From 3c49e6967050cfc367b3c826c9eac86a48528af5 Mon Sep 17 00:00:00 2001
From: Max Edwards <youwontforgetthis@gmail.com>
Date: Fri, 1 Mar 2024 11:43:36 +0000
Subject: [PATCH] test: fix weight estimates in functional tests

Updates the weight estimate to be more accurate by removing byte buffers and calculating the length of the count of scriptWitnesses rather than just using the count itself.
---
 test/functional/rpc_psbt.py    | 13 +++++++++----
 test/functional/wallet_send.py | 13 +++++++++----
 2 files changed, 18 insertions(+), 8 deletions(-)

diff --git a/test/functional/rpc_psbt.py b/test/functional/rpc_psbt.py
index 1fd938d18a9..016aa3ba119 100755
--- a/test/functional/rpc_psbt.py
+++ b/test/functional/rpc_psbt.py
@@ -753,11 +753,16 @@ class PSBTTest(BitcoinTestFramework):
                 break
         psbt_in = dec["inputs"][input_idx]
         # Calculate the input weight
-        # (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
+        # (prevout + sequence + length of scriptSig + scriptsig) * WITNESS_SCALE_FACTOR + len of num scriptWitness stack items + (length of stack item + stack item) * N stack items
+        # Note that occasionally this weight estimate may be slightly larger or smaller than the real weight
+        # as sometimes ECDSA signatures are one byte shorter than expected with a probability of 1/128
         len_scriptsig = len(psbt_in["final_scriptSig"]["hex"]) // 2 if "final_scriptSig" in psbt_in else 0
-        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
+        len_scriptsig += len(ser_compact_size(len_scriptsig))
+        len_scriptwitness = (sum([(len(x) // 2) + len(ser_compact_size(len(x) // 2)) for x in psbt_in["final_scriptwitness"]]) + len(ser_compact_size(len(psbt_in["final_scriptwitness"])))) if "final_scriptwitness" in psbt_in else 0
+        len_prevout_txid = 32
+        len_prevout_index = 4
+        len_sequence = 4
+        input_weight = ((len_prevout_txid + len_prevout_index + len_sequence + len_scriptsig) * WITNESS_SCALE_FACTOR) + len_scriptwitness
         low_input_weight = input_weight // 2
         high_input_weight = input_weight * 2
 
diff --git a/test/functional/wallet_send.py b/test/functional/wallet_send.py
index 6ce2a56bfcb..c5942578a44 100755
--- a/test/functional/wallet_send.py
+++ b/test/functional/wallet_send.py
@@ -543,11 +543,16 @@ class WalletSendTest(BitcoinTestFramework):
                 break
         psbt_in = dec["inputs"][input_idx]
         # Calculate the input weight
-        # (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
+        # (prevout + sequence + length of scriptSig + scriptsig) * WITNESS_SCALE_FACTOR + len of num scriptWitness stack items + (length of stack item + stack item) * N stack items
+        # Note that occasionally this weight estimate may be slightly larger or smaller than the real weight
+        # as sometimes ECDSA signatures are one byte shorter than expected with a probability of 1/128
         len_scriptsig = len(psbt_in["final_scriptSig"]["hex"]) // 2 if "final_scriptSig" in psbt_in else 0
-        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
+        len_scriptsig += len(ser_compact_size(len_scriptsig))
+        len_scriptwitness = (sum([(len(x) // 2) + len(ser_compact_size(len(x) // 2)) for x in psbt_in["final_scriptwitness"]]) + len(ser_compact_size(len(psbt_in["final_scriptwitness"])))) if "final_scriptwitness" in psbt_in else 0
+        len_prevout_txid = 32
+        len_prevout_index = 4
+        len_sequence = 4
+        input_weight = ((len_prevout_txid + len_prevout_index + len_sequence + len_scriptsig) * WITNESS_SCALE_FACTOR) + len_scriptwitness
 
         # Input weight error conditions
         assert_raises_rpc_error(