Here's what's happening: after one process unlinks the file, other processes are still able to flock their open file handles after the first process closes its file handle:

          child 1      child 2
          open(...)
                       open(...)
          flock(...)
          unlink(...)
          close(...)
                       flock(...) (succeeds!)
          ...          ...
[download]

In general flock and unlink on the same file do not work well together. You really only want to use flock on a file which will always exist. Assuming that in this case you are simulating a pool of worker threads trying to grab a unit of work, you can fix things by checking to see if the file still exists after the flock succeeds.

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