feat(mempool): add pending queue to tx queue

crates/mempool/src/transaction_queue.rs

@@ -11,6 +11,9 @@ use crate::mempool::TransactionReference;
 pub struct TransactionQueue {
     // Priority queue of transactions with associated priority.
     priority_queue: BTreeSet<QueuedTransaction>,
+    // queue of transaction with low gas price.
+    pending_queue: BTreeSet<PendingTransaction>,
+    gas_price_threshold: u128,
     // Set of account addresses for efficient existence checks.
     address_to_tx: HashMap<ContractAddress, TransactionReference>,
 }
@@ -27,10 +30,17 @@ impl TransactionQueue {
             "Only a single transaction from the same contract class can be in the mempool at a \
              time."
         );
-        assert!(
-            self.priority_queue.insert(tx.into()),
-            "Keys should be unique; duplicates are checked prior."
-        );
+        if tx.get_l2_gas_price() < Some(self.gas_price_threshold) {
+            assert!(
+                self.pending_queue.insert(tx.into()),
+                "Keys should be unique; duplicates are checked prior."
+            );
+        } else {
+            assert!(
+                self.priority_queue.insert(tx.into()),
+                "Keys should be unique; duplicates are checked prior."
+            );
+        }
     }
 
     // TODO(gilad): remove collect
@@ -58,13 +68,14 @@ impl TransactionQueue {
     /// This is well-defined, since there is at most one transaction per address in the queue.
     pub fn remove(&mut self, address: ContractAddress) -> bool {
         if let Some(tx) = self.address_to_tx.remove(&address) {
-            return self.priority_queue.remove(&tx.into());
+            return self.priority_queue.remove(&tx.clone().into())
+                || self.pending_queue.remove(&tx.into());
         }
         false
     }
 
     pub fn is_empty(&self) -> bool {
-        self.priority_queue.is_empty()
+        self.priority_queue.is_empty() && self.pending_queue.is_empty()
     }
 }