wire: remove erroneous witness size check in wire parsing

In this commit, we fix a bug that would cause nodes to be unable to
parse a given block from the wire. The block would be properly accepted
if fed in via other mechanisms.

The issue here is that the old checks for the maximum witness size,
circa segwit v0 where placed in the wire package _as well_ as the tx
engine. This check should only be in the engine, since it's properly
gated by other related scrip validation flags.

The fix itself is simple: limit witnesses only based on the maximum
block size in bytes, or ~4MB.

blockchain/weight.go

@@ -7,9 +7,9 @@ package blockchain
 import (
 	"fmt"
 
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcd/btcutil"
 )
 
 const (

wire/msgtx.go

@@ -106,24 +106,26 @@ const (
 	// an input's witness data. This number is derived from the fact that
 	// for script validation, each pushed item onto the stack must be less
 	// than 10k bytes.
-	maxWitnessItemSize = 11000
+	maxWitnessItemSize = 4_000_000
 )
 
 // TxFlagMarker is the first byte of the FLAG field in a bitcoin tx
 // message. It allows decoders to distinguish a regular serialized
 // transaction from one that would require a different parsing logic.
 //
 // Position of FLAG in a bitcoin tx message:
-//   ┌─────────┬────────────────────┬─────────────┬─────┐
-//   │ VERSION │ FLAG               │ TX-IN-COUNT │ ... │
-//   │ 4 bytes │ 2 bytes (optional) │ varint      │     │
-//   └─────────┴────────────────────┴─────────────┴─────┘
+//
+//	┌─────────┬────────────────────┬─────────────┬─────┐
+//	│ VERSION │ FLAG               │ TX-IN-COUNT │ ... │
+//	│ 4 bytes │ 2 bytes (optional) │ varint      │     │
+//	└─────────┴────────────────────┴─────────────┴─────┘
 //
 // Zooming into the FLAG field:
-//   ┌── FLAG ─────────────┬────────┐
-//   │ TxFlagMarker (0x00) │ TxFlag │
-//   │ 1 byte              │ 1 byte │
-//   └─────────────────────┴────────┘
+//
+//	┌── FLAG ─────────────┬────────┐
+//	│ TxFlagMarker (0x00) │ TxFlag │
+//	│ 1 byte              │ 1 byte │
+//	└─────────────────────┴────────┘
 const TxFlagMarker = 0x00
 
 // TxFlag is the second byte of the FLAG field in a bitcoin tx message.
@@ -586,8 +588,9 @@ func (msg *MsgTx) BtcDecode(r io.Reader, pver uint32, enc MessageEncoding) error
 			// item itself.
 			txin.Witness = make([][]byte, witCount)
 			for j := uint64(0); j < witCount; j++ {
-				txin.Witness[j], err = readScript(r, pver,
-					maxWitnessItemSize, "script witness item")
+				txin.Witness[j], err = readScript(
+					r, pver, maxWitnessItemSize, "script witness item",
+				)
 				if err != nil {
 					returnScriptBuffers()
 					return err