pytest: fix flake in test_closing_anchorspend_htlc_tx_rbf
What the developer wrote
Authored by Rusty Russell
pytest: fix flake in test_closing_anchorspend_htlc_tx_rbf
```
2026-04-02T04:45:50.5882651Z > assert 3000 - 1 < total_feerate_perkw < 3000 + 1
2026-04-02T04:45:50.5883085Z E assert 3001.3083296990844 < (3000 + 1)
2026-04-02T04:45:50.5883354Z
2026-04-02T04:45:50.5883508Z tests/test_closing.py:3853: AssertionError
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
What changed, and why it matters
This is a test-only change that makes a flaky test more tolerant. It does not change any production code, so it has no direct security impact on users of Core Lightning.
No action required; this is a test reliability fix. Reviewers may optionally verify that did_short_sig correctly detects low-R signatures and that the relaxed assertion still covers the intended feerate behavior.
Security signals we found
No strong security signals were identified.
Evidence from the diff
The commit modifies tests/test_closing.py to fix a flaky assertion in test_closing_anchorspend_htlc_tx_rbf. When a low-R (short) ECDSA signature is produced, the transaction weight is slightly lower, causing the computed feerate per kiloweight to exceed the expected ±1 tolerance. The fix adds a helper check did_short_sig() and skips the strict feerate assertion when a short signature occurs. No node logic, protocol handling, or wallet code is changed.
Changed components
tests/test_closing.pyInspect captured patch +5 / −4
diff --git a/tests/test_closing.py b/tests/test_closing.py
index 5c4a17ac..63ff6ad7 100644
--- a/tests/test_closing.py
+++ b/tests/test_closing.py
@@ -8,7 +8,7 @@ from utils import (
scriptpubkey_addr, calc_lease_fee,
check_utxos_channel, check_coin_moves,
mine_funding_to_announce, check_inspect_channel,
- first_scid, check_feerate
+ first_scid, check_feerate, did_short_sig
)
from typing import List, Optional
@@ -3787,7 +3787,8 @@ def test_closing_anchorspend_htlc_tx_rbf(node_factory, bitcoind):
# We want an outstanding HTLC for l1, so it uses anchor to push.
# Set feerates to lowball for now.
l1, l2 = node_factory.line_graph(2, opts=[{'feerates': (1000,) * 4,
- 'min-emergency-msat': 546000},
+ 'min-emergency-msat': 546000,
+ 'broken_log': 'overgrind: short signature length'},
{'feerates': (1000,) * 4,
'disconnect': ['-WIRE_UPDATE_FAIL_HTLC']}])
assert 'anchors/even' in only_one(l1.rpc.listpeerchannels()['channels'])['channel_type']['names']
@@ -3835,7 +3836,7 @@ def test_closing_anchorspend_htlc_tx_rbf(node_factory, bitcoind):
total_weight = sum([d['weight'] for d in details])
total_fees = sum([float(d['fees']['base']) * 100_000_000 for d in details])
total_feerate_perkw = total_fees / total_weight * 1000
- assert 2000 - 1 < total_feerate_perkw < 2000 + 1
+ assert did_short_sig(l1) or 2000 - 1 < total_feerate_perkw < 2000 + 1
# But we don't mine it! And fees go up again!
l1.set_feerates((3000, 3000, 3000, 3000))
@@ -3850,7 +3851,7 @@ def test_closing_anchorspend_htlc_tx_rbf(node_factory, bitcoind):
total_weight = sum([d['weight'] for d in details])
total_fees = sum([float(d['fees']['base']) * 100_000_000 for d in details])
total_feerate_perkw = total_fees / total_weight * 1000
- assert 3000 - 1 < total_feerate_perkw < 3000 + 1
+ assert did_short_sig(l1) or 3000 - 1 < total_feerate_perkw < 3000 + 1
# And now we'll get it in (there's some rounding, so feerate a bit lower!)
bitcoind.generate_block(1, needfeerate=2990)
Why this scored 13/100
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