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+#!/usr/bin/env python
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+"""
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+Licensed to the Apache Software Foundation (ASF) under one
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+or more contributor license agreements. See the NOTICE file
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+distributed with this work for additional information
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+regarding copyright ownership. The ASF licenses this file
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+to you under the Apache License, Version 2.0 (the
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+"License"); you may not use this file except in compliance
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+with the License. You may obtain a copy of the License at
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+
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+ http://www.apache.org/licenses/LICENSE-2.0
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+
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+Unless required by applicable law or agreed to in writing, software
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+distributed under the License is distributed on an "AS IS" BASIS,
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+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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+See the License for the specific language governing permissions and
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+limitations under the License.
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+
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+Ambari Agent
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+
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+"""
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+
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+from resource_management.core.logger import Logger
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+
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+class FcntlBasedProcessLock(object):
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+ """A file descriptor based lock for interprocess locking.
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+ The lock is automatically released when process dies.
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+
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+ WARNING: A file system and OS must support fcntl.lockf.
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+ Doesn't work on Windows systems. Doesn't work properly on
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+ some NFS implementations.
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+
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+ Currently Ambari uses FcntlBasedProcessLock only when parallel
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+ execution is enabled on the agent.
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+
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+ WARNING: Do not use this lock for synchronization between threads.
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+ Multiple threads in a same process can simultaneously acquire this lock.
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+ It should be used only for locking between processes.
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+ """
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+
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+ def __init__(self, lock_file_path, skip_fcntl_failures, enabled = True):
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+ """
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+ :param lock_file_path: The path to the file used for locking
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+ :param skip_fcntl_failures: Use this only if the lock is not mandatory.
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+ If set to True, the lock will ignore fcntl call failures.
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+ Locking will not work, if fcntl is not supported.
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+ skip_fcntl_failures prevents exceptions raising in this case.
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+ :param enabled: If is set to False, fcntl will not be imported and lock/unlock methods return immediately.
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+ """
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+ self.lock_file_name = lock_file_path
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+ self.lock_file = None
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+ self.acquired = False
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+ self.skip_fcntl_failures = skip_fcntl_failures
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+ self.enabled = enabled
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+
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+ def blocking_lock(self):
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+ """
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+ Creates the lock file if it doesn't exist.
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+ Waits to acquire an exclusive lock on the lock file descriptor.
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+ """
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+ if not self.enabled:
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+ return
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+ import fcntl
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+ Logger.info("Trying to acquire a lock on {0}".format(self.lock_file_name))
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+ if self.lock_file is None or self.lock_file.closed:
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+ self.lock_file = open(self.lock_file_name, 'a')
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+ try:
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+ fcntl.lockf(self.lock_file, fcntl.LOCK_EX)
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+ except:
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+ if self.skip_fcntl_failures:
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+ Logger.exception("Fcntl call raised an exception. A lock was not aquired. "
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+ "Continuing as skip_fcntl_failures is set to True")
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+ else:
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+ raise
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+ else:
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+ self.acquired = True
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+ Logger.info("Acquired the lock on {0}".format(self.lock_file_name))
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+
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+ def unlock(self):
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+ """
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+ Unlocks the lock file descriptor.
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+ """
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+ if not self.enabled:
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+ return
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+ import fcntl
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+ Logger.info("Releasing the lock on {0}".format(self.lock_file_name))
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+ if self.acquired:
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+ try:
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+ fcntl.lockf(self.lock_file, fcntl.LOCK_UN)
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+ except:
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+ if self.skip_fcntl_failures:
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+ Logger.exception("Fcntl call raised an exception. The lock was not released. "
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+ "Continuing as skip_fcntl_failures is set to True")
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+ else:
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+ raise
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+ else:
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+ self.acquired = False
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+ try:
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+ self.lock_file.close()
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+ self.lock_file = None
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+ except IOError:
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+ Logger.warning("Failed to close {0}".format(self.lock_file_name))
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+
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+ def __enter__(self):
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+ self.blocking_lock()
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+ return None
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+
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+ def __exit__(self, exc_type, exc_val, exc_tb):
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+ self.unlock()
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+ return False
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+
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Andrew Onishuk