fix(core): Fix TxWeightCalculator.

- Refactor TxWeightCalculator to count inputs and outputs itself.
- Fix witness data weight by adding the weight of the witness stack item count
  for each input in segwit transactions and removing the weight of the
  nonsensical extra inputs count.
- Get multisig pubkey count from multisig.nodes or multisig.pubkeys like in
  multisig_get_pubkeys().
- Fix size of multisig script length encoding in segwit (varint vs. OP_PUSH).
- Improve comments.

core/src/apps/bitcoin/sign_tx/approvers.py

@@ -27,7 +27,7 @@ if False:
 class Approver:
     def __init__(self, tx: SignTx, coin: CoinInfo) -> None:
         self.coin = coin
-        self.weight = tx_weight.TxWeightCalculator(tx.inputs_count, tx.outputs_count)
+        self.weight = tx_weight.TxWeightCalculator()
 
         # amounts in the current transaction
         self.total_in = 0  # sum of input amounts
@@ -267,9 +267,7 @@ class CoinJoinApprover(Approver):
             raise wire.DataError("Coin name does not match authorization.")
 
         # Upper bound on the user's contribution to the weight of the transaction.
-        self.our_weight = tx_weight.TxWeightCalculator(
-            tx.inputs_count, tx.outputs_count
-        )
+        self.our_weight = tx_weight.TxWeightCalculator()
 
         # base for coordinator fee to be multiplied by fee_per_anonymity
         self.coordinator_fee_base = 0

core/src/apps/bitcoin/sign_tx/tx_weight.py

@@ -27,7 +27,7 @@ _TXSIZE_INPUT = const(40)
 _TXSIZE_OUTPUT = const(8)
 # size of a pubkey
 _TXSIZE_PUBKEY = const(33)
-# size of a DER signature (3 type bytes, 3 len bytes, 33 R, 32 S, 1 sighash
+# maximum size of a DER signature (3 type bytes, 3 len bytes, 33 R, 32 S, 1 sighash)
 _TXSIZE_SIGNATURE = const(72)
 # size of a multiscript without pubkey (1 M, 1 N, 1 checksig)
 _TXSIZE_MULTISIGSCRIPT = const(3)
@@ -38,32 +38,26 @@ _TXSIZE_WITNESSSCRIPT = const(34)
 
 
 class TxWeightCalculator:
-    def __init__(self, inputs_count: int, outputs_count: int):
-        self.inputs_count = inputs_count
-        self.counter = 4 * (
-            _TXSIZE_HEADER
-            + _TXSIZE_FOOTER
-            + self.ser_length_size(inputs_count)
-            + self.ser_length_size(outputs_count)
-        )
-        self.segwit = False
-
-    def add_witness_header(self) -> None:
-        if not self.segwit:
-            self.counter += _TXSIZE_SEGWIT_OVERHEAD
-            self.counter += self.ser_length_size(self.inputs_count)
-            self.segwit = True
+    def __init__(self) -> None:
+        self.inputs_count = 0
+        self.outputs_count = 0
+        self.counter = 4 * (_TXSIZE_HEADER + _TXSIZE_FOOTER)
+        self.segwit_inputs_count = 0
 
     def add_input(self, i: TxInput) -> None:
+        self.inputs_count += 1
 
         if i.multisig:
-            multisig_script_size = _TXSIZE_MULTISIGSCRIPT + len(i.multisig.pubkeys) * (
-                1 + _TXSIZE_PUBKEY
-            )
+            n = len(i.multisig.nodes) if i.multisig.nodes else len(i.multisig.pubkeys)
+            multisig_script_size = _TXSIZE_MULTISIGSCRIPT + n * (1 + _TXSIZE_PUBKEY)
+            if i.script_type in common.SEGWIT_INPUT_SCRIPT_TYPES:
+                multisig_script_size += self.varint_size(multisig_script_size)
+            else:
+                multisig_script_size += self.op_push_size(multisig_script_size)
+
             input_script_size = (
-                1
-                + i.multisig.m * (1 + _TXSIZE_SIGNATURE)  # the OP_FALSE bug in multisig
-                + self.op_push_size(multisig_script_size)
+                1  # the OP_FALSE bug in multisig
+                + i.multisig.m * (1 + _TXSIZE_SIGNATURE)
                 + multisig_script_size
             )
         else:
@@ -72,29 +66,38 @@ class TxWeightCalculator:
         self.counter += 4 * _TXSIZE_INPUT
 
         if i.script_type in common.NONSEGWIT_INPUT_SCRIPT_TYPES:
-            input_script_size += self.ser_length_size(input_script_size)
+            input_script_size += self.varint_size(input_script_size)
             self.counter += 4 * input_script_size
-
         elif i.script_type in common.SEGWIT_INPUT_SCRIPT_TYPES:
-            self.add_witness_header()
+            self.segwit_inputs_count += 1
             if i.script_type == InputScriptType.SPENDP2SHWITNESS:
+                # add script_sig size
                 if i.multisig:
                     self.counter += 4 * (2 + _TXSIZE_WITNESSSCRIPT)
                 else:
                     self.counter += 4 * (2 + _TXSIZE_WITNESSPKHASH)
             else:
-                self.counter += 4  # empty
-            self.counter += input_script_size  # discounted witness
+                self.counter += 4  # empty script_sig (1 byte)
+            self.counter += 1 + input_script_size  # discounted witness
 
     def add_output(self, script: bytes) -> None:
-        size = len(script) + self.ser_length_size(len(script))
-        self.counter += 4 * (_TXSIZE_OUTPUT + size)
+        self.outputs_count += 1
+        script_size = self.varint_size(len(script)) + len(script)
+        self.counter += 4 * (_TXSIZE_OUTPUT + script_size)
 
     def get_total(self) -> int:
-        return self.counter
+        total = self.counter
+        total += 4 * self.varint_size(self.inputs_count)
+        total += 4 * self.varint_size(self.outputs_count)
+        if self.segwit_inputs_count:
+            total += _TXSIZE_SEGWIT_OVERHEAD
+            # add one byte of witness stack item count per non-segwit input
+            total += self.inputs_count - self.segwit_inputs_count
+
+        return total
 
     @staticmethod
-    def ser_length_size(length: int) -> int:
+    def varint_size(length: int) -> int:
         if length < 253:
             return 1
         if length < 0x1_0000:

core/tests/test_apps.bitcoin.txweight.py

@@ -32,7 +32,7 @@ class TestCalculateTxWeight(unittest.TestCase):
                             address_n=[],
                             multisig=None)
 
-        calculator = TxWeightCalculator(1, 1)
+        calculator = TxWeightCalculator()
         calculator.add_input(inp1)
         calculator.add_output(output_derive_script(out1.address, coin))
 
@@ -71,14 +71,14 @@ class TestCalculateTxWeight(unittest.TestCase):
             multisig=None,
         )
 
-        calculator = TxWeightCalculator(1, 2)
+        calculator = TxWeightCalculator()
         calculator.add_input(inp1)
         calculator.add_output(output_derive_script(out1.address, coin))
         calculator.add_output(output_derive_script(out2.address, coin))
 
         self.assertEqual(calculator.get_total(), 670)
         # non-segwit: header, inputs, outputs, locktime 4*(4+65+67+4) = 560
-        # segwit: segwit header, witness count, 2x witness 1*(2+1+107) = 110
+        # segwit: segwit header, witness stack item count, witness 1*(2+1+107) = 110
         # total 670
 
     def test_native_p2wpkh_txweight(self):
@@ -111,14 +111,14 @@ class TestCalculateTxWeight(unittest.TestCase):
             multisig=None,
         )
 
-        calculator = TxWeightCalculator(1, 2)
+        calculator = TxWeightCalculator()
         calculator.add_input(inp1)
         calculator.add_output(output_derive_script(out1.address, coin))
         calculator.add_output(output_derive_script(out2.address, coin))
 
         self.assertEqual(calculator.get_total(), 566)
         # non-segwit: header, inputs, outputs, locktime 4*(4+42+64+4) = 456
-        # segwit: segwit header, witness count, 2x witness 1*(2+1+107) = 110
+        # segwit: segwit header, witness stack item count, witness 1*(2+1+107) = 110
         # total 566