tests: Calculate input weight more accurately

The external input test with specifying input weight would make a
pessimistic estimate of the input weight. However this would result in a
test failure as it is sometimes too pessimistic when an ECDSA signature
ends up being smaller than usual. To correct this, we can calculate the
input weight more accurately.

Author: achow101

test/functional/rpc_psbt.py

@@ -10,6 +10,10 @@
 
 from test_framework.descriptors import descsum_create
 from test_framework.key import ECKey
+from test_framework.messages import (
+    ser_compact_size,
+    WITNESS_SCALE_FACTOR,
+)
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_approx,
@@ -655,10 +659,11 @@ def test_psbt_input_keys(psbt_input, keys):
                 break
         psbt_in = dec["inputs"][input_idx]
         # Calculate the input weight
-        # (prevout + sequence + length of scriptSig + 2 bytes buffer) * 4 + len of scriptwitness
+        # (prevout + sequence + length of scriptSig + scriptsig + 1 byte buffer) * WITNESS_SCALE_FACTOR + num scriptWitness stack items + (length of stack item + stack item) * N stack items + 1 byte buffer
         len_scriptsig = len(psbt_in["final_scriptSig"]["hex"]) // 2 if "final_scriptSig" in psbt_in else 0
-        len_scriptwitness = len(psbt_in["final_scriptwitness"]["hex"]) // 2 if "final_scriptwitness" in psbt_in else 0
-        input_weight = ((41 + len_scriptsig + 2) * 4) + len_scriptwitness
+        len_scriptsig += len(ser_compact_size(len_scriptsig)) + 1
+        len_scriptwitness = (sum([(len(x) // 2) + len(ser_compact_size(len(x) // 2)) for x in psbt_in["final_scriptwitness"]]) + len(psbt_in["final_scriptwitness"]) + 1) if "final_scriptwitness" in psbt_in else 0
+        input_weight = ((40 + len_scriptsig) * WITNESS_SCALE_FACTOR) + len_scriptwitness
         low_input_weight = input_weight // 2
         high_input_weight = input_weight * 2
 

test/functional/wallet_send.py

@@ -10,6 +10,10 @@
 from test_framework.authproxy import JSONRPCException
 from test_framework.descriptors import descsum_create
 from test_framework.key import ECKey
+from test_framework.messages import (
+    ser_compact_size,
+    WITNESS_SCALE_FACTOR,
+)
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_equal,
@@ -526,10 +530,11 @@ def run_test(self):
                 break
         psbt_in = dec["inputs"][input_idx]
         # Calculate the input weight
-        # (prevout + sequence + length of scriptSig + 2 bytes buffer) * 4 + len of scriptwitness
+        # (prevout + sequence + length of scriptSig + scriptsig + 1 byte buffer) * WITNESS_SCALE_FACTOR + num scriptWitness stack items + (length of stack item + stack item) * N stack items + 1 byte buffer
         len_scriptsig = len(psbt_in["final_scriptSig"]["hex"]) // 2 if "final_scriptSig" in psbt_in else 0
-        len_scriptwitness = len(psbt_in["final_scriptwitness"]["hex"]) // 2 if "final_scriptwitness" in psbt_in else 0
-        input_weight = ((41 + len_scriptsig + 2) * 4) + len_scriptwitness
+        len_scriptsig += len(ser_compact_size(len_scriptsig)) + 1
+        len_scriptwitness = (sum([(len(x) // 2) + len(ser_compact_size(len(x) // 2)) for x in psbt_in["final_scriptwitness"]]) + len(psbt_in["final_scriptwitness"]) + 1) if "final_scriptwitness" in psbt_in else 0
+        input_weight = ((40 + len_scriptsig) * WITNESS_SCALE_FACTOR) + len_scriptwitness
 
         # Input weight error conditions
         assert_raises_rpc_error(