RPC simulateTransaction endpoint now returns program log output

client/src/rpc_client.rs

@@ -129,7 +129,7 @@ impl RpcClient {
         &self,
         transaction: &Transaction,
         sig_verify: bool,
-    ) -> RpcResult<TransactionStatus> {
+    ) -> RpcResult<RpcSimulateTransactionResult> {
         let serialized_encoded = bs58::encode(serialize(transaction).unwrap()).into_string();
         self.send(
             RpcRequest::SimulateTransaction,

client/src/rpc_response.rs

@@ -211,6 +211,13 @@ pub struct RpcSignatureConfirmation {
     pub status: Result<()>,
 }
 
+#[derive(Serialize, Deserialize, Clone, Debug)]
+#[serde(rename_all = "camelCase")]
+pub struct RpcSimulateTransactionResult {
+    pub err: Option<TransactionError>,
+    pub logs: Option<Vec<String>>,
+}
+
 #[derive(Serialize, Deserialize, Clone, Debug)]
 #[serde(rename_all = "camelCase")]
 pub struct RpcStorageTurn {

core/src/banking_stage.rs

@@ -514,7 +514,7 @@ impl BankingStage {
             vec![]
         };
         let (mut loaded_accounts, results, mut retryable_txs, tx_count, signature_count) =
-            bank.load_and_execute_transactions(batch, MAX_PROCESSING_AGE);
+            bank.load_and_execute_transactions(batch, MAX_PROCESSING_AGE, None);
         load_execute_time.stop();
 
         let freeze_lock = bank.freeze_lock();

core/src/rpc.rs

@@ -25,7 +25,7 @@ use solana_ledger::{
     bank_forks::BankForks, blockstore::Blockstore, blockstore_db::BlockstoreError,
 };
 use solana_perf::packet::PACKET_DATA_SIZE;
-use solana_runtime::{accounts::AccountAddressFilter, bank::Bank};
+use solana_runtime::{accounts::AccountAddressFilter, bank::Bank, log_collector::LogCollector};
 use solana_sdk::{
     clock::{Epoch, Slot, UnixTimestamp},
     commitment_config::{CommitmentConfig, CommitmentLevel},
@@ -718,14 +718,22 @@ fn verify_signature(input: &str) -> Result<Signature> {
 }
 
 /// Run transactions against a frozen bank without committing the results
-fn run_transaction_simulation(bank: &Bank, transaction: Transaction) -> transaction::Result<()> {
+fn run_transaction_simulation(
+    bank: &Bank,
+    transaction: Transaction,
+) -> (transaction::Result<()>, Vec<String>) {
     assert!(bank.is_frozen(), "simulation bank must be frozen");
 
     let txs = &[transaction];
     let batch = bank.prepare_simulation_batch(txs);
+    let log_collector = LogCollector::default();
     let (_loaded_accounts, executed, _retryable_transactions, _transaction_count, _signature_count) =
-        bank.load_and_execute_transactions(&batch, solana_sdk::clock::MAX_PROCESSING_AGE);
-    executed[0].0.clone().map(|_| ())
+        bank.load_and_execute_transactions(
+            &batch,
+            solana_sdk::clock::MAX_PROCESSING_AGE,
+            Some(&log_collector),
+        );
+    (executed[0].0.clone().map(|_| ()), log_collector.output())
 }
 
 #[derive(Clone)]
@@ -932,7 +940,7 @@ pub trait RpcSol {
         meta: Self::Metadata,
         data: String,
         config: Option<RpcSimulateTransactionConfig>,
-    ) -> RpcResponse<TransactionStatus>;
+    ) -> RpcResponse<RpcSimulateTransactionResult>;
 
     #[rpc(meta, name = "getSlotLeader")]
     fn get_slot_leader(
@@ -1452,7 +1460,7 @@ impl RpcSol for RpcSolImpl {
             }
 
             let bank = &*meta.request_processor.read().unwrap().bank(None)?;
-            if let Err(err) = run_transaction_simulation(&bank, transaction) {
+            if let (Err(err), _log_output) = run_transaction_simulation(&bank, transaction) {
                 // Note: it's possible that the transaction simulation failed but the actual
                 // transaction would succeed, such as when a transaction depends on an earlier
                 // transaction that has yet to reach max confirmations. In these cases the user
@@ -1486,7 +1494,7 @@ impl RpcSol for RpcSolImpl {
         meta: Self::Metadata,
         data: String,
         config: Option<RpcSimulateTransactionConfig>,
-    ) -> RpcResponse<TransactionStatus> {
+    ) -> RpcResponse<RpcSimulateTransactionResult> {
         let (_, transaction) = deserialize_bs58_transaction(data)?;
         let config = config.unwrap_or_default();
 
@@ -1497,18 +1505,19 @@ impl RpcSol for RpcSolImpl {
         };
 
         let bank = &*meta.request_processor.read().unwrap().bank(None)?;
-
-        if result.is_ok() {
-            result = run_transaction_simulation(&bank, transaction);
-        }
+        let logs = if result.is_ok() {
+            let sim_result = run_transaction_simulation(&bank, transaction);
+            result = sim_result.0;
+            Some(sim_result.1)
+        } else {
+            None
+        };
 
         new_response(
             &bank,
-            TransactionStatus {
-                slot: bank.slot(),
-                confirmations: Some(0),
-                status: result.clone(),
+            RpcSimulateTransactionResult {
                 err: result.err(),
+                logs,
             },
         )
     }
@@ -2400,7 +2409,7 @@ pub mod tests {
             "jsonrpc": "2.0",
             "result": {
                 "context":{"slot":0},
-                "value":{"confirmations":0,"slot": 0,"status":{"Ok":null},"err":null}
+                "value":{"err":null, "logs":[]}
             },
             "id": 1,
         });
@@ -2420,7 +2429,7 @@ pub mod tests {
             "jsonrpc": "2.0",
             "result": {
                 "context":{"slot":0},
-                "value":{"confirmations":0,"slot":0,"status":{"Err":"SignatureFailure"},"err":"SignatureFailure"}
+                "value":{"err":"SignatureFailure", "logs":null}
             },
             "id": 1,
         });
@@ -2440,7 +2449,7 @@ pub mod tests {
             "jsonrpc": "2.0",
             "result": {
                 "context":{"slot":0},
-                "value":{"confirmations":0,"slot": 0,"status":{"Ok":null},"err":null}
+                "value":{"err":null, "logs":[]}
             },
             "id": 1,
         });
@@ -2460,7 +2469,7 @@ pub mod tests {
             "jsonrpc": "2.0",
             "result": {
                 "context":{"slot":0},
-                "value":{"confirmations":0,"slot": 0,"status":{"Ok":null},"err":null}
+                "value":{"err":null, "logs":[]}
             },
             "id": 1,
         });

docs/src/apps/jsonrpc-api.md

@@ -1107,6 +1107,10 @@ Simulate sending a transaction
 #### Results:
 
 An RpcResponse containing a TransactionStatus object
+The result will be an RpcResponse JSON object with `value` set to a JSON object with the following fields:
+
+* `err: <object | null>` - Error if transaction failed, null if transaction succeeded. [TransactionError definitions](https://github.com/solana-labs/solana/blob/master/sdk/src/transaction.rs#L14)
+* `logs: <array | null>` - Array of log messages the transaction instructions output during execution, null if simulation failed before the transaction was able to execute (for example due to an invalid blockhash or signature verification failure)
 
 #### Example:
 

programs/bpf/benches/bpf_loader.rs

@@ -155,4 +155,8 @@ impl InvokeContext for MockInvokeContext {
     fn get_programs(&self) -> &[(Pubkey, ProcessInstruction)] {
         &[]
     }
+    fn log_enabled(&self) -> bool {
+        false
+    }
+    fn log(&mut self, _message: &str) {}
 }

programs/bpf_loader/src/lib.rs

@@ -267,6 +267,7 @@ mod tests {
     #[derive(Debug, Default)]
     pub struct MockInvokeContext {
         key: Pubkey,
+        pub log: Vec<String>,
     }
     impl InvokeContext for MockInvokeContext {
         fn push(&mut self, _key: &Pubkey) -> Result<(), InstructionError> {
@@ -287,6 +288,13 @@ mod tests {
         fn get_programs(&self) -> &[(Pubkey, ProcessInstruction)] {
             &[]
         }
+        fn log_enabled(&self) -> bool {
+            true
+        }
+        fn log(&mut self, message: &str) {
+            info!("[MockInvokeContext::log] {}", message);
+            self.log.push(message.to_string());
+        }
     }
 
     #[test]