pytest: make test_peer_anchor_push robust against short sig generation.

This happens one time in 256:

```
2025-05-06T11:00:43.2201832Z >           assert feerate - 1 < total_feerate_perkw < feerate + 1
2025-05-06T11:00:43.2202133Z E           assert 12005.233318796338 < (12000 + 1)
```

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

tests/test_closing.py

@@ -8,7 +8,7 @@
     scriptpubkey_addr, calc_lease_fee,
     check_utxos_channel, check_coin_moves,
     mine_funding_to_announce, check_inspect_channel,
-    first_scid
+    first_scid, check_feerate
 )
 
 import bitcoin
@@ -3981,7 +3981,9 @@ def test_closing_minfee(node_factory, bitcoind):
 def test_peer_anchor_push(node_factory, bitcoind, executor, chainparams):
     """Test that we use anchor on peer's commit to CPFP tx"""
     l1, l2, l3 = node_factory.line_graph(3, opts=[{},
-                                                  {'min-emergency-msat': 546000},
+                                                  {'min-emergency-msat': 546000,
+                                                   'dev-warn-on-overgrind': None,
+                                                   'broken_log': 'overgrind: short signature length'},
                                                   {'disconnect': ['-WIRE_UPDATE_FULFILL_HTLC']}],
                                          wait_for_announce=True)
 
@@ -4045,7 +4047,7 @@ def test_peer_anchor_push(node_factory, bitcoind, executor, chainparams):
         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 feerate - 1 < total_feerate_perkw < feerate + 1
+        check_feerate(l2, total_feerate_perkw, feerate)
         bitcoind.generate_block(1, needfeerate=16000)
         sync_blockheight(bitcoind, [l2])
         assert len(bitcoind.rpc.getrawmempool()) == 2

tests/test_wallet.py

@@ -6,7 +6,7 @@
 from utils import (
     only_one, wait_for, sync_blockheight,
     VALGRIND, check_coin_moves, TailableProc, scriptpubkey_addr,
-    check_utxos_channel
+    check_utxos_channel, check_feerate, did_short_sig
 )
 
 import os
@@ -304,21 +304,6 @@ def feerate_from_psbt(chainparams, bitcoind, node, psbt):
     return fee / weight * 1000
 
 
-# I wish we could force libwally to use different entropy and thus force it to
-# create 71-byte sigs always!
-def did_short_sig(node):
-    # This can take a moment to appear in the log!
-    time.sleep(1)
-    return node.daemon.is_in_log('overgrind: short signature length')
-
-
-def check_feerate(node, actual_feerate, expected_feerate):
-    # Feerate can't be lower.
-    assert actual_feerate > expected_feerate - 2
-    if not did_short_sig(node):
-        assert actual_feerate < expected_feerate + 2
-
-
 def test_txprepare(node_factory, bitcoind, chainparams):
     amount = 1000000
     l1 = node_factory.get_node(random_hsm=True, options={'dev-warn-on-overgrind': None},

tests/utils.py

@@ -658,3 +658,18 @@ def serialize_payload_final_tlv(amount_msat, delay, total_msat, blockheight, pay
     #        * [`tu64`:`total_msat`]
     payload.add_field(8, bytes.fromhex(payment_secret) + tu64_encode(int(total_msat)))
     return payload.to_bytes()
+
+
+# I wish we could force libwally to use different entropy and thus force it to
+# create 71-byte sigs always!
+def did_short_sig(node):
+    # This can take a moment to appear in the log!
+    time.sleep(1)
+    return node.daemon.is_in_log('overgrind: short signature length')
+
+
+def check_feerate(node, actual_feerate, expected_feerate):
+    # Feerate can't be lower.
+    assert actual_feerate > expected_feerate - 2
+    if not did_short_sig(node):
+        assert actual_feerate < expected_feerate + 2