openlock 1.0.3

A Python locking library not depending on inter-process locking primitives in the OS

openlock

A locking library not depending on inter-process locking primitives in the OS.

Api

• FileLock(lock_file, timeout=None). Constructor. The optional timeout argument is the default for the corresponding argument of acquire() (see below).
• FileLock.acquire(timeout=None). Attempts to acquires the lock. The optional timeout argument specifies the maximum waiting time before a Timeout exception is thrown.
• FileLock.release(). Releases the lock.
• FileLock.locked(). Indicates if the lock is held by a process.
• FileLock.getpid(). The PID of the process that holds the lock, if any. Otherwise returns None.

How does it work

A valid lock file has two lines of text containing respectively:

• pid: the PID of the process holding the lock;
• name: the content of argv[0] of the process holding the lock.

A lock file is considered stale if the pair (pid, name) does not belong to a process in the process table.

A process that seeks to acquire a lock first looks for an existing valid lock file. If there is no lock file, or if it is stale or unparsable, then the process atomically creates a new lock file with its own data. It then sleeps 0.5 seconds (configurable) and checks if the lock file has not been overwritten by a different process. If not then it has acquired the lock.

Issues

The algorithm fails if a process needs more than 0.5 seconds to create a new lock file after detecting the absence of a valid one.

History

This is a refactored version of the locking algorithm used by the worker for the Fishtest web application https://tests.stockfishchess.org/tests.