0" to machine: cannot assign
            unit "dummy-source/0" to new machine or container: cannot assign unit
            "dummy-source/0" to new machine: unit is already assigned to a machine'
          since: 05 Nov 2015 06:43:14Z
        agent-status:
          current: lost
          message: agent is not communicating with the server
          since: 05 Nov 2015 06:43:14Z
          version: 1.26-alpha2
        agent-state: error
        agent-state-info: 'cannot assign unit "dummy-source/0" to machine: cannot
          assign unit "dummy-source/0" to new machine or container: cannot assign
          unit "dummy-source/0" to new machine: unit is already assigned to a machine'
        agent-version: 1.26-alpha2
        machine: "1"
        public-address: 54.254.229.81

So, this appears to be a race condition where doing an expose and/or add-relation immediately after deploy causes the error. This is likely due to those commands being issued while the unit exists but before it is assigned to a machine.

If you just juju deploy a charm, it deploys just fine.

Note that the charms actually are deployed just fine (you can see the unit logs running without error in the CI results). They also appear exposed and related as expected. If you wait for a while, the errors actually go away, and everything appears fine.

Obviously, we shouldn't get into this error state in the first place, even if we do eventually get out of it.

I've submitted a revert of the merge that caused this issue, though I'm still not exactly clear on why it's happening, but at least the code is out of master for now.

revert PR: https://github.com/juju/juju/pull/3679

Juju-CI verified that this issue is Fix Released in juju-core master:
    http://reports.vapour.ws/releases/3278