If memory serves, I coded up a primitive two-stage, file-based, self-help locking mechanism. Something akin to what follows. In this example, for file test.dat:

  1. Create a file (lock request token) based on the original filename
    • Include its creation timestamp
    • Include its modification timestamp
    • Include its server and unique PID
    • Make these sortable by creation, server, pid, modification
  2. Loop until it is the oldest lock request for this file
    • Re-create your lock request token file if it's been removed.
    • Update your lock request token modification timestamp
    • Delete requests older than the lock request timeout
    • sleep a short bit (don't burn the CPU)
  3. Create another file (lock bid token) based on the original filename
    • Include its creation timestamp
    • Include its modification timestamp
    • Include its server and unique PID
    • Make these sortable by creation, server, pid, modification
  4. Loop until it is the oldest lock bid for this file
    • Re-create your lock bid token file if it's been removed.
    • Update your lock bid token modification timestamp
    • Delete bids older than the lock bid timeout
    • sleep a short bit (don't burn the CPU)
  5. Unlocking consists of removing the lock request and lock bid files
  6. Long-running processes need to have a mechanism for updating the modification timestamp on the bid token to avoid it being deleted by others using the same self-help system.

Then requests for lock simply queue up. This is not collision-proof, but in a low-volume environment was sufficiently collision-resistent for my needs (the math was favorable).

In reply to Re^3: How to do atomic file locking? by marinersk
in thread How to do atomic file locking? by Acapulco

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