pytest: Configure logging in a fixture to match stdout capturing

pytest captures the output by monkey patching out `sys.stdout`. This may
conflict with our use of `sys.stdout` when configuring logging, resulting in
the "Write to closed file" issue that is spamming the logs. By making the
logging configuration a fixture hopefully we always use the correct
stdout (after pytest has monkey-patched it).

contrib/pyln-testing/pyln/testing/fixtures.py

@@ -1,12 +1,13 @@
 from concurrent import futures
 from pyln.testing.db import SqliteDbProvider, PostgresDbProvider
-from pyln.testing.utils import NodeFactory, BitcoinD, ElementsD, env, DEVELOPER, LightningNode
+from pyln.testing.utils import NodeFactory, BitcoinD, ElementsD, env, DEVELOPER, LightningNode, TEST_DEBUG
 
 import logging
 import os
 import pytest
 import re
 import shutil
+import sys
 import tempfile
 
 
@@ -28,6 +29,18 @@ def test_base_dir():
         shutil.rmtree(directory)
 
 
+@pytest.fixture(autouse=True)
+def setup_logging():
+    logger = logging.getLogger()
+    before_handlers = list(logger.handlers)
+
+    if TEST_DEBUG:
+        logging.basicConfig(level=logging.DEBUG, stream=sys.stdout)
+
+    yield
+    logger.handlers = before_handlers
+
+
 @pytest.fixture
 def directory(request, test_base_dir, test_name):
     """Return a per-test specific directory.

contrib/pyln-testing/pyln/testing/utils.py

@@ -17,7 +17,6 @@ import sqlite3
 import string
 import struct
 import subprocess
-import sys
 import threading
 import time
 
@@ -69,10 +68,6 @@ SLOW_MACHINE = env("SLOW_MACHINE", "0") == "1"
 TIMEOUT = int(env("TIMEOUT", 180 if SLOW_MACHINE else 60))
 
 
-if TEST_DEBUG:
-    logging.basicConfig(level=logging.DEBUG, stream=sys.stdout)
-
-
 def wait_for(success, timeout=TIMEOUT):
     start_time = time.time()
     interval = 0.25

tests/fixtures.py

@@ -1,5 +1,5 @@
 from utils import DEVELOPER, TEST_NETWORK  # noqa: F401,F403
-from pyln.testing.fixtures import directory, test_base_dir, test_name, chainparams, node_factory, bitcoind, teardown_checks, db_provider, executor  # noqa: F401,F403
+from pyln.testing.fixtures import directory, test_base_dir, test_name, chainparams, node_factory, bitcoind, teardown_checks, db_provider, executor, setup_logging  # noqa: F401,F403
 from pyln.testing import utils
 
 import pytest