test: fixing stateful test edge cases

* removed sanity checks from ramping tests since continuous running could lead to xcp being less than 1.

* virtual price invariant is now checked only after at least a swap has been done. In some cases you can have a balanced deposit followed by an unbalanced withdrawal which would break this invariant (tentative fix).

* removed assertion about equilibrium since it wasn't sensitive enough for small amounts.

tests/unitary/pool/stateful/stateful_base.py

@@ -7,6 +7,7 @@
     RuleBasedStateMachine,
     initialize,
     invariant,
+    precondition,
     rule,
 )
 from hypothesis.strategies import integers
@@ -57,6 +58,8 @@ def initialize_pool(self, pool, amount, user):
 
         self.equilibrium = 5e17
 
+        self.swapped_once = False
+
         self.fee_receiver = factory.at(pool.factory()).fee_receiver()
         self.admin = factory.at(pool.factory()).admin()
 
@@ -130,10 +133,6 @@ def report_equilibrium(self):
 
         self.equilibrium = (xp + yp) / self.pool.D()
 
-        assert (
-            self.equilibrium != old_equilibrium
-        ), "equlibrium didn't change after an imbalanced operation"
-
         # we compute the percentage change from the old equilibrium
         # to have a sense of how much an operation changed the pool
         percentage_change = (
@@ -288,6 +287,7 @@ def exchange(self, dx: int, i: int, user: str) -> bool:
             )
         )
 
+        self.swapped_once = True
         return True
 
     def remove_liquidity(self, amount: int, user: str):
@@ -558,6 +558,7 @@ def sanity_check(self):
                 self.pool.balanceOf(d) > 0
             ), "tracked depositors should not have 0 lp tokens"
 
+    @precondition(lambda self: self.swapped_once)
     @invariant()
     def virtual_price(self):
         assert (self.pool.virtual_price() - 1e18) * 2 >= (

tests/unitary/pool/stateful/test_stateful.py

@@ -333,6 +333,10 @@ def up_only_profit(self):
         # we disable this invariant because ramping can lead to losses
         pass
 
+    def sanity_check(self):
+        # we disable this invariant because ramping can lead to losses
+        pass
+
 
 TestOnlySwap = OnlySwapStateful.TestCase
 TestUpOnlyLiquidity = UpOnlyLiquidityStateful.TestCase