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.

1 files changed, 9 insertions, 4 deletions

diff --git a/test/functional/rpc_psbt.py b/test/functional/rpc_psbt.py
index 1fd938d18a..016aa3ba11 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