update negative interest check to use relative tolerance for log level (

#1735)

    If the rate went down, post a log
    If the pool is paused, log is warning regardless of relative rate change
    If the pool is not paused and the rate went down by more than the relative tol, log is critical
    If the pool is not paused, but the rate did not go down by more than the relative tol, log is warning

src/agent0/hyperfuzz/system_fuzz/invariant_checks.py

@@ -23,7 +23,8 @@
 
 LP_SHARE_PRICE_EPSILON = 1e-4
 TOTAL_SHARES_EPSILON = 1e-9
-NEGATIVE_INTEREST_EPSILON = FixedPoint(scaled_value=10)  # 10 wei
+NEGATIVE_INTEREST_ATOL = FixedPoint(scaled_value=10)  # absolute tolerance 10 wei
+NEGATIVE_INTEREST_RTOL = FixedPoint(0.001)  # relative tolerance 10bp
 PRESENT_VALUE_EPSILON = FixedPoint(scaled_value=1)  # 1 wei
 EZETH_NEG_INTEREST_TIME_DELTA = 12 * 60 * 60
 
@@ -279,6 +280,13 @@ def _check_price_spike(interface: HyperdriveReadInterface, pool_state: PoolState
 
 
 def _check_negative_interest(interface: HyperdriveReadInterface, pool_state: PoolState) -> InvariantCheckResults:
+    """Check for negative interest in the pool.
+
+    If the rate went down, post a log
+    If the pool is paused, log is warning regardless of relative rate change
+    If the pool is not paused and the rate went down by more than the relative tol, log is critical
+    If the pool is not paused, but the rate did not go down by more than the relative tol, log is warning
+    """
     # Hyperdrive base & eth balances should always be zero
     failed = False
     exception_message: str | None = None
@@ -303,13 +311,16 @@ def _check_negative_interest(interface: HyperdriveReadInterface, pool_state: Poo
     previous_pool_state = interface.get_hyperdrive_state(block_identifier=previous_block_number)
     previous_vault_share_price = previous_pool_state.pool_info.vault_share_price
 
-    if (current_vault_share_price - previous_vault_share_price) <= -NEGATIVE_INTEREST_EPSILON:
+    # Absolute check to see if the rate went down
+    if current_vault_share_price - previous_vault_share_price <= -NEGATIVE_INTEREST_ATOL:
+        log_level = logging.WARNING
         if interface.get_pool_is_paused():
             paused_str = "paused"
-            log_level = logging.WARNING
         else:
             paused_str = "unpaused"
-            log_level = logging.CRITICAL
+            # relative check to see if the rate went down by a lot
+            if previous_vault_share_price * (FixedPoint(1) - NEGATIVE_INTEREST_RTOL) >= current_vault_share_price:
+                log_level = logging.CRITICAL
         failed = True
         exception_data["invariance_check:current_vault_share_price"] = current_vault_share_price
         exception_data["invariance_check:previous_vault_share_price"] = previous_vault_share_price