Look more closely at how you're obtaining a lock:
- You are opening a lock to a semaphore file (lock.tmp).
- Once that lock is obtained you are opening an output file (somefile$_.tmp).
- You unlock/close your semaphore file.
- You write to somefile$_.tmp
- You close somefile$_.tmp.
- You copy from your tmp file to a new file.
Steps 4, 5, and 6 are unprotected.
You should not be releasing your semaphore file lock until after you're done writing to somefile$_.tmp, and in fact, probably not even until after the copy operation. And probably shouldn't be using LOCK_UN as a matter of habit (simply closing a filehandle will terminate the lock, and explicitly unlocking before closing can create a race condition, though not in this usage.)
You may already know about this set of slides from Mark Jason Dominus, but just in case: File Locking Tricks and Traps.
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