Add checks to prevent concurrent executions of local-cname.

local_cname/cli.py

@@ -1,9 +1,11 @@
 import argparse
 import socket
+import sys
 import time
 from pathlib import Path
 
-from clickclick import Action, info
+from clickclick import Action, info, error
+from filelock import FileLock, Timeout
 
 
 def main():
@@ -13,11 +15,30 @@ def main():
  args = parser.parse_args()
 
  hosts_file = Path('/etc/hosts')
+ backup_file = hosts_file.with_suffix('.local-cname-backup')
+ lock_file = hosts_file.with_suffix('.local-cname-lock')
+
+ lock = FileLock(str(lock_file))
+ try:
+ with lock.acquire(timeout=1):
+ edit_etc_hosts(hosts_file, backup_file, args)
+ except Timeout:
+ error('Another instance of local-cname seems to be running.')
+ info('(if a previous process crashed, remove {} '
+ 'and check the contents of {} and {})'.format(lock_file, hosts_file, backup_file))
+ sys.exit(1)
 
+
+def edit_etc_hosts(hosts_file, backup_file, args):
  with hosts_file.open() as fd:
  old_contents = fd.read()
 
- backup_file = hosts_file.with_suffix('.local-cname-backup')
+ HEADER = '#### Start of entries generated by local-cname'
+ if HEADER in old_contents:
+ error('{} seems to have already been modified by local-cname.'.format(hosts_file))
+ info('Remove the local-cname header line from this file to proceed.')
+ sys.exit(1)
+
  with backup_file.open('w') as fd:
  fd.write(old_contents)
 
@@ -40,7 +61,7 @@ def main():
  with Action('Writing {} ..'.format(hosts_file)):
  with hosts_file.open('w') as fd:
  fd.write(old_contents)
- fd.write('#### Start of entries generated by local-cname\n')
+ fd.write('{}\n'.format(HEADER))
  for hostname, ip in entries:
  fd.write('{} {}\n'.format(ip, hostname))
 

setup.py

@@ -22,7 +22,7 @@ def readme():
  keywords='dns hosts local',
  license='GNU General Public License v3 (GPLv3)',
  setup_requires=['flake8'],
- install_requires=['clickclick'],
+ install_requires=['clickclick', 'filelock'],
  tests_require=[],
  classifiers=[
  'Programming Language :: Python',