mixer_module: send a last sample out after all outputs were disabled

This matters for PWM when the last output gets disabled on either FMU or IO
it would just keep on running.

Also when rebooting with a parameters reset or new airframe with no mapped outputs
it would previously keep outputting PWM with the disarmed value of the new airframe
e.g. 1000us which is a safety hazard because servos could break the physical limit of the
model or miscalibrated ESCs spinning motors.

src/lib/mixer_module/mixer_module.cpp

@@ -455,14 +455,17 @@ bool MixingOutput::update()
 		}
 	}
 
-	if (!all_disabled) {
+	// Send output if any function mapped or one last disabling sample
+	if (!all_disabled || !_was_all_disabled) {
 		if (!_armed.armed && !_armed.manual_lockdown) {
 			_actuator_test.overrideValues(outputs, _max_num_outputs);
 		}
 
 		limitAndUpdateOutputs(outputs, has_updates);
 	}
 
+	_was_all_disabled = all_disabled;
+
 	return true;
 }
 

src/lib/mixer_module/mixer_module.hpp

@@ -288,6 +288,7 @@ class MixingOutput : public ModuleParams
 	hrt_abstime _lowrate_schedule_interval{300_ms};
 	ActuatorTest _actuator_test{_function_assignment};
 	uint32_t _reversible_mask{0}; ///< per-output bits. If set, the output is configured to be reversible (motors only)
+	bool _was_all_disabled{false};
 
 	uORB::SubscriptionCallbackWorkItem *_subscription_callback{nullptr}; ///< current scheduling callback
 

src/lib/mixer_module/mixer_module_tests.cpp

@@ -191,11 +191,15 @@ TEST_F(MixerModuleTest, basic)
 	mixing_output.setAllMaxValues(MAX_VALUE);
 	EXPECT_EQ(test_module.num_updates, 0);
 
-	// all functions disabled: not expected to get an update
+	// all functions disabled: expect to get one single update to process disabling the output signal
 	mixing_output.update();
 	mixing_output.updateSubscriptions(false);
 	mixing_output.update();
-	EXPECT_EQ(test_module.num_updates, 0);
+	EXPECT_EQ(test_module.num_updates, 1);
+	mixing_output.update();
+	mixing_output.updateSubscriptions(false);
+	mixing_output.update();
+	EXPECT_EQ(test_module.num_updates, 1);
 	test_module.reset();
 
 	// configure motor, ensure all still disarmed