Fixed bug in relay payouts

When a relay initiates a payout to the next hop in the circuit, the values were computed incorrectly. This PR fixes the behaviour and extends the payout tests to capture such bugs.

src/tribler-core/tribler_core/modules/tunnel/community/triblertunnel_community.py

@@ -354,7 +354,7 @@ def on_payout(self, source_address: Address, data: bytes) -> None:
         if payload.circuit_id in self.relay_from_to and tx.amount > payload.base_amount:
             relay = self.relay_from_to[payload.circuit_id]
             self._logger.info("Sending next payout to peer %s", relay.peer)
-            self.do_payout(relay.peer, relay.circuit_id, tx.amount - payload.base_amount * 2, payload.base_amount)
+            self.do_payout(relay.peer, relay.circuit_id, payload.base_amount * 2, payload.base_amount)
 
     def clean_from_slots(self, circuit_id):
         """

src/tribler-core/tribler_core/modules/tunnel/tests/test_triblertunnel_community.py

@@ -282,6 +282,15 @@ async def test_payouts(self):
         self.add_node_to_experiment(self.create_node())
         self.add_node_to_experiment(self.create_node())
 
+        # Make sure that every node has some initial transactions. This will help us to detect bugs in the
+        # relay payout logic, e.g. https://github.com/Tribler/tribler/issues/5789.
+        for node in self.nodes:
+            for other_node in self.nodes:
+                if node == other_node:
+                    continue
+
+                await node.overlay.bandwidth_community.do_payout(other_node.my_peer, 100 * 1024 * 1024)
+
         # Build a tunnel
         self.nodes[2].overlay.settings.peer_flags.add(PEER_FLAG_EXIT_BT)
         await self.introduce_nodes()
@@ -302,6 +311,15 @@ async def test_payouts(self):
         self.assertTrue(self.nodes[1].overlay.bandwidth_community.database.get_my_balance() > 0)
         self.assertTrue(self.nodes[2].overlay.bandwidth_community.database.get_my_balance() > 0)
 
+        balances = []
+        for node_nr in [0, 1, 2]:
+            balances.append(self.nodes[node_nr].overlay.bandwidth_community.database.get_my_balance())
+
+        balances.sort()
+        self.assertEqual(balances[0], -750 * 1024 * 1024)
+        self.assertEqual(balances[1], 250 * 1024 * 1024)
+        self.assertEqual(balances[2], 500 * 1024 * 1024)
+
     async def test_invalid_payout(self):
         """
         Test whether an invalid payout to another peer is ignored