From 96cef5260c4ed0d1b1fec9ba648f8eeea24e619e Mon Sep 17 00:00:00 2001
From: Justin Starry <justin@solana.com>
Date: Tue, 25 May 2021 13:01:21 -0700
Subject: [PATCH] Add a flag to simulateTransaction to use most recent
 blockhash

---
 client/src/rpc_config.rs                   | 2 ++
 core/src/rpc.rs                            | 5 ++++-
 docs/src/developing/clients/jsonrpc-api.md | 1 +
 3 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/client/src/rpc_config.rs b/client/src/rpc_config.rs
index dba7bdd200..03c5370413 100644
--- a/client/src/rpc_config.rs
+++ b/client/src/rpc_config.rs
@@ -28,6 +28,8 @@ pub struct RpcSendTransactionConfig {
 pub struct RpcSimulateTransactionConfig {
     #[serde(default)]
     pub sig_verify: bool,
+    #[serde(default)]
+    pub use_most_recent_blockhash: bool,
     #[serde(flatten)]
     pub commitment: Option<CommitmentConfig>,
     pub encoding: Option<UiTransactionEncoding>,
diff --git a/core/src/rpc.rs b/core/src/rpc.rs
index 78521da701..e229563815 100644
--- a/core/src/rpc.rs
+++ b/core/src/rpc.rs
@@ -2969,7 +2969,7 @@ pub mod rpc_full {
             debug!("simulate_transaction rpc request received");
             let config = config.unwrap_or_default();
             let encoding = config.encoding.unwrap_or(UiTransactionEncoding::Base58);
-            let (_, transaction) = deserialize_transaction(data, encoding)?;
+            let (_, mut transaction) = deserialize_transaction(data, encoding)?;
 
             if config.sig_verify {
                 if let Err(e) = verify_transaction(&transaction) {
@@ -2978,6 +2978,9 @@ pub mod rpc_full {
             }
 
             let bank = &*meta.bank(config.commitment);
+            if config.use_most_recent_blockhash {
+                transaction.message.recent_blockhash = bank.last_blockhash();
+            }
             let (result, logs) = bank.simulate_transaction(transaction);
 
             Ok(new_response(
diff --git a/docs/src/developing/clients/jsonrpc-api.md b/docs/src/developing/clients/jsonrpc-api.md
index 6fb0113e0c..67606b5ec7 100644
--- a/docs/src/developing/clients/jsonrpc-api.md
+++ b/docs/src/developing/clients/jsonrpc-api.md
@@ -3206,6 +3206,7 @@ Simulate sending a transaction
   - `sigVerify: <bool>` - if true the transaction signatures will be verified (default: false)
   - `commitment: <string>` - (optional) [Commitment](jsonrpc-api.md#configuring-state-commitment) level to simulate the transaction at (default: `"finalized"`).
   - `encoding: <string>` - (optional) Encoding used for the transaction data. Either `"base58"` (*slow*, **DEPRECATED**), or `"base64"`. (default: `"base58"`).
+  - `useMostRecentBlockhash: <bool>` - (optional) if true the transaction recent blockhash will be ignored and overridden with the most recent blockhash. (default: false)
 
 #### Results: