Pid File

This repo contains a small python module that is intended to allow multiple instances of a program to lock a resource and in the event that the resurce is already locked to report who has locked the resource.

Installing

I have recently added the resource_lock project to pypi. As a result the module can be installed with

cd to your project folder
source venv/bin/activate
python3 -m pip install resource_lock

Installing - backdoor way

Without using pypi the simplest way to install this module into a project is:

cd .../Projects/my_project
cd ..
git clone git@github.com:robertblackwell/resource_lock.git
cd ../my_project
source ./env/bin/activate
pip install ../resource_lock

Where yout python project that wants to use resource_lock resides in .../Projects/my_project

The repo github.com:robertblackwell/resource_lock_example demonstrates this

Usage

Consider a situation where an application wishes to perform an action on an external resource. For example sending a configuration file to an external device.

The following points define the situations:

• there may be multiple instances of the application running at any time for multiple users.

• only one instance of the application is permitted to try sending to the device at the same time as there is no locking mechanism in the remote device.

Thus this external device is a resource that cannot be shared.

A mechanism is required to ensure this device is "locked" while one instance of the app sends the file.

The basic mechansim is as follows:

• use the Linux advisory locking mechanism flock.
• provide a globally unique name for the resource to be protected resource_name
• provide a path to a directory in which a lock file and pid file will be created
• derive the name of the lock file as f"{resource_name}.lock"
• derive the name of the pid file as f"{resource_name}.pid"
• apply flock to the lock file and
  • if successful write the process pid into the pid file
  • if flock failed read the pid of the holder of the lock from the pid file and use that pid to get the user name of the lock holder

Then the following code segment demonstrates how to use this module

from resource_lock import ResourceLock
# or from resource_lock.resource_lock import ResourceLock if using version lower than 0.4.0

resource_name = "....."
lockfile_dir_path = "....."

lock = ResourceLock(resource_name, lockfile_dir_path)

token = lock.acquire():
if token is not None:
    # the lock is successfully acquired
    send_file_to_remote_device(.....)
    # or whatever other stuff you need to do here

    lock.release(token)
else:
    # failed to acquire lock - print an error message

    print(f"Failed to acquire lock details are : {lock.error_msg}")