Add new Instructions to already existing transactions

[Moses-Alero]

I would like to know if it is possible to add new instructions to pre-existing transactions similar to transaction.add() in the @solana/web3.js package.

I tried to do something to make it work using the example below

var feePayer, _ = types.AccountFromBase58("4TMFNY9ntAn3CHzguSAvDNLPRoQTaK3sWbQQXdDXaE6KWRBLufGL6PJdsD2koiEe3gGmMdRK3aAw7sikGNksHJrN")

// 9aE476sH92Vz7DMPyq5WLPkrKWivxeuTKEFKd2sZZcde
var alice, _ = types.AccountFromBase58("4voSPg3tYuWbKzimpQK9EbXHmuyy5fUrtXvpLDMLkmY6TRncaTHAKGD8jUg3maB5Jbrd9CkQg4qjJMyN6sQvnEF2")

func main() {
	c := client.NewClient(rpc.DevnetRPCEndpoint)

	// to fetch recent blockhash
	recentBlockhashResponse, err := c.GetLatestBlockhash(context.Background())
	if err != nil {
		log.Fatalf("failed to get recent blockhash, err: %v", err)
	}

	// create a transfer tx
	tx, err := types.NewTransaction(types.NewTransactionParam{
		Signers: []types.Account{feePayer, alice},
		Message: types.NewMessage(types.NewMessageParam{
			FeePayer:        feePayer.PublicKey,
			RecentBlockhash: recentBlockhashResponse.Blockhash,
			Instructions: []types.Instruction{
				system.Transfer(system.TransferParam{
					From:   alice.PublicKey,
					To:     common.PublicKeyFromString("2xNweLHLqrbx4zo1waDvgWJHgsUpPj8Y8icbAFeR4a8i"),
					Amount: 1e8, // 0.1 SOL
				}),
			},
		}),
	})
	if err != nil {
		log.Fatalf("failed to new a transaction, err: %v", err)
	}
	//new instruction
	newInstruction := system.Transfer(system.TransferParam{
		From:   alice.PublicKey,
		To:     common.PublicKeyFromString("2xNweLHLqrbx4zo1waDvgWJHgsUpPj8Y8icbAFeR4a8i"),
		Amount: 1e8, // 0.1 SOL
	})

	//add new instruction to pre-existing Transaction
	msg := types.NewMessage(types.NewMessageParam{
		Instructions: []types.Instruction{newInstruction},
	})
	msgByte, _ := msg.Serialize()

	newMsg, _ := types.MessageDeserialize(msgByte)
	
	tx.Message.Instructions = append(tx.Message.Instructions, newMsg.Instructions...)

        //get transaction hash
	txhash, err := c.SendTransaction(context.Background(), tx)
	if err != nil {
		log.Fatalf("failed to send tx, err: %v", err)
	}

	log.Println("txhash:", txhash)
}

I tried to make it work using the example above.
It returns tx hash but the second instruction isn't added to the transaction when viewed on the explorar
Is there a way to do this?

[yihau]

your example might not work. tx.Message.Instructions is compiled instruction. you need the same account list to generate a correct compiled instruction. we can do something similar to transaction.add() by this SDK but it's a bit tricky atm. could you provide more details about your scenario? does it help if you pass the instructions instead then compose them into a transaction in the finial step?

[Moses-Alero]

@yihau my current use case requires the transaction to be passed in as an argument to a function then an additional instruction which is the new instruction is then added to the transaction before it is then used.
Here's a simple example

func CreatePostRes(tx types.Transaction, ref common.PublicKey){
     //do simple check
     if len(tx.Message.Instruction) < 1 {
        return
     }
     instruction := fetchNewInstruction(ref)
     
     //append instruction to tx
     
     tx = AddIxToTransaction(tx, instruction) //we do the tricky stuff here
   
   ...
}

Something like the above.

If only there was a way to decompile instructions then compile them back with the data intact. I think that might be helpful too.
you said that doing something similar to transaction.add() is possible but tricky. I'm pretty sure I can handle tricky :)

[debugger84]

I also need an analog of the transaction.add() in the @solana/web3.js package.

I want to send a transaction with the instruction compute_budget.SetComputeUnitPrice. To send it I need to call a Helius API to get the right number of lamports of fee (https://docs.helius.dev/solana-apis/priority-fee-api#examples). But this API gets the serialized transaction for the right prediction of the fee amount. And only after that I should add the instruction to the transaction, sign it and send.