For those who are interested.
At the moment my patch runs quiet good.

@mfalkvidd maybe you can copy this patch to the developement branch of mysensors.

I don't know if the CORE_DEBUG text meets the guidelines.
This patch needs a new #define to wait some time before sending a new awake message to the controller.
If time is to short sometimes asleep and awake where executed reversed by the controller.

#define MY_SMART_SLEEP_REVOKE_WAIT_DURATION_MS (300ul)

//Tmandel: MySensorsCore.cpp Line 619
// wait(MY_SMART_SLEEP_WAIT_DURATION_MS);		// listen for incoming messages
if (wait(MY_SMART_SLEEP_WAIT_DURATION_MS, C_SET, I_VERSION)) //cancel sleeping request if there is a new C_SET Command
{
	#define MY_SMART_SLEEP_REVOKE_WAIT_DURATION_MS (300ul) 	// 300 seems to be OK (smaller values let the controller think that the node is still sleeping)
	CORE_DEBUG(PSTR("!MCO:SLP:RVKE\n")); //sleep revoked
	wait(MY_SMART_SLEEP_REVOKE_WAIT_DURATION_MS);
	CORE_DEBUG(PSTR("MCO:SLP:WUP=%" PRIi8 "\n"), MY_SLEEP_NOT_POSSIBLE);	// inform controller about wake-up
	(void)_sendRoute(build(_msgTmp, GATEWAY_ADDRESS, NODE_SENSOR_ID, C_INTERNAL,
					   I_POST_SLEEP_NOTIFICATION).set(MY_SMART_SLEEP_WAIT_DURATION_MS + MY_SMART_SLEEP_REVOKE_WAIT_DURATION_MS));
	return MY_SLEEP_NOT_POSSIBLE;
}
				

@rejoe2 Thank you for your effort with patching the FHEM module.

@rejoe2
would it be possible to send out all queued messages if the controllers receives a heartbeat from the node? Maybe this should be a good solution, event if last POST_SLEEP_MESSAGE got lost. We dont have to invent new message types.

I could test your code on my development FHEM system.

Like @jlaraujo said I would like to know what the main-develepers of mysensors have to say about the prefered design of smartSleep.

@rejoe2 Thanks for your effort with trying to change the FHEM Module.
I think I found a way to cancel the smartSleep counter.

In MySensorsCore.cpp in line 619 I changed

wait(MY_SMART_SLEEP_WAIT_DURATION_MS);		// listen for incoming messages

to

if (wait(MY_SMART_SLEEP_WAIT_DURATION_MS, C_SET, I_VERSION)) //cancel sleeping request if there is a new C_SET Command
{
	CORE_DEBUG(PSTR("smartsleep canceled\n"));
	return MY_SLEEP_NOT_POSSIBLE;
}

This seems to work in MySenors but in FHEM the state says "asleep".
FHEM sends no more messages. Instead it increases the retainedMessages counter.

Even if I change the code to send I_POST_SLEEP_NOTIFICATION before cancel the smartSleep.

if (wait(MY_SMART_SLEEP_WAIT_DURATION_MS, C_SET, I_VERSION)) //cancel sleeping request if there is a new C_SET Command
{
	(void)_sendRoute(build(_msgTmp, GATEWAY_ADDRESS, NODE_SENSOR_ID, C_INTERNAL,
					   I_POST_SLEEP_NOTIFICATION).set(MY_SMART_SLEEP_WAIT_DURATION_MS + 20));
	CORE_DEBUG(PSTR("smartsleep canceled\n"));
	return MY_SLEEP_NOT_POSSIBLE;
}

Maybe we need a new internal message I_SMARTSLEEP_CANCELED.

Do you have an other idea?

@mfalkvidd

I made some more investigation and sortet my code a little bit. The problem still exists.
FHEM sends the next commant to my node when the node sends I_PRE_SLEEP_NOTIFICATION.
But in this situation the smartSleep counter is already counting down.
The node receives the value of 1 but goes to sleep right after that. After Next wakeup the node runs through the loop and stays awake.

In my opinion there should be 2 solutions for that problem

1. FHEM sends new values after I_POST_SLEEP_NOTIFICATION
2. in mysensors there is a method to cancel smartSleep countdown

//	###################   Debugging   #####################
#define MY_DEBUG
#define SER_DEBUG
#define MY_RF24_PA_LEVEL 					RF24_PA_LOW
#define MY_RADIO_RF24
#define MY_RF24_CHANNEL 					96
#define MY_TRANSPORT_WAIT_READY_MS 			(5000ul)
#define MY_TRANSPORT_SANITY_CHECK

#define MY_NODE_ID 							110
#define MY_PARENT_NODE_ID 					0		//without this passive node broadcasts everything to parent 255 (dont know what happens, if 2 repeater receive this at the same time)
// #define MY_PARENT_NODE_IS_STATIC
// #define MY_PASSIVE_NODE


// ###################   Node Spezifisch   #####################
#define SKETCH_VER            				"1.0-002"        			// Sketch version
#define SKETCH_NAME           				"LEDSwitch"   		// Optional child sensor name

#define SLEEP_TIME							13333
#define	TOGGLE_BUTTON						3

#include <MySensors.h>


#ifdef SER_DEBUG
#define DEBUG_SERIAL(x) Serial.begin(x)
#define DEBUG_PRINT(x) Serial.print(x)
#define DEBUG_PRINTLN(x) Serial.println(x)
#else
#define DEBUG_SERIAL(x)
#define DEBUG_PRINT(x) 
#define DEBUG_PRINTLN(x) 
#endif

#define LED_PWM_PIN							5

#define CHILD_SINGLE_LED_SWITCH				51
#define CHILD_SINGLE_LED_SWITCH_TEXT		(F("Switch"))




#define MAX_LED_LEVEL						1
#define LED_LEVEL_EEPROM					0
#define	BLINK_DELAY							180


bool justReceived = false;
uint8_t	lastLevel = 1;
uint32_t wakeupTime = 0;
uint32_t waitBeforeNextSleep = 5000;
volatile uint8_t currentLevel = 0; 			// Current LED level 0 or 1

MyMessage msgSwitchState					(CHILD_SINGLE_LED_SWITCH,		V_STATUS);			//51


void preHwInit() 
{
	DEBUG_SERIAL(MY_BAUD_RATE);
	pinMode(LED_PWM_PIN, OUTPUT);   // sets the pin as output
	pinMode(TOGGLE_BUTTON, INPUT_PULLUP);   // sets the pin as output
	DEBUG_PRINTLN("preHwInit done");

}

void before() 
{
	digitalWrite( LED_PWM_PIN, currentLevel );
}


void setup()
{
	DEBUG_PRINTLN("Setup");
}

void presentation()
{
	sendSketchInfo(SKETCH_NAME, SKETCH_VER );
	present(CHILD_SINGLE_LED_SWITCH, 	S_BINARY, 		CHILD_SINGLE_LED_SWITCH_TEXT);
}


void loop()
{
	uint32_t currentTime = millis();
	if (justReceived)//
	{
		DEBUG_PRINTLN("justReceived");
		justReceived = false;
	}
	
	if (lastLevel != currentLevel) 
	{
		lastLevel = currentLevel;
		setLED(currentLevel);
	}
	
	if ((currentTime - wakeupTime) > waitBeforeNextSleep)
	{
		DEBUG_PRINTLN("_");
		if (currentLevel == 0)
		{
			DEBUG_PRINTLN("prepare to sleep");
			int8_t wakeupReason = sleep(digitalPinToInterrupt(TOGGLE_BUTTON), FALLING , SLEEP_TIME, true);
			if (wakeupReason == digitalPinToInterrupt(TOGGLE_BUTTON))
			{
				currentLevel = 1;
				DEBUG_PRINTLN("LED on after wake by ButtonIRQ");
			}
			wakeupTime = millis();
			DEBUG_PRINT("leaving sleep: wakeup reason: ");
			DEBUG_PRINTLN(wakeupReason);
		}
	}
	else
	{
		DEBUG_PRINT(".");
	}
}



void receive(const MyMessage &message)
{
	DEBUG_PRINT( "receive: " );
	DEBUG_PRINTLN( message.getByte() );	
	if (message.type == V_STATUS) 
	{
		justReceived = true;
		uint8_t requestedLevel = message.getByte();
		if (requestedLevel == 0)
		{
			currentLevel=0;
		}
		else
		{
			currentLevel=1;
		}
	}
}


void setLED(uint8_t newLevel)
{
	DEBUG_PRINT( "setLED: " );
	DEBUG_PRINTLN( newLevel );	
	if (newLevel == 0)
	{
		analogWrite(LED_PWM_PIN,0);
	}
	else
	{
		analogWrite(LED_PWM_PIN,newLevel<<3);
	}
	send( msgSwitchState.set(currentLevel) );
}

15:06:44.843 -> setLED: 0
15:06:44.843 -> 3224 TSF:MSG:SEND,110-110-0-0,s=51,c=1,t=2,pt=1,l=1,sg=0,ft=0,st=OK:0
15:06:44.843 -> prepare to sleep
15:06:44.843 -> 3230 MCO:SLP:MS=13333,SMS=1,I1=1,M1=2,I2=255,M2=255
15:06:44.843 -> 3238 TSF:MSG:SEND,110-110-0-0,s=255,c=3,t=32,pt=5,l=4,sg=0,ft=0,st=OK:500
15:06:45.346 -> 3745 TSF:TDI:TSL
15:07:00.209 -> 3746 MCO:SLP:WUP=-1
15:07:00.209 -> 3748 TSF:TRI:TSB
15:07:00.209 -> 3757 TSF:MSG:SEND,110-110-0-0,s=255,c=3,t=33,pt=5,l=4,sg=0,ft=0,st=OK:13333
15:07:00.209 -> leaving sleep: wakeup reason: -1
15:07:00.209 -> prepare to sleep
15:07:00.209 -> 3764 MCO:SLP:MS=13333,SMS=1,I1=1,M1=2,I2=255,M2=255
15:07:00.209 -> 3774 TSF:MSG:SEND,110-110-0-0,s=255,c=3,t=32,pt=5,l=4,sg=0,ft=0,st=OK:500
15:07:00.275 -> 3817 TSF:MSG:READ,0-0-110,s=51,c=1,t=2,pt=0,l=1,sg=0:1
15:07:00.275 -> receive: 1
15:07:00.711 -> 4281 TSF:TDI:TSL
15:07:15.543 -> 4282 MCO:SLP:WUP=-1
15:07:15.543 -> 4284 TSF:TRI:TSB
15:07:15.577 -> 4292 TSF:MSG:SEND,110-110-0-0,s=255,c=3,t=33,pt=5,l=4,sg=0,ft=0,st=OK:13333
15:07:15.577 -> leaving sleep: wakeup reason: -1
15:07:15.577 -> justReceived
15:07:15.577 -> setLED: 1
15:07:15.577 -> 4301 TSF:MSG:SEND,110-110-0-0,s=51,c=1,t=2,pt=1,l=1,sg=0,ft=0,st=OK:1

I am developing a LED switch node that should sleep as much as possible while the LED is off.
As a controller I use FHEM which supports the smartSleep feature.
When the node wakes up the controller sends new commands to the node (turn LED on). The node turns on the LED, but directly turns it off and goes to sleep mode again.
Afer sleeptime the node wakes up, turns on the LED and stays on.
I think the node has initiated to go to sleep while receiving the turn-on-command.
Is it possible to turn smartSleep off (or cancel while the smartsleep countdown is running) in the receive method.
I counld not find any method to do that. I tried sleep_disable() but this seems not to work.

I use Mysensors 2.3.1 on an arduino nano.

volatile uint8_t currentLevel = 0;  // Current dim level...

void receive(const MyMessage &message)
{
	DEBUG_PRINT( "receive" );
	DEBUG_PRINTLN( String(message.data) );	
	if (message.type == V_STATUS) 
	{
		uint8_t requestedLevel = atoi( message.data );

		requestedLevel = requestedLevel > 1 ? 1 : requestedLevel;

		currentLevel = requestedLevel;
		if (currentLevel == 1)
		{
			sleep_disable();	
		}

		DEBUG_PRINT(F("analogWrite: "));
		DEBUG_PRINTLN(currentLevel);
		setLED(currentLevel);

		// saveState(LED_LEVEL_EEPROM,	currentLevel);
		DEBUG_PRINT(F("LED_LEVEL_EEPROM New Value: "));
		DEBUG_PRINTLN(currentLevel);
		// Inform the gateway
		send(msgSwitchState.set(currentLevel > 0));
	}
}