blockchain: Combine ErrDoubleSpend & ErrMissingTx.

This replaces the ErrDoubleSpend and ErrMissingTx error codes with a
single error code named ErrMissingTxOut and updates the relevant errors
and expected test results accordingly.

Once upon a time, the code relied on a transaction index, so it was able
to definitively differentiate between a transaction output that
legitimately did not exist and one that had already been spent.

However, since the code now uses a pruned utxoset, it is no longer
possible to reliably differentiate since once all outputs of a
transaction are spent, it is removed from the utxoset completely.
Consequently, a missing transaction could be either because the
transaction never existed or because it is fully spent.

blockchain/chain.go

@@ -348,9 +348,7 @@ type SequenceLock struct {
 // the candidate transaction to be included in a block.
 //
 // This function is safe for concurrent access.
-func (b *BlockChain) CalcSequenceLock(tx *btcutil.Tx, utxoView *UtxoViewpoint,
-	mempool bool) (*SequenceLock, error) {
-
+func (b *BlockChain) CalcSequenceLock(tx *btcutil.Tx, utxoView *UtxoViewpoint, mempool bool) (*SequenceLock, error) {
 	b.chainLock.Lock()
 	defer b.chainLock.Unlock()
 
@@ -361,9 +359,7 @@ func (b *BlockChain) CalcSequenceLock(tx *btcutil.Tx, utxoView *UtxoViewpoint,
 // transaction. See the exported version, CalcSequenceLock for further details.
 //
 // This function MUST be called with the chain state lock held (for writes).
-func (b *BlockChain) calcSequenceLock(node *blockNode, tx *btcutil.Tx,
-	utxoView *UtxoViewpoint, mempool bool) (*SequenceLock, error) {
-
+func (b *BlockChain) calcSequenceLock(node *blockNode, tx *btcutil.Tx, utxoView *UtxoViewpoint, mempool bool) (*SequenceLock, error) {
 	// A value of -1 for each relative lock type represents a relative time
 	// lock value that will allow a transaction to be included in a block
 	// at any given height or time. This value is returned as the relative
@@ -411,10 +407,11 @@ func (b *BlockChain) calcSequenceLock(node *blockNode, tx *btcutil.Tx,
 	for txInIndex, txIn := range mTx.TxIn {
 		utxo := utxoView.LookupEntry(&txIn.PreviousOutPoint.Hash)
 		if utxo == nil {
-			str := fmt.Sprintf("unable to find unspent output "+
-				"%v referenced from transaction %s:%d",
-				txIn.PreviousOutPoint, tx.Hash(), txInIndex)
-			return sequenceLock, ruleError(ErrMissingTx, str)
+			str := fmt.Sprintf("output %v referenced from "+
+				"transaction %s:%d either does not exist or "+
+				"has already been spent", txIn.PreviousOutPoint,
+				tx.Hash(), txInIndex)
+			return sequenceLock, ruleError(ErrMissingTxOut, str)
 		}
 
 		// If the input height is set to the mempool height, then we

blockchain/error.go

@@ -128,9 +128,9 @@ const (
 	// range or not referencing one at all.
 	ErrBadTxInput
 
-	// ErrMissingTx indicates a transaction referenced by an input is
-	// missing.
-	ErrMissingTx
+	// ErrMissingTxOut indicates a transaction output referenced by an input
+	// either does not exist or has already been spent.
+	ErrMissingTxOut
 
 	// ErrUnfinalizedTx indicates a transaction has not been finalized.
 	// A valid block may only contain finalized transactions.
@@ -150,10 +150,6 @@ const (
 	// coinbase that has not yet reached the required maturity.
 	ErrImmatureSpend
 
-	// ErrDoubleSpend indicates a transaction is attempting to spend coins
-	// that have already been spent.
-	ErrDoubleSpend
-
 	// ErrSpendTooHigh indicates a transaction is attempting to spend more
 	// value than the sum of all of its inputs.
 	ErrSpendTooHigh
@@ -242,12 +238,11 @@ var errorCodeStrings = map[ErrorCode]string{
 	ErrBadTxOutValue:             "ErrBadTxOutValue",
 	ErrDuplicateTxInputs:         "ErrDuplicateTxInputs",
 	ErrBadTxInput:                "ErrBadTxInput",
-	ErrMissingTx:                 "ErrMissingTx",
+	ErrMissingTxOut:              "ErrMissingTxOut",
 	ErrUnfinalizedTx:             "ErrUnfinalizedTx",
 	ErrDuplicateTx:               "ErrDuplicateTx",
 	ErrOverwriteTx:               "ErrOverwriteTx",
 	ErrImmatureSpend:             "ErrImmatureSpend",
-	ErrDoubleSpend:               "ErrDoubleSpend",
 	ErrSpendTooHigh:              "ErrSpendTooHigh",
 	ErrBadFees:                   "ErrBadFees",
 	ErrTooManySigOps:             "ErrTooManySigOps",

blockchain/error_test.go

@@ -38,12 +38,11 @@ func TestErrorCodeStringer(t *testing.T) {
 		{blockchain.ErrDuplicateTxInputs, "ErrDuplicateTxInputs"},
 		{blockchain.ErrBadTxInput, "ErrBadTxInput"},
 		{blockchain.ErrBadCheckpoint, "ErrBadCheckpoint"},
-		{blockchain.ErrMissingTx, "ErrMissingTx"},
+		{blockchain.ErrMissingTxOut, "ErrMissingTxOut"},
 		{blockchain.ErrUnfinalizedTx, "ErrUnfinalizedTx"},
 		{blockchain.ErrDuplicateTx, "ErrDuplicateTx"},
 		{blockchain.ErrOverwriteTx, "ErrOverwriteTx"},
 		{blockchain.ErrImmatureSpend, "ErrImmatureSpend"},
-		{blockchain.ErrDoubleSpend, "ErrDoubleSpend"},
 		{blockchain.ErrSpendTooHigh, "ErrSpendTooHigh"},
 		{blockchain.ErrBadFees, "ErrBadFees"},
 		{blockchain.ErrTooManySigOps, "ErrTooManySigOps"},

blockchain/fullblocktests/generate.go

@@ -997,7 +997,7 @@ func Generate(includeLargeReorg bool) (tests [][]TestInstance, err error) {
 	acceptedToSideChainWithExpectedTip("b6")
 
 	g.nextBlock("b8", outs[4])
-	rejected(blockchain.ErrMissingTx)
+	rejected(blockchain.ErrMissingTxOut)
 
 	// ---------------------------------------------------------------------
 	// Too much proof-of-work coinbase tests.
@@ -1078,7 +1078,7 @@ func Generate(includeLargeReorg bool) (tests [][]TestInstance, err error) {
 	//    \-> b3(1) -> b4(2)
 	g.setTip("b15")
 	g.nextBlock("b17", &b3Tx1Out)
-	rejected(blockchain.ErrMissingTx)
+	rejected(blockchain.ErrMissingTxOut)
 
 	// Create block that forks and spends a tx created on a third fork.
 	//
@@ -1090,7 +1090,7 @@ func Generate(includeLargeReorg bool) (tests [][]TestInstance, err error) {
 	acceptedToSideChainWithExpectedTip("b15")
 
 	g.nextBlock("b19", outs[6])
-	rejected(blockchain.ErrMissingTx)
+	rejected(blockchain.ErrMissingTxOut)
 
 	// ---------------------------------------------------------------------
 	// Immature coinbase tests.
@@ -1278,11 +1278,11 @@ func Generate(includeLargeReorg bool) (tests [][]TestInstance, err error) {
 	doubleSpendTx := createSpendTx(outs[11], lowFee)
 	g.nextBlock("b37", outs[11], additionalTx(doubleSpendTx))
 	b37Tx1Out := makeSpendableOut(g.tip, 1, 0)
-	rejected(blockchain.ErrDoubleSpend)
+	rejected(blockchain.ErrMissingTxOut)
 
 	g.setTip("b35")
 	g.nextBlock("b38", &b37Tx1Out)
-	rejected(blockchain.ErrMissingTx)
+	rejected(blockchain.ErrMissingTxOut)
 
 	// ---------------------------------------------------------------------
 	// Pay-to-script-hash signature operation count tests.
@@ -1536,7 +1536,7 @@ func Generate(includeLargeReorg bool) (tests [][]TestInstance, err error) {
 		b.Transactions[1].TxIn[0].PreviousOutPoint.Hash = *hash
 		b.Transactions[1].TxIn[0].PreviousOutPoint.Index = 0
 	})
-	rejected(blockchain.ErrMissingTx)
+	rejected(blockchain.ErrMissingTxOut)
 
 	// ---------------------------------------------------------------------
 	// Block header median time tests.
@@ -1688,7 +1688,7 @@ func Generate(includeLargeReorg bool) (tests [][]TestInstance, err error) {
 	g.nextBlock("b58", outs[17], func(b *wire.MsgBlock) {
 		b.Transactions[1].TxIn[0].PreviousOutPoint.Index = 42
 	})
-	rejected(blockchain.ErrMissingTx)
+	rejected(blockchain.ErrMissingTxOut)
 
 	// Create block with transaction that pays more than its inputs.
 	//
@@ -1816,7 +1816,7 @@ func Generate(includeLargeReorg bool) (tests [][]TestInstance, err error) {
 		b.AddTransaction(tx3)
 		b.AddTransaction(tx2)
 	})
-	rejected(blockchain.ErrMissingTx)
+	rejected(blockchain.ErrMissingTxOut)
 
 	// Create block that double spends a transaction created in the same
 	// block.
@@ -1831,7 +1831,7 @@ func Generate(includeLargeReorg bool) (tests [][]TestInstance, err error) {
 		b.AddTransaction(tx3)
 		b.AddTransaction(tx4)
 	})
-	rejected(blockchain.ErrDoubleSpend)
+	rejected(blockchain.ErrMissingTxOut)
 
 	// ---------------------------------------------------------------------
 	// Extra subsidy tests.
@@ -2022,7 +2022,7 @@ func Generate(includeLargeReorg bool) (tests [][]TestInstance, err error) {
 	// to effective negate that behavior.
 	b75OpReturnOut.amount++
 	g.nextBlock("b80", &b75OpReturnOut)
-	rejected(blockchain.ErrMissingTx)
+	rejected(blockchain.ErrMissingTxOut)
 
 	// Create a block that has a transaction with multiple OP_RETURNs.  Even
 	// though it's not considered a standard transaction, it is still valid

blockchain/scriptval.go

@@ -65,7 +65,7 @@ out:
 					"transaction %v referenced from "+
 					"transaction %v", originTxHash,
 					txVI.tx.Hash())
-				err := ruleError(ErrMissingTx, str)
+				err := ruleError(ErrMissingTxOut, str)
 				v.sendResult(err)
 				break out
 			}

blockchain/validate.go

@@ -389,10 +389,11 @@ func CountP2SHSigOps(tx *btcutil.Tx, isCoinBaseTx bool, utxoView *UtxoViewpoint)
 		originTxIndex := txIn.PreviousOutPoint.Index
 		txEntry := utxoView.LookupEntry(originTxHash)
 		if txEntry == nil || txEntry.IsOutputSpent(originTxIndex) {
-			str := fmt.Sprintf("unable to find unspent output "+
-				"%v referenced from transaction %s:%d",
-				txIn.PreviousOutPoint, tx.Hash(), txInIndex)
-			return 0, ruleError(ErrMissingTx, str)
+			str := fmt.Sprintf("output %v referenced from "+
+				"transaction %s:%d either does not exist or "+
+				"has already been spent", txIn.PreviousOutPoint,
+				tx.Hash(), txInIndex)
+			return 0, ruleError(ErrMissingTxOut, str)
 		}
 
 		// We're only interested in pay-to-script-hash types, so skip
@@ -897,12 +898,14 @@ func CheckTransactionInputs(tx *btcutil.Tx, txHeight int32, utxoView *UtxoViewpo
 	for txInIndex, txIn := range tx.MsgTx().TxIn {
 		// Ensure the referenced input transaction is available.
 		originTxHash := &txIn.PreviousOutPoint.Hash
+		originTxIndex := txIn.PreviousOutPoint.Index
 		utxoEntry := utxoView.LookupEntry(originTxHash)
-		if utxoEntry == nil {
-			str := fmt.Sprintf("unable to find unspent output "+
-				"%v referenced from transaction %s:%d",
-				txIn.PreviousOutPoint, tx.Hash(), txInIndex)
-			return 0, ruleError(ErrMissingTx, str)
+		if utxoEntry == nil || utxoEntry.IsOutputSpent(originTxIndex) {
+			str := fmt.Sprintf("output %v referenced from "+
+				"transaction %s:%d either does not exist or "+
+				"has already been spent", txIn.PreviousOutPoint,
+				tx.Hash(), txInIndex)
+			return 0, ruleError(ErrMissingTxOut, str)
 		}
 
 		// Ensure the transaction is not spending coins which have not
@@ -922,15 +925,6 @@ func CheckTransactionInputs(tx *btcutil.Tx, txHeight int32, utxoView *UtxoViewpo
 			}
 		}
 
-		// Ensure the transaction is not double spending coins.
-		originTxIndex := txIn.PreviousOutPoint.Index
-		if utxoEntry.IsOutputSpent(originTxIndex) {
-			str := fmt.Sprintf("transaction %s:%d tried to double "+
-				"spend output %v", txHash, txInIndex,
-				txIn.PreviousOutPoint)
-			return 0, ruleError(ErrDoubleSpend, str)
-		}
-
 		// Ensure the transaction amounts are in range.  Each of the
 		// output values of the input transactions must not be negative
 		// or more than the max allowed per transaction.  All amounts in
@@ -1017,7 +1011,7 @@ func (b *BlockChain) checkConnectBlock(node *blockNode, block *btcutil.Block, vi
 	// an error now.
 	if node.hash.IsEqual(b.chainParams.GenesisHash) {
 		str := "the coinbase for the genesis block is not spendable"
-		return ruleError(ErrMissingTx, str)
+		return ruleError(ErrMissingTxOut, str)
 	}
 
 	// Ensure the view is for the node being checked.

blockchain/weight.go

@@ -91,10 +91,12 @@ func GetSigOpCost(tx *btcutil.Tx, isCoinBaseTx bool, utxoView *UtxoViewpoint,
 			originTxIndex := txIn.PreviousOutPoint.Index
 			txEntry := utxoView.LookupEntry(originTxHash)
 			if txEntry == nil || txEntry.IsOutputSpent(originTxIndex) {
-				str := fmt.Sprintf("unable to find unspent output "+
-					"%v referenced from transaction %s:%d",
-					txIn.PreviousOutPoint, tx.Hash(), txInIndex)
-				return 0, ruleError(ErrMissingTx, str)
+				str := fmt.Sprintf("output %v referenced from "+
+					"transaction %s:%d either does not "+
+					"exist or has already been spent",
+					txIn.PreviousOutPoint, tx.Hash(),
+					txInIndex)
+				return 0, ruleError(ErrMissingTxOut, str)
 			}
 
 			witness := txIn.Witness

mempool/error.go

@@ -74,8 +74,6 @@ func extractRejectCode(err error) (wire.RejectCode, bool) {
 		switch err.ErrorCode {
 		// Rejected due to duplicate.
 		case blockchain.ErrDuplicateBlock:
-			fallthrough
-		case blockchain.ErrDoubleSpend:
 			code = wire.RejectDuplicate
 
 		// Rejected due to obsolete version.

rpcserver.go

@@ -1995,7 +1995,7 @@ func chainErrToGBTErrString(err error) string {
 		return "bad-txns-dupinputs"
 	case blockchain.ErrBadTxInput:
 		return "bad-txns-badinput"
-	case blockchain.ErrMissingTx:
+	case blockchain.ErrMissingTxOut:
 		return "bad-txns-missinginput"
 	case blockchain.ErrUnfinalizedTx:
 		return "bad-txns-unfinalizedtx"
@@ -2005,8 +2005,6 @@ func chainErrToGBTErrString(err error) string {
 		return "bad-txns-overwrite"
 	case blockchain.ErrImmatureSpend:
 		return "bad-txns-maturity"
-	case blockchain.ErrDoubleSpend:
-		return "bad-txns-dblspend"
 	case blockchain.ErrSpendTooHigh:
 		return "bad-txns-highspend"
 	case blockchain.ErrBadFees: