Environment.assertAliveOp() in juju-1.17.0 incompatible with juju-1.16.*

[John A Meinel]

The new code in AddOneMachine has:
 env, err := st.Environment()
 if err != nil {
  return nil, err
 } else if env.Life() != Alive {
  return nil, fmt.Errorf("environment is no longer alive")
 }
...
 ops = append(ops, env.assertAliveOp())
 if err := st.runTransaction(ops); err != nil {
  return nil, onAbort(err, fmt.Errorf("environment is no longer alive (aborted transaction)"))
 }

However, if you are connecting to a 1.16 environment, there is no Life attribute in the environment doc. Which means that the assertion op:
func (e *Environment) assertAliveOp() txn.Op {
 return txn.Op{
  C: e.st.environments.Name,
  Id: e.UUID(),
  Assert: isAliveDoc,
 }
}

fails. Because null != "0". So even though st.environment.Life() returns 0 == Alive, it isn't actually 0 in the database.

This is a compatibility regression for 1.17 vs a 1.16 environment (you can't add-machines to a 1.16 environment once my patch to support falling back is finished).

[Andrew Wilkins (axwalk)]

The assertion was originally !dead, which worked if the attribute didn't exist; this got broken during review changes. I'll see if I can whip up a unit test for this first.