From c8332cc9a7ecb05812e25a9b3bac565c9656f701 Mon Sep 17 00:00:00 2001
From: Dave Collins <davec@conformal.com>
Date: Sun, 16 Mar 2014 23:07:20 -0500
Subject: [PATCH] Correct num expected inputs calc for multisig.

This commit corrects the number of expected inputs for a multi-sig script
to include the additional item that is popped from the stack due to the
OP_CHECKMULTISIG consensus bug (which is required and properly performed).

Note this issue did NOT affect the consensus critical code and hence would
not cause a chain fork.  It did however, cause standard p2sh multisig txns
to be rejected from the mempool as nonstandard.

The tx rejected as non-standard which prompted this was spotted by
@mbelshe on IRC.

ok @owainga
---
 script.go      | 12 +++++++-----
 script_test.go |  7 ++++---
 2 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/script.go b/script.go
index 0854931b..11f0dab3 100644
--- a/script.go
+++ b/script.go
@@ -1128,11 +1128,13 @@ func expectedInputs(pops []parsedOpcode, class ScriptClass) int {
 		return 1
 	case MultiSigTy:
 		// Standard multisig has a push a small number for the number
-		// of sigs and number of keys.
-		// Check the first push instruction to see how many arguments
-		// are expected. typeOfScript already checked this so we know
-		// it'll be a small int.
-		return asSmallInt(pops[0].opcode)
+		// of sigs and number of keys.  Check the first push instruction
+		// to see how many arguments are expected. typeOfScript already
+		// checked this so we know it'll be a small int.  Also, due to
+		// the original bitcoind bug where OP_CHECKMULTISIG pops an
+		// additional item from the stack, add an extra expected input
+		// for the extra push that is required to compensate.
+		return asSmallInt(pops[0].opcode) + 1
 	case NullDataTy:
 		fallthrough
 	default:
diff --git a/script_test.go b/script_test.go
index d1eb604e..1fb6e7a3 100644
--- a/script_test.go
+++ b/script_test.go
@@ -1200,7 +1200,7 @@ var txTests = []txTest{
 		scriptInfo: btcscript.ScriptInfo{
 			PkScriptClass:  btcscript.ScriptHashTy,
 			NumInputs:      2,
-			ExpectedInputs: 1,
+			ExpectedInputs: 2,
 			SigOps:         1,
 		},
 	},
@@ -1780,6 +1780,7 @@ func TestScriptInfo(t *testing.T) {
 			name: "multisig script",
 			sigScript: []byte{btcscript.OP_TRUE,
 				btcscript.OP_TRUE, btcscript.OP_TRUE,
+				btcscript.OP_0, // Extra arg for OP_CHECKMULTISIG bug
 			},
 			pkScript: []byte{
 				btcscript.OP_3, btcscript.OP_DATA_33,
@@ -1802,8 +1803,8 @@ func TestScriptInfo(t *testing.T) {
 			bip16: true,
 			scriptInfo: btcscript.ScriptInfo{
 				PkScriptClass:  btcscript.MultiSigTy,
-				NumInputs:      3,
-				ExpectedInputs: 3,
+				NumInputs:      4,
+				ExpectedInputs: 4,
 				SigOps:         3,
 			},
 		},