Compute volume based protocol fee in surplus token (#346)

This PR changes the behavior of protocol fees on volume and volume based
cap be consistent with the change from
cowprotocol/services#2465.

Instead of charging volume based fees in the sell token, they are now
charged in the surplus token. This simplifies the computation of
protocol fees.

The code in dune-sync needs to be changed as well.

queries/orderbook/batch_rewards.sql

@@ -86,7 +86,7 @@ order_protocol_fee AS (
                 -- impossible anyways. This query will return a division by
                 -- zero error in that case.
                 LEAST(
-                    fp.max_volume_factor * os.sell_amount * os.buy_amount / (os.sell_amount - os.observed_fee),
+                    fp.max_volume_factor / (1 - fp.max_volume_factor) * os.buy_amount,
                     -- at most charge a fraction of volume
                     fp.surplus_factor / (1 - fp.surplus_factor) * surplus -- charge a fraction of surplus
                 )
@@ -96,12 +96,14 @@ order_protocol_fee AS (
                     fp.surplus_factor / (1 - fp.surplus_factor) * surplus -- charge a fraction of surplus
                 )
             END
-            WHEN fp.kind = 'volume' THEN fp.volume_factor / (1 + fp.volume_factor) * os.sell_amount
+            WHEN fp.kind = 'volume' THEN CASE
+                WHEN os.kind = 'sell' THEN
+                    fp.volume_factor / (1 - fp.volume_factor) * os.buy_amount
+                WHEN os.kind = 'buy' THEN
+                    fp.volume_factor / (1 + fp.volume_factor) * os.sell_amount
+            END
         END AS protocol_fee,
-        CASE
-            WHEN fp.kind = 'surplus' THEN os.surplus_token
-            WHEN fp.kind = 'volume' THEN os.sell_token
-        END AS protocol_fee_token
+        os.surplus_token AS protocol_fee_token
     FROM
         order_surplus os
         JOIN fee_policies fp -- contains protocol fee policy
@@ -268,4 +270,4 @@ select
 from
     aggregate_results
 order by
-    solver
+    solver

tests/queries/test_batch_rewards.py

@@ -29,7 +29,7 @@ def test_get_batch_rewards(self):
                     "0x5444444444444444444444444444444444444444",
                 ],
                 "payment_eth": [
-                    9.534313722772278e15,
+                    9535064849462312.0,
                     10.5e15,
                     6600000000000000.00000,
                     12450000000000000.00000,
@@ -53,7 +53,7 @@ def test_get_batch_rewards(self):
                     6,
                 ],
                 "protocol_fee_eth": [
-                    5.748876684972541e14,  # 0.5 / (1 - 0.5) * 1e18 * 5e14 / 1e18 + 0.0015 / (1 + 0.0015) * 1e8 * 5e26 / 1e18
+                    571357035553330.0,  # 0.5 / (1 - 0.5) * 1e18 * 5e14 / 1e18 + 0.0015 / (1 - 0.0015) * 95e18 * 5e14 / 1e18
                     2.0198019801980198e15,  # 0.75 / (1 - 0.75) * 1e6 * 5e26 / 1e18 + 0.01 / (1 + 0.01) * 105e6 * 5e26 / 1e18
                     0.0,
                     0.0,