Merge bitcoin/bitcoin#29615: test: fix accurate multisig sigop count (BIP16), add unit test

3e9c736 test: fix accurate multisig sigop count (BIP16), add unit test (Sebastian Falbesoner)

Pull request description:

  In the course of reviewing #29589 I noticed the following buggy call-site of `CScriptOp.decode_op_n` in the CScript's `GetSigOpCount` method:
  https://github.com/bitcoin/bitcoin/blob/4cc99df44aec4d104590aee46cf18318e22a8568/test/functional/test_framework/script.py#L591-L593
  This should be `lastOpcode` rather than `opcode`. The latter is either OP_CHECKMULTISIG or OP_CHECKMULTISIGVERIFY at this point, so `decode_op_n` would result in an error. Also, in `CScript.raw_iter`, we have to return the op as `CScriptOp` type instead of a bare integer, otherwise we can't call the decode method on it. To prevent this in the future, add some simple unit tests for `GetSigOpCount`.

  Note that this was unnoticed, as the code part was never hit so far in the test framework.

ACKs for top commit:
  achow101:
    ACK 3e9c736
  Christewart:
    ACK 3e9c736
  rkrux:
    tACK [3e9c736](bitcoin/bitcoin@3e9c736)
  hernanmarino:
    tACK 3e9c736

Tree-SHA512: 51647bb6d462fbd101effd851afdbd6ad198c0567888cd4fdcac389a9fb4bd3d7e648095c6944fd8875d36272107ebaabdc62d0e2423289055588c12294d05a7

Author: achow101

test/functional/test_framework/script.py

@@ -483,7 +483,7 @@ def raw_iter(self):
         i = 0
         while i < len(self):
             sop_idx = i
-            opcode = self[i]
+            opcode = CScriptOp(self[i])
             i += 1
 
             if opcode > OP_PUSHDATA4:
@@ -590,7 +590,7 @@ def GetSigOpCount(self, fAccurate):
                 n += 1
             elif opcode in (OP_CHECKMULTISIG, OP_CHECKMULTISIGVERIFY):
                 if fAccurate and (OP_1 <= lastOpcode <= OP_16):
-                    n += opcode.decode_op_n()
+                    n += lastOpcode.decode_op_n()
                 else:
                     n += 20
             lastOpcode = opcode
@@ -782,6 +782,20 @@ def test_cscriptnum_encoding(self):
         for value in values:
             self.assertEqual(CScriptNum.decode(CScriptNum.encode(CScriptNum(value))), value)
 
+    def test_legacy_sigopcount(self):
+        # test repeated single sig ops
+        for n_ops in range(1, 100, 10):
+            for singlesig_op in (OP_CHECKSIG, OP_CHECKSIGVERIFY):
+                singlesigs_script = CScript([singlesig_op]*n_ops)
+                self.assertEqual(singlesigs_script.GetSigOpCount(fAccurate=False), n_ops)
+                self.assertEqual(singlesigs_script.GetSigOpCount(fAccurate=True), n_ops)
+        # test multisig op (including accurate counting, i.e. BIP16)
+        for n in range(1, 16+1):
+            for multisig_op in (OP_CHECKMULTISIG, OP_CHECKMULTISIGVERIFY):
+                multisig_script = CScript([CScriptOp.encode_op_n(n), multisig_op])
+                self.assertEqual(multisig_script.GetSigOpCount(fAccurate=False), 20)
+                self.assertEqual(multisig_script.GetSigOpCount(fAccurate=True), n)
+
 def BIP341_sha_prevouts(txTo):
     return sha256(b"".join(i.prevout.serialize() for i in txTo.vin))