Add a global lock file to Certbot (#4369)

* add fasteners as a dependency

* add LOCK_FILE constant

* Add lock file to Certbot

* Move code to _run_subcommand

* move lock file path into CLI_CONSTANTS

* add --lock-path flag

* move locking code to separate function

* Add TestAcquireFileLock

* assert we log

* test lock contention

* add fasteners to certbot-auto

* Use a different lock file for each test in MainTest

certbot/main.py

@@ -8,6 +8,7 @@ import sys
 import time
 import traceback
 
+import fasteners
 import zope.component
 
 from acme import jose
@@ -866,6 +867,56 @@ def _post_logging_setup(config, plugins, cli_args):
     logger.debug("Discovered plugins: %r", plugins)
 
 
+def acquire_file_lock(lock_path):
+    """Obtain a lock on the file at the specified path.
+
+    :param str lock_path: path to the file to be locked
+
+    :returns: lock file object representing the acquired lock
+    :rtype: fasteners.InterProcessLock
+
+    :raises .Error: if the lock is held by another process
+
+    """
+    lock = fasteners.InterProcessLock(lock_path)
+    logger.debug("Attempting to acquire lock file %s", lock_path)
+
+    try:
+        lock.acquire(blocking=False)
+    except IOError as err:
+        logger.debug(err)
+        logger.warning(
+            "Unable to access lock file %s. You should set --lock-file "
+            "to a writeable path to ensure multiple instances of "
+            "Certbot don't attempt modify your configuration "
+            "simultaneously.", lock_path)
+    else:
+        if not lock.acquired:
+            raise errors.Error(
+                "Another instance of Certbot is already running.")
+
+    return lock
+
+
+def _run_subcommand(config, plugins):
+    """Executes the Certbot subcommand specified in the configuration.
+
+    :param .IConfig config: parsed configuration object
+    :param .PluginsRegistry plugins: available plugins
+
+    :returns: return value from the specified subcommand
+    :rtype: str or int
+
+    """
+    lock = acquire_file_lock(config.lock_path)
+
+    try:
+        return config.func(config, plugins)
+    finally:
+        if lock.acquired:
+            lock.release()
+
+
 def main(cli_args=sys.argv[1:]):
     """Command line argument parsing and main script execution."""
     sys.excepthook = functools.partial(_handle_exception, config=None)
@@ -893,7 +944,7 @@ def main(cli_args=sys.argv[1:]):
     zope.component.provideUtility(report)
     atexit.register(report.atexit_print_messages)
 
-    return config.func(config, plugins)
+    return _run_subcommand(config, plugins)
 
 
 if __name__ == "__main__":