old_contents: #!/usr/bin/env python3
  # Copyright (c) 2018-2020 The Bitcoin Core developers
  # Distributed under the MIT software license, see the accompanying
  # file COPYING or http://www.opensource.org/licenses/mit-license.php.
  """Check that it's not possible to start a second bitcoind instance using the same datadir or wallet."""
  import os
  import random
  import string
  
  from test_framework.test_framework import BitcoinTestFramework
  from test_framework.test_node import ErrorMatch
  
  class FilelockTest(BitcoinTestFramework):
      def set_test_params(self):
          self.setup_clean_chain = True
          self.num_nodes = 2
  
      def setup_network(self):
          self.add_nodes(self.num_nodes, extra_args=None)
          self.nodes[0].start()
          self.nodes[0].wait_for_rpc_connection()
  
      def run_test(self):
          datadir = os.path.join(self.nodes[0].datadir, self.chain)
          self.log.info("Using datadir {}".format(datadir))
  
          self.log.info("Check that we can't start a second bitcoind instance using the same datadir")
          expected_msg = "Error: Cannot obtain a lock on data directory {0}. {1} is probably already running.".format(datadir, self.config['environment']['PACKAGE_NAME'])
          self.nodes[1].assert_start_raises_init_error(extra_args=['-datadir={}'.format(self.nodes[0].datadir), '-noserver'], expected_msg=expected_msg)
  
          if self.is_wallet_compiled():
              def check_wallet_filelock(descriptors):
                  wallet_name = ''.join([random.choice(string.ascii_lowercase) for _ in range(6)])
                  self.nodes[0].createwallet(wallet_name=wallet_name, descriptors=descriptors)
                  wallet_dir = os.path.join(datadir, 'wallets')
                  self.log.info("Check that we can't start a second bitcoind instance using the same wallet")
                  if descriptors:
                      expected_msg = "Error: SQLiteDatabase: Unable to obtain an exclusive lock on the database, is it being used by another bitcoind?"
                  else:
                      expected_msg = "Error: Error initializing wallet database environment"
                  self.nodes[1].assert_start_raises_init_error(extra_args=['-walletdir={}'.format(wallet_dir), '-wallet=' + wallet_name, '-noserver'], expected_msg=expected_msg, match=ErrorMatch.PARTIAL_REGEX)
  
              if self.is_bdb_compiled():
                  check_wallet_filelock(False)
              if self.is_sqlite_compiled():
                  check_wallet_filelock(True)
  
  if __name__ == '__main__':
      FilelockTest().main()
  
new_contents: #!/usr/bin/env python3
  # Copyright (c) 2018-2020 The Bitcoin Core developers
  # Distributed under the MIT software license, see the accompanying
  # file COPYING or http://www.opensource.org/licenses/mit-license.php.
  """Check that it's not possible to start a second bitcoind instance using the same datadir or wallet."""
  import os
  import random
  import string
  
  from test_framework.test_framework import BitcoinTestFramework
  from test_framework.test_node import ErrorMatch
  
  class FilelockTest(BitcoinTestFramework):
      def set_test_params(self):
          self.setup_clean_chain = True
          self.num_nodes = 2
  
      def setup_network(self):
          self.add_nodes(self.num_nodes, extra_args=None)
          self.nodes[0].start()
          self.nodes[0].wait_for_rpc_connection()
  
      def run_test(self):
          datadir = os.path.join(self.nodes[0].datadir, self.chain)
          self.log.info(f"Using datadir {datadir}")
  
          self.log.info("Check that we can't start a second bitcoind instance using the same datadir")
          expected_msg = f"Error: Cannot obtain a lock on data directory {datadir}. {self.config['environment']['PACKAGE_NAME']} is probably already running."
          self.nodes[1].assert_start_raises_init_error(extra_args=[f'-datadir={self.nodes[0].datadir}', '-noserver'], expected_msg=expected_msg)
  
          if self.is_wallet_compiled():
              def check_wallet_filelock(descriptors):
                  wallet_name = ''.join([random.choice(string.ascii_lowercase) for _ in range(6)])
                  self.nodes[0].createwallet(wallet_name=wallet_name, descriptors=descriptors)
                  wallet_dir = os.path.join(datadir, 'wallets')
                  self.log.info("Check that we can't start a second bitcoind instance using the same wallet")
                  if descriptors:
                      expected_msg = "Error: SQLiteDatabase: Unable to obtain an exclusive lock on the database, is it being used by another bitcoind?"
                  else:
                      expected_msg = "Error: Error initializing wallet database environment"
                  self.nodes[1].assert_start_raises_init_error(extra_args=[f'-walletdir={wallet_dir}', f'-wallet={wallet_name}', '-noserver'], expected_msg=expected_msg, match=ErrorMatch.PARTIAL_REGEX)
  
              if self.is_bdb_compiled():
                  check_wallet_filelock(False)
              if self.is_sqlite_compiled():
                  check_wallet_filelock(True)
  
  if __name__ == '__main__':
      FilelockTest().main()
  
subject: use f-strings in feature_filelock.py