README

This component provides a named system lock that allows for synchronization across multiple processes without relying on lockfiles. Linux, MacOS, and Windows are supported.

Example usage

Implementation

The lock is implemented per platform:

• Linux - As a pthread mutex.
• MacOS - Using bootstrap_check_in(), interpreting ownership of receive rights on a Mach service name as ownership of a lock.
• Windows - As a kernel mutex.

Linux-specific notes

The lock is implemented using a pthread mutex in shared memory. Contenders attempt to open a POSIX shared memory object, creating the object if it does not exist. The mutex is configured with the PTHREAD_MUTEX_ROBUST attribute to ensure that it remains recoverable if the process holding the lock exits abnormally.

Due to the nature of the shm_unlink system call, it is impossible for any contending process to determine if it is safe to destroy the shared memory object. Consider the following sequence of processes A, B, and C:

1. A: Shared memory foo does not exist. Create the shared memory object.
2. A: Creates and acquires the mutex lock in shared memory.
3. B: Shared memory foo exists. Open the existing shared memory object.
4. A: Release the mutex lock and shm_unlink “foo”. Note: Process B can still use the shared memory until it closes it. Future attempts to open foo will fail with ENOENT. foo can be recreated.
5. C: Shared memory foo does not exist. Create the shared memory object.
6. B: Acquire the mutex lock in shared memory.
7. C: Creates and acquires the mutex lock in shared memory.

In the sequence above, unlinking the shared memory created a situation in which processes B and C hold the lock simultaneously. Thus, by design, the lock uses a leaky mutex in shared memory. The leak occurs once per named lock and is around 40 bytes.