pyln-testing: add old_hsmsecret param to get_node.

diff --git a/contrib/pyln-testing/pyln/testing/utils.py b/contrib/pyln-testing/pyln/testing/utils.py
index 732bd7b4..2159ef87 100644
--- a/contrib/pyln-testing/pyln/testing/utils.py
+++ b/contrib/pyln-testing/pyln/testing/utils.py
@@ -17,6 +17,7 @@ import json
 import logging
 import lzma
 import math
+import mnemonic
 import os
 import random
 import re
@@ -664,6 +665,14 @@ class ElementsD(BitcoinD):
         return info['unconfidential']
 
 
+def mnemonic_from_seed(seed):
+    m = mnemonic.Mnemonic('english')
+    mnem = m.to_mnemonic(seed)
+    if not m.check(mnem):
+        raise RuntimeError("Generated mnemonic failed BIP39 validation (unexpected).")
+    return mnem
+
+
 class LightningD(TailableProc):
     def __init__(
             self,
@@ -673,6 +682,7 @@ class LightningD(TailableProc):
             random_hsm=False,
             node_id=0,
             executable=None,
+            old_hsmsecret=None,
     ):
         # We handle our own version of verbose, below.
         TailableProc.__init__(self, lightning_dir, verbose=False)
@@ -714,11 +724,27 @@ class LightningD(TailableProc):
         if not os.path.exists(os.path.join(lightning_dir, TEST_NETWORK)):
             os.makedirs(os.path.join(lightning_dir, TEST_NETWORK))
 
-        # Last 32-bytes of final part of dir -> seed.
-        seed = (bytes(re.search('([^/]+)/*$', lightning_dir).group(1), encoding='utf-8') + bytes(32))[:32]
+        # Default: use old-timey hsm_secret.
+        if old_hsmsecret is None:
+            old_hsmsecret = True
+
+        # BIP 39 secrets were only added in v25.12.
+        if old_hsmsecret is True:
+            assert self.cln_version >= "v25.12"
+
         if not random_hsm:
+            # Last 32-bytes of final part of dir -> seed.
+            seed = (bytes(re.search('([^/]+)/*$', lightning_dir).group(1), encoding='utf-8') + bytes(32))[:32]
+            # Modern style is 32 zeroes then a 12-word mnemonic phrase.
+            if not old_hsmsecret:
+                # Use first 16 bytes (128 bits) for 12-word mnemonic
+                entropy_128 = seed[:16]
+                mnemonic_phrase = mnemonic_from_seed(entropy_128)
+                seed = bytes(32) + bytes(mnemonic_phrase, encoding='utf-8')
+
             with open(os.path.join(lightning_dir, TEST_NETWORK, 'hsm_secret'), 'wb') as f:
                 f.write(seed)
+
         self.opts['dev-fast-gossip'] = None
         self.opts['dev-bitcoind-poll'] = 1
         self.prefix = 'lightningd-%d' % (node_id)
@@ -851,6 +877,7 @@ class LightningNode(object):
                  valgrind_plugins=True,
                  executable=None,
                  bad_notifications=False,
+                 old_hsmsecret=None,
                  **kwargs):
         self.bitcoin = bitcoind
         self.executor = executor
@@ -875,6 +902,7 @@ class LightningNode(object):
             lightning_dir, bitcoindproxy=bitcoind.get_proxy(),
             port=port, random_hsm=random_hsm, node_id=node_id,
             executable=executable,
+            old_hsmsecret=old_hsmsecret,
         )
         self.cln_version = self.daemon.cln_version
 
@@ -1689,6 +1717,7 @@ class NodeFactory(object):
             'allow_bad_gossip',
             'start',
             'gossip_store_file',
+            'old_hsmsecret',
         ]
         node_opts = {k: v for k, v in opts.items() if k in node_opt_keys}
         cli_opts = {k: v for k, v in opts.items() if k not in node_opt_keys}
diff --git a/pyproject.toml b/pyproject.toml
index b86ffef5..76335c2e 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -19,6 +19,7 @@ dependencies = [
   "pytest-rerunfailures>=16.0.1",
   "pyln-testing",
   "pyln-proto",
+  "mnemonic>=0.21",
 ]
 package-mode = false
 [dependency-groups]
diff --git a/uv.lock b/uv.lock
index 9373dedf..0c75e49f 100644
--- a/uv.lock
+++ b/uv.lock
@@ -460,6 +460,7 @@ dependencies = [
     { name = "grpcio" },
     { name = "grpcio-tools" },
     { name = "mako" },
+    { name = "mnemonic" },
     { name = "protobuf" },
     { name = "pyln-client" },
     { name = "pyln-grpc-proto" },
@@ -501,6 +502,7 @@ requires-dist = [
     { name = "grpcio", specifier = "==1.75.1" },
     { name = "grpcio-tools", specifier = "==1.75.1" },
     { name = "mako", specifier = ">=1.1.6" },
+    { name = "mnemonic", specifier = ">=0.21" },
     { name = "protobuf", specifier = "==6.32.1" },
     { name = "pyln-client", editable = "contrib/pyln-client" },
     { name = "pyln-grpc-proto", editable = "contrib/pyln-grpc-proto" },
@@ -1264,6 +1266,15 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/27/1a/1f68f9ba0c207934b35b86a8ca3aad8395a3d6dd7921c0686e23853ff5a9/mccabe-0.7.0-py2.py3-none-any.whl", hash = "sha256:6c2d30ab6be0e4a46919781807b4f0d834ebdd6c6e3dca0bda5a15f863427b6e", size = 7350, upload-time = "2022-01-24T01:14:49.62Z" },
 ]
 
+[[package]]
+name = "mnemonic"
+version = "0.21"
+source = { registry = "https://pypi.org/simple" }
+sdist = { url = "https://files.pythonhosted.org/packages/ff/77/e6232ed59fbd7b90208bb8d4f89ed5aabcf30a524bc2fb8f0dafbe8e7df9/mnemonic-0.21.tar.gz", hash = "sha256:1fe496356820984f45559b1540c80ff10de448368929b9c60a2b55744cc88acf", size = 152462, upload-time = "2024-01-05T10:46:14.895Z" }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/57/48/5abb16ce7f9d97b728e6b97c704ceaa614362e0847651f379ed0511942a0/mnemonic-0.21-py3-none-any.whl", hash = "sha256:72dc9de16ec5ef47287237b9b6943da11647a03fe7cf1f139fc3d7c4a7439288", size = 92701, upload-time = "2024-01-05T10:46:12.703Z" },
+]
+
 [[package]]
 name = "more-itertools"
 version = "10.8.0"