Merge pull request #573 from cderici/reliable-unit-public-address

#573

### Description

Assigning a unit's public address may take a little time, which makes the result of `unit.public_address` depend on when it's called, thereby creating inconsistency. This PR adds a method, namely `get_public_address`, that waits (with a timeout) until the unit is assigned an address and returns the address when it's assigned.

Fixes #551

Jira card [#141](https://warthogs.atlassian.net/browse/JUJU-141)

### QA Steps

```sh
tox -e integration -- tests/integration/test_unit.py::test_unit_public_address
```

### Notes & Discussion

**A minor design decision:** is that in the case of a timeout, `get_public_address` returns a `None`, rather than a `TimeoutError`.

juju/unit.py

@@ -2,7 +2,7 @@
 
 import pyrfc3339
 
-from . import model, tag
+from . import model, tag, jasyncio
 from .annotationhelper import _get_annotations, _set_annotations
 from .client import client
 
@@ -102,6 +102,28 @@ async def destroy(self):
         return await app_facade.DestroyUnits(unit_names=[self.name])
     remove = destroy
 
+    async def get_public_address(self, timeout=60):
+        """Return the public address of this unit. Waits until the unit is
+        assigned a public address.
+
+        In case of a timeout, a None is returned (instead of a
+        TimeoutError)
+
+        :param int timeout (60): Maximum seconds to wait for unit to
+        be assigned an address.
+
+        :return int public-address
+
+        """
+        try:
+            if self.public_address is None:
+                await self.model.block_until(
+                    lambda: self.public_address,
+                    timeout=timeout)
+        except jasyncio.TimeoutError:
+            return None
+        return self.public_address
+
     def get_resources(self, details=False):
         """Return resources for this unit.
 

tests/integration/test_unit.py

@@ -7,6 +7,29 @@
 from .. import base
 
 
+@base.bootstrapped
+@pytest.mark.asyncio
+async def test_unit_public_address(event_loop):
+    async with base.CleanModel() as model:
+        app = await model.deploy(
+            'ch:ubuntu',
+            application_name='ubuntu',
+            series='bionic',
+            channel='stable',
+            num_units=2,
+        )
+
+        # wait for the units to come up
+        await model.block_until(lambda: app.units, timeout=480)
+
+        # make sure we have some units to test with
+        assert len(app.units) >= 1
+
+        for unit in app.units:
+            addr = await unit.get_public_address(timeout=480)
+            assert addr, 'unit public address not set'
+
+
 @base.bootstrapped
 @pytest.mark.asyncio
 async def test_run(event_loop):