Spend down the block budget limit by 25% every block

Signed-off-by: Jacinta Ferrant <[email protected]>

clarity/src/vm/costs/mod.rs

@@ -1170,6 +1170,7 @@ pub trait CostOverflowingMath<T> {
     fn cost_overflow_mul(self, other: T) -> Result<T>;
     fn cost_overflow_add(self, other: T) -> Result<T>;
     fn cost_overflow_sub(self, other: T) -> Result<T>;
+    fn cost_overflow_div(self, other: T) -> Result<T>;
 }
 
 impl CostOverflowingMath<u64> for u64 {
@@ -1185,6 +1186,10 @@ impl CostOverflowingMath<u64> for u64 {
         self.checked_sub(other)
             .ok_or_else(|| CostErrors::CostOverflow)
     }
+    fn cost_overflow_div(self, other: u64) -> Result<u64> {
+        self.checked_div(other)
+            .ok_or_else(|| CostErrors::CostOverflow)
+    }
 }
 
 impl ExecutionCost {
@@ -1293,6 +1298,15 @@ impl ExecutionCost {
         Ok(())
     }
 
+    pub fn divide(&mut self, divisor: u64) -> Result<()> {
+        self.runtime = self.runtime.cost_overflow_div(divisor)?;
+        self.read_count = self.read_count.cost_overflow_div(divisor)?;
+        self.read_length = self.read_length.cost_overflow_div(divisor)?;
+        self.write_length = self.write_length.cost_overflow_div(divisor)?;
+        self.write_count = self.write_count.cost_overflow_div(divisor)?;
+        Ok(())
+    }
+
     /// Returns whether or not this cost exceeds any dimension of the
     ///  other cost.
     pub fn exceeds(&self, other: &ExecutionCost) -> bool {

stackslib/src/chainstate/stacks/miner.rs

@@ -2222,10 +2222,22 @@ impl StacksBlockBuilder {
         let mempool_settings = settings.mempool_settings.clone();
         let ts_start = get_epoch_time_ms();
         let stacks_epoch_id = epoch_tx.get_epoch();
-        let block_limit = epoch_tx
+        let remaining_limit = epoch_tx
             .block_limit()
             .expect("Failed to obtain block limit from miner's block connection");
 
+        // Attempt to spread the cost limit across the tenure by always using only 25% of the remaining budget
+        // until the remaining budget is less than 100
+        let mut target_limit = remaining_limit.clone();
+        let block_limit = if target_limit.divide(100).is_ok() {
+            target_limit.multiply(25).expect(
+                "BUG: Successfully divided by 100, but failed to multiply block limit by 25",
+            );
+            target_limit
+        } else {
+            remaining_limit
+        };
+
         let mut tx_events = Vec::new();
 
         for initial_tx in initial_txs.iter() {