Bug: z_createbuildinstructionscoincontrol returns TransactionBuilder without setting checksum

[HackZhong]

Bug: z_createbuildinstructionscoincontrol returns TransactionBuilder without setting checksum
Version

Pirate v6.0.3-842c598

Commit:

842c598

Description

After upgrading to v6.0.3, using the following RPC flow fails:

z_createbuildinstructionscoincontrol
-> z_buildrawtransaction

z_createbuildinstructionscoincontrol successfully returns a hex-encoded TransactionBuilder, but passing that result directly to z_buildrawtransaction returns:

{
"result": null,
"error": {
"code": -22,
"message": "Transaction builder checksum is invalid."
}
}
Steps to reproduce
Call z_createbuildinstructionscoincontrol with valid Sapling inputs and outputs:
z_createbuildinstructionscoincontrol inputs outputs fee expiryBlocks
The RPC successfully returns a hex string.
Pass that returned hex string directly to:
z_buildrawtransaction "<builder_hex>"
z_buildrawtransaction fails with:
Transaction builder checksum is invalid.

[HackZhong]

This is the suggestion from GPT. Hope it's helpful:

In z_createbuildinstructions, the checksum is explicitly set before serializing the TransactionBuilder:

tb.SetChecksum();

CDataStream ssTb(SER_NETWORK, PROTOCOL_VERSION);
ssTb << tb;
return HexStr(ssTb.begin(), ssTb.end());

However, z_createbuildinstructionscoincontrol serializes the builder without calling SetChecksum():

if (total < 0)
throw JSONRPCError(
RPC_INVALID_PARAMETER,
"Output values plus tx fee are greater than input values"
);

CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
ssTx << tx;
return HexStr(ssTx.begin(), ssTx.end());

Then z_buildrawtransaction deserializes the returned builder and immediately validates its checksum:

if (!tb.ValidateChecksum()) {
throw JSONRPCError(
RPC_DESERIALIZATION_ERROR,
"Transaction builder checksum is invalid."
);
}

Therefore, the output generated by z_createbuildinstructionscoincontrol cannot be consumed by z_buildrawtransaction.

Suggested fix

Call SetChecksum() before serializing the TransactionBuilder in z_createbuildinstructionscoincontrol:

if (total < 0)
throw JSONRPCError(
RPC_INVALID_PARAMETER,
"Output values plus tx fee are greater than input values"
);

• tx.SetChecksum();

CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
ssTx << tx;
return HexStr(ssTx.begin(), ssTx.end());

This would make the behavior consistent with z_createbuildinstructions and allow the returned builder hex to pass z_buildrawtransaction checksum validation.

Expected behavior

The TransactionBuilder returned by z_createbuildinstructionscoincontrol should contain a valid checksum and should be accepted by z_buildrawtransaction.

Actual behavior

z_createbuildinstructionscoincontrol returns a TransactionBuilder without setting its checksum, causing z_buildrawtransaction to always reject it with error -22.