Add test for subordinate charm num-unit auto adjustment

Author: cderici

tests/integration/test_charmhub.py

@@ -1,7 +1,7 @@
 import pytest
 
 from .. import base
-from juju.errors import JujuAPIError
+from juju.errors import JujuAPIError, JujuError
 
 
 @base.bootstrapped
@@ -69,3 +69,26 @@ async def test_find_all(event_loop):
         for resp in result.result:
             assert resp.name != ""
             assert resp.type_ in ["charm", "bundle"]
+
+
+@base.bootstrapped
+@pytest.mark.asyncio
+async def test_subordinate_charm_zero_units(event_loop):
+    # normally in pylibjuju deploy num_units defaults to 1, we switch
+    # that to 0 behind the scenes if we see that the charmhub charm
+    # we're deploying is a subordinate charm
+    async with base.CleanModel() as model:
+
+        # rsyslog-forwarder-ha is a subordinate charm
+        app = await model.deploy('rsyslog-forwarder-ha')
+        assert len(app.units) == 0
+        await app.destroy()
+
+        # note that it'll error if the user tries to use num_units
+        with pytest.raises(JujuError):
+            await model.deploy('rsyslog-forwarder-ha', num_units=175)
+
+        # (full disclosure: it'll quitely switch to 0 if user enters
+        # num_units=1)
+        app2 = await model.deploy('rsyslog-forwarder-ha', num_units=1)
+        assert len(app2.units) == 0