Lock Folder

Obtain a lock by posting a bid into a lock folder, and checking for rivals.

Installation

pip install lockfolder

How to Use It

import pathlib
from lockfolder import lockfolder

lockfolder.init()

p = pathlib.Path("path/to/lockfolder")

if lockfolder.lock(p):
    print("aquired lock")
    ...
    ...
    lockfolder.unlock(p)
else:
    print("failed to aquire lock")

Adapt to context manager, throw-finally systems, decorator, what have you, as you like.

Lock Files

Lock files are JSON files that are logically equivalent to:

----------[ filename: <GUID>.json ]-----------------------------------
  {
    "PID": <integer PID>,
    "CREATED": <integer timestamp>
  }
----------------------------------------------------------------------

Concept

This is a mutex system, that ensures that only one process has access to a resource.

It's defining features are:

• a filesystem folder is used to bid for a lock
• it is cross-platform to contemporary operating systems
• it's very simple
• it has only one external dependency: psutil
• it is robust to recycled PIDs

Some of the limitations of this system, are:

• live-locks are possible, which occurs when too many processes all try to get a lock at the same time
  • all petitioners will fail to achieve a lock
  • insistent petitioners should repeat efforts at connection with an exponential backoff
• security and file permissions are not taken account of
  • it is possible for a malicious user or process to manually delete or create lock files
• there is an extremely unlikely possibility of a v4 GUID collision
• PID recycling fails if the same PID is recycled to a competitor within the second (highly unusual)

Basic Strategy:

• each process self-assigns a GUID
• each process notes its PID
• each process notes its create-time
• a folder keeps the locks for the resource ("bids" and "locks" are the same thing in this system)
• the procedure is "check-bid-check", and if any check shows another bid/lock, the process deletes its own bid (if it got that far) and returns "fail"
• if the "check-bid-check" passes, then the bid is left until the process completes, at which point the bid is deleted
• bids contain the process PID and creation time of the process, and may be deleted by any process that wishes to challenge a prior bid, provided that it can determine that a process created at with process ID is no longer running
• upon a fail, at the programmer's discretion, processes may delay and retry, with delays having a random component, and increasing duration between attempts

Procedure:

Here is the basic procedure more specifically:

STEP 10. CHECK -- check the lock folder for the presence of any files; if there are files, END (error state: FAIL); if there are no files, proceed to step 20

STEP 20. BID -- write a file into the lock folder, containing the following, then proceed to step 30

• filename: (self-selected-GUID-for-this-process).json
• JSON file content:
  • PID: (process-id)
  • PROCESS-START-TIME: (timestamp-for-this-process)

STEP 30. CHECK -- check the lock folder for the presence of any files, other than my own: if there are other files, proceed to step 40, otherwise, proceed to step 50

STEP 40. DELETE & FAIL -- delete the bid file that was created in step 20, then END (error state: FAIL)

STEP 50. OPERATE -- the lock has been acquired (it's the same file as the bid created in step 20) -- do whatever you please with it

STEP 60. DELETE & END -- delete the bid file that was created in step 20, then END (error state: SUCCESS)

Additional Resources

• https://www.reddit.com/r/AskProgramming/comments/186ot93/is_this_checkbidcheck_mutex_strategy_okay/ -- a Reddit thread, in which I requested verification of correctness for this system.

written: 2023-11-29