fix!: adjust gas distribution formula for NEP264 (#6536)

Adjusting formula based on @matklad suggestion. It more closely represents distributing gas based on weights if the sum of weights is greater than the amount of gas remaining at the cost of performing slightly more arithmetic.

I think this makes more sense, but hard to think about the exact implications without some benchmark of the two.

runtime/near-vm-logic/src/receipt_manager.rs

@@ -447,26 +447,22 @@ impl ReceiptManager {
             return GasDistribution::NoRatios;
         }
 
-        // Floor division that will ensure gas allocated is <= gas to distribute
-        let gas_per_weight = (unused_gas as u128 / gas_weight_sum) as Gas;
-
         let mut distribute_gas = |index: &FunctionCallActionIndex, assigned_gas: Gas| {
             let FunctionCallAction { gas, .. } =
                 get_fuction_call_action_mut(&mut self.action_receipts, *index);
 
-            // This operation cannot overflow because the gas_per_weight calculation is a floor
-            // division of the total amount of gas by the weight sum and the remainder is
-            // distributed exactly.
+            // Operation cannot overflow because the amount of assigned gas is a fraction of
+            // the unused gas and is using floor division.
             *gas += assigned_gas;
         };
 
         let mut distributed = 0;
         for (action_index, GasWeight(weight)) in &self.gas_weights {
-            // This can't overflow because the gas_per_weight is floor division
-            // of the weight sum.
-            let assigned_gas = gas_per_weight * weight;
+            // Multiplication is done in u128 with max values of u64::MAX so this cannot overflow.
+            // Division result can be truncated to 64 bits because gas_weight_sum >= weight.
+            let assigned_gas = (unused_gas as u128 * *weight as u128 / gas_weight_sum) as u64;
 
-            distribute_gas(action_index, assigned_gas);
+            distribute_gas(action_index, assigned_gas as u64);
 
             distributed += assigned_gas
         }

runtime/near-vm-logic/src/tests/gas_counter.rs

@@ -195,7 +195,14 @@ fn function_call_weight_basic_cases_test() {
     ]);
 
     // Weight over u64 bounds
-    function_call_weight_check(&[(0, u64::MAX, 0), (0, 1000, 10_000_000_000)]);
+    function_call_weight_check(&[(0, u64::MAX, 9_999_999_999), (0, 1000, 1)]);
+
+    // Weight over gas limit with three function calls
+    function_call_weight_check(&[
+        (0, 10_000_000_000, 4_999_999_999),
+        (0, 1, 0),
+        (0, 10_000_000_000, 5_000_000_001),
+    ]);
 
     // Weights with one zero and one non-zero
     function_call_weight_check(&[(0, 0, 0), (0, 1, 10_000_000_000)])