Support for returnData in simulate transaction

web3.js/src/connection.ts

@@ -711,13 +711,20 @@ export type SimulatedTransactionAccountInfo = {
   rentEpoch?: number;
 };
 
+export type TransactionReturnDataEncoding = 'base64';
+
+export type TransactionReturnData = {
+  programId: string;
+  data: [string, TransactionReturnDataEncoding];
+};
+
 export type SimulatedTransactionResponse = {
   err: TransactionError | string | null;
   logs: Array<string> | null;
   accounts?: (SimulatedTransactionAccountInfo | null)[] | null;
   unitsConsumed?: number;
+  returnData?: TransactionReturnData | null;
 };
-
 const SimulatedTransactionResponseStruct = jsonRpcResultAndContext(
   pick({
     err: nullable(union([pick({}), string()])),
@@ -738,6 +745,14 @@ const SimulatedTransactionResponseStruct = jsonRpcResultAndContext(
       ),
     ),
     unitsConsumed: optional(number()),
+    returnData: optional(
+      nullable(
+        pick({
+          programId: string(),
+          data: tuple([string(), literal('base64')]),
+        }),
+      ),
+    ),
   }),
 );
 

web3.js/test/bpf-loader.test.ts

@@ -147,6 +147,25 @@ if (process.env.TEST_LIVE) {
         );
       });
 
+      it('simulate transaction with returnData', async () => {
+        const simulatedTransaction = new Transaction().add({
+          keys: [
+            {pubkey: payerAccount.publicKey, isSigner: true, isWritable: true},
+          ],
+          programId: program.publicKey,
+        });
+        const {err, returnData} = (
+          await connection.simulateTransaction(simulatedTransaction, [
+            payerAccount,
+          ])
+        ).value;
+        const expectedReturnData = new Uint8Array([1, 2, 3]);
+        var decodedData = Buffer.from(returnData.data[0], returnData.data[1]);
+        expect(err).to.be.null;
+        expect(returnData.programId).to.eql(program.publicKey.toString());
+        expect(decodedData).to.eql(expectedReturnData);
+      });
+
       it('deprecated - simulate transaction without signature verification', async () => {
         const simulatedTransaction = new Transaction().add({
           keys: [

web3.js/test/connection.test.ts

@@ -3756,6 +3756,53 @@ describe('Connection', function () {
     verifySignatureStatus(response, expectedErr);
   });
 
+  if (mockServer) {
+    it('returnData on simulateTransaction', async () => {
+      const tx = new Transaction();
+      tx.feePayer = Keypair.generate().publicKey;
+
+      const getLatestBlockhashResponse = {
+        method: 'getLatestBlockhash',
+        params: [],
+        value: {
+          blockhash: 'CSymwgTNX1j3E4qhKfJAUE41nBWEwXufoYryPbkde5RR',
+          feeCalculator: {
+            lamportsPerSignature: 5000,
+          },
+          lastValidBlockHeight: 51,
+        },
+        withContext: true,
+      };
+      const simulateTransactionResponse = {
+        method: 'simulateTransaction',
+        params: [],
+        value: {
+          err: null,
+          accounts: null,
+          logs: [
+            'Program 83astBRguLMdt2h5U1Tpdq5tjFoJ6noeGwaY3mDLVcri invoke [1]',
+            'Program 83astBRguLMdt2h5U1Tpdq5tjFoJ6noeGwaY3mDLVcri consumed 2366 of 1400000 compute units',
+            'Program return: 83astBRguLMdt2h5U1Tpdq5tjFoJ6noeGwaY3mDLVcri KgAAAAAAAAA=',
+            'Program 83astBRguLMdt2h5U1Tpdq5tjFoJ6noeGwaY3mDLVcri success',
+          ],
+          returnData: {
+            data: ['KgAAAAAAAAA==', 'base64'],
+            programId: '83astBRguLMdt2h5U1Tpdq5tjFoJ6noeGwaY3mDLVcri',
+          },
+          unitsConsumed: 2366,
+        },
+        withContext: true,
+      };
+      await mockRpcResponse(getLatestBlockhashResponse);
+      await mockRpcResponse(simulateTransactionResponse);
+      const response = (await connection.simulateTransaction(tx)).value;
+      expect(response.returnData).to.eql({
+        data: ['KgAAAAAAAAA==', 'base64'],
+        programId: '83astBRguLMdt2h5U1Tpdq5tjFoJ6noeGwaY3mDLVcri',
+      });
+    });
+  }
+
   if (process.env.TEST_LIVE) {
     it('simulate transaction with message', async () => {
       connection._commitment = 'confirmed';

web3.js/test/fixtures/noop-program/src/lib.rs

@@ -1,7 +1,8 @@
 //! Example Rust-based BPF program that prints out the parameters passed to it
 
 use solana_program::{
-    account_info::AccountInfo, entrypoint::ProgramResult, log::*, msg, pubkey::Pubkey,
+    account_info::AccountInfo, entrypoint::ProgramResult, log::*, msg, program::set_return_data,
+    pubkey::Pubkey,
 };
 
 #[derive(Debug, PartialEq)]
@@ -55,6 +56,12 @@ fn process_instruction(
         panic!();
     }
 
+    {
+        // return data in simulate transaction
+        let return_data: &[u8] = &[1, 2, 3];
+        set_return_data(return_data);
+    }
+
     Ok(())
 }