Set zero power for PV inverters not neccessary to reach target power (#946)

If desired power is already reached without the need to use some
inverters, their powers need to be set to zero.

If not, their original powers would remain and the target powers would
be wrong.

In case the requested power is zero, this was also leading to crashes
because there were no allocations. This change also fixes that issue.

Closes #933

Author: shsms

RELEASE_NOTES.md

@@ -16,4 +16,4 @@
 
 ## Bug Fixes
 
-<!-- Here goes notable bug fixes that are worth a special mention or explanation -->
+- When the PowerDistributor receives a zero power request for PV inverters, it now correctly sets zero power to the inverters, and no longer crashes.

src/frequenz/sdk/actor/power_distributing/_component_managers/_pv_inverter_manager/_pv_inverter_manager.py

@@ -136,7 +136,8 @@ async def distribute_power(self, request: Request) -> None:
             if remaining_power > Power.zero() or is_close_to_zero(
                 remaining_power.as_watts()
             ):
-                break
+                allocations[inv_id] = Power.zero()
+                continue
             distribution = remaining_power / float(num_components - idx)
             inv_data = self._component_data_caches[inv_id]
             if not inv_data.has_value():

tests/timeseries/_pv_pool/test_pv_pool_control_methods.py

@@ -131,7 +131,7 @@ async def _recv_reports_until(
             if check(report):
                 break
 
-    async def test_setting_power(
+    async def test_setting_power(  # pylint: disable=too-many-statements
         self,
         mocks: _Mocks,
         mocker: MockerFixture,
@@ -260,3 +260,24 @@ async def test_setting_power(
             mocker.call(inv_ids[2], -30000.0),
             mocker.call(inv_ids[3], -30000.0),
         ]
+
+        # Setting 0 power should set all inverters to 0
+        set_power.reset_mock()
+        await pv_pool.propose_power(Power.zero())
+        await self._recv_reports_until(
+            bounds_rx,
+            lambda x: x.target_power is not None and x.target_power.as_watts() == 0.0,
+        )
+        self._assert_report(
+            await bounds_rx.receive(), power=0.0, lower=-100000.0, upper=0.0
+        )
+        await asyncio.sleep(0.0)
+
+        assert set_power.call_count == 4
+        inv_ids = mocks.microgrid.pv_inverter_ids
+        assert sorted(set_power.call_args_list, key=lambda x: x.args[0]) == [
+            mocker.call(inv_ids[0], 0.0),
+            mocker.call(inv_ids[1], 0.0),
+            mocker.call(inv_ids[2], 0.0),
+            mocker.call(inv_ids[3], 0.0),
+        ]