Use gather to improve performance of wait_for_idle (#421)

We found that the wait_for_idle execution time grows proportionally to
the number of applications, so we used asyncio.gather. see [1]

fixes: #419

---
[1]: juju/python-libjuju#1055

cou/utils/juju_utils.py

@@ -605,19 +605,29 @@ async def wait_for_active_idle(
                                  WaitForApplicationsTimeout, defaults to False.
         :type raise_on_blocked: bool
         """
+        if apps is None:
+            apps = await self._get_supported_apps()
 
         @retry(timeout=timeout, no_retry_exceptions=(WaitForApplicationsTimeout,))
         @wraps(self.wait_for_active_idle)
         async def _wait_for_active_idle() -> None:
             # NOTE(rgildein): Defining wrapper so we can use retry with proper timeout
             model = await self._get_model()
             try:
-                await model.wait_for_idle(
-                    apps=apps,
-                    timeout=timeout,
-                    idle_period=idle_period,
-                    raise_on_blocked=raise_on_blocked,
-                    status="active",
+                # NOTE(rgildein): Use asyncio.gather because we always go through the application
+                # list (apps will never be None) and libjuju is very slow here.
+                # https://github.com/juju/python-libjuju/issues/1055
+                await asyncio.gather(
+                    *(
+                        model.wait_for_idle(
+                            apps=[app],
+                            timeout=timeout,
+                            idle_period=idle_period,
+                            raise_on_blocked=raise_on_blocked,
+                            status="active",
+                        )
+                        for app in apps
+                    )
                 )
             except (asyncio.exceptions.TimeoutError, JujuAppError, JujuUnitError) as error:
                 # NOTE(rgildein): Catching TimeoutError raised as exception when wait_for_idle
@@ -631,7 +641,4 @@ async def _wait_for_active_idle() -> None:
                 msg = str(error).replace("\n", "\n  ", 1)
                 raise WaitForApplicationsTimeout(msg) from error
 
-        if apps is None:
-            apps = await self._get_supported_apps()
-
         await _wait_for_active_idle()

tests/unit/utils/test_juju_utils.py

@@ -46,6 +46,7 @@ def mocked_model(mocker):
     model.connection.return_value.is_open = True  # simulate already connected model
     model.disconnect = AsyncMock()
     model.connect = AsyncMock()
+    model.wait_for_idle = AsyncMock()
     yield model
 
 
@@ -506,12 +507,23 @@ async def test_coumodel_wait_for_active_idle(mock_get_supported_apps, mocked_mod
 
     await model.wait_for_active_idle(timeout)
 
-    mocked_model.wait_for_idle.assert_awaited_once_with(
-        apps=["app1", "app2"],
-        timeout=timeout,
-        idle_period=juju_utils.DEFAULT_MODEL_IDLE_PERIOD,
-        raise_on_blocked=False,
-        status="active",
+    mocked_model.wait_for_idle.assert_has_awaits(
+        [
+            call(
+                apps=["app1"],
+                timeout=timeout,
+                idle_period=juju_utils.DEFAULT_MODEL_IDLE_PERIOD,
+                raise_on_blocked=False,
+                status="active",
+            ),
+            call(
+                apps=["app2"],
+                timeout=timeout,
+                idle_period=juju_utils.DEFAULT_MODEL_IDLE_PERIOD,
+                raise_on_blocked=False,
+                status="active",
+            ),
+        ]
     )
     mock_get_supported_apps.assert_awaited_once_with()
 
@@ -547,12 +559,17 @@ async def test_coumodel_wait_for_active_idle_timeout(mock_get_supported_apps, mo
     with pytest.raises(WaitForApplicationsTimeout):
         await model.wait_for_active_idle(timeout, apps=exp_apps)
 
-    mocked_model.wait_for_idle.assert_awaited_once_with(
-        apps=exp_apps,
-        timeout=timeout,
-        idle_period=juju_utils.DEFAULT_MODEL_IDLE_PERIOD,
-        raise_on_blocked=False,
-        status="active",
+    mocked_model.wait_for_idle.assert_has_awaits(
+        [
+            call(
+                apps=[app],
+                timeout=timeout,
+                idle_period=juju_utils.DEFAULT_MODEL_IDLE_PERIOD,
+                raise_on_blocked=False,
+                status="active",
+            )
+            for app in exp_apps
+        ]
     )
     mock_get_supported_apps.assert_not_awaited()