From f0f4784c1ccf8cdafb3c72b8f251fa5290174297 Mon Sep 17 00:00:00 2001
From: "Johan T. Halseth" <johanth@gmail.com>
Date: Mon, 26 Apr 2021 13:53:22 +0200
Subject: [PATCH 1/2] mining: extract witness commitment add into method

---
 mining/mining.go | 78 +++++++++++++++++++++++++++---------------------
 1 file changed, 44 insertions(+), 34 deletions(-)

diff --git a/mining/mining.go b/mining/mining.go
index 44ec7dc7..e918328d 100644
--- a/mining/mining.go
+++ b/mining/mining.go
@@ -803,40 +803,7 @@ mempoolLoop:
 	// OP_RETURN output within the coinbase transaction.
 	var witnessCommitment []byte
 	if witnessIncluded {
-		// The witness of the coinbase transaction MUST be exactly 32-bytes
-		// of all zeroes.
-		var witnessNonce [blockchain.CoinbaseWitnessDataLen]byte
-		coinbaseTx.MsgTx().TxIn[0].Witness = wire.TxWitness{witnessNonce[:]}
-
-		// Next, obtain the merkle root of a tree which consists of the
-		// wtxid of all transactions in the block. The coinbase
-		// transaction will have a special wtxid of all zeroes.
-		witnessMerkleTree := blockchain.BuildMerkleTreeStore(blockTxns,
-			true)
-		witnessMerkleRoot := witnessMerkleTree[len(witnessMerkleTree)-1]
-
-		// The preimage to the witness commitment is:
-		// witnessRoot || coinbaseWitness
-		var witnessPreimage [64]byte
-		copy(witnessPreimage[:32], witnessMerkleRoot[:])
-		copy(witnessPreimage[32:], witnessNonce[:])
-
-		// The witness commitment itself is the double-sha256 of the
-		// witness preimage generated above. With the commitment
-		// generated, the witness script for the output is: OP_RETURN
-		// OP_DATA_36 {0xaa21a9ed || witnessCommitment}. The leading
-		// prefix is referred to as the "witness magic bytes".
-		witnessCommitment = chainhash.DoubleHashB(witnessPreimage[:])
-		witnessScript := append(blockchain.WitnessMagicBytes, witnessCommitment...)
-
-		// Finally, create the OP_RETURN carrying witness commitment
-		// output as an additional output within the coinbase.
-		commitmentOutput := &wire.TxOut{
-			Value:    0,
-			PkScript: witnessScript,
-		}
-		coinbaseTx.MsgTx().TxOut = append(coinbaseTx.MsgTx().TxOut,
-			commitmentOutput)
+		witnessCommitment = AddWitnessCommitment(coinbaseTx, blockTxns)
 	}
 
 	// Calculate the required difficulty for the block.  The timestamp
@@ -895,6 +862,49 @@ mempoolLoop:
 	}, nil
 }
 
+// AddWitnessCommitment adds the witness commitment as an OP_RETURN outpout
+// within the coinbase tx.  The raw commitment is returned.
+func AddWitnessCommitment(coinbaseTx *btcutil.Tx,
+	blockTxns []*btcutil.Tx) []byte {
+
+	// The witness of the coinbase transaction MUST be exactly 32-bytes
+	// of all zeroes.
+	var witnessNonce [blockchain.CoinbaseWitnessDataLen]byte
+	coinbaseTx.MsgTx().TxIn[0].Witness = wire.TxWitness{witnessNonce[:]}
+
+	// Next, obtain the merkle root of a tree which consists of the
+	// wtxid of all transactions in the block. The coinbase
+	// transaction will have a special wtxid of all zeroes.
+	witnessMerkleTree := blockchain.BuildMerkleTreeStore(blockTxns,
+		true)
+	witnessMerkleRoot := witnessMerkleTree[len(witnessMerkleTree)-1]
+
+	// The preimage to the witness commitment is:
+	// witnessRoot || coinbaseWitness
+	var witnessPreimage [64]byte
+	copy(witnessPreimage[:32], witnessMerkleRoot[:])
+	copy(witnessPreimage[32:], witnessNonce[:])
+
+	// The witness commitment itself is the double-sha256 of the
+	// witness preimage generated above. With the commitment
+	// generated, the witness script for the output is: OP_RETURN
+	// OP_DATA_36 {0xaa21a9ed || witnessCommitment}. The leading
+	// prefix is referred to as the "witness magic bytes".
+	witnessCommitment := chainhash.DoubleHashB(witnessPreimage[:])
+	witnessScript := append(blockchain.WitnessMagicBytes, witnessCommitment...)
+
+	// Finally, create the OP_RETURN carrying witness commitment
+	// output as an additional output within the coinbase.
+	commitmentOutput := &wire.TxOut{
+		Value:    0,
+		PkScript: witnessScript,
+	}
+	coinbaseTx.MsgTx().TxOut = append(coinbaseTx.MsgTx().TxOut,
+		commitmentOutput)
+
+	return witnessCommitment
+}
+
 // UpdateBlockTime updates the timestamp in the header of the passed block to
 // the current time while taking into account the median time of the last
 // several blocks to ensure the new time is after that time per the chain

From 37a6e8485b69f9514d74d153843477c73d1b0473 Mon Sep 17 00:00:00 2001
From: "Johan T. Halseth" <johanth@gmail.com>
Date: Mon, 26 Apr 2021 13:53:51 +0200
Subject: [PATCH 2/2] rpctest: add witness commitment when calling CreateBlock

If we tried to include transactions having witnesses, the block would be
invalid since the witness commitment was not added.
---
 integration/rpctest/blockgen.go | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/integration/rpctest/blockgen.go b/integration/rpctest/blockgen.go
index de5821b0..0d802f5a 100644
--- a/integration/rpctest/blockgen.go
+++ b/integration/rpctest/blockgen.go
@@ -14,6 +14,7 @@ import (
 	"github.com/btcsuite/btcd/blockchain"
 	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
+	"github.com/btcsuite/btcd/mining"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/btcsuite/btcutil"
@@ -181,6 +182,21 @@ func CreateBlock(prevBlock *btcutil.Block, inclusionTxs []*btcutil.Tx,
 	if inclusionTxs != nil {
 		blockTxns = append(blockTxns, inclusionTxs...)
 	}
+
+	// We must add the witness commitment to the coinbase if any
+	// transactions are segwit.
+	witnessIncluded := false
+	for i := 1; i < len(blockTxns); i++ {
+		if blockTxns[i].MsgTx().HasWitness() {
+			witnessIncluded = true
+			break
+		}
+	}
+
+	if witnessIncluded {
+		_ = mining.AddWitnessCommitment(coinbaseTx, blockTxns)
+	}
+
 	merkles := blockchain.BuildMerkleTreeStore(blockTxns, false)
 	var block wire.MsgBlock
 	block.Header = wire.BlockHeader{