Developer builds atomic file-locking guard to prevent duplicate charges in distributed systems

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A developer spent over a week designing a concise exactly-once execution guard using POSIX file primitives, specifically the O_CREAT|O_EXCL flag combination, to prevent duplicate operations such as double charges in distributed systems. The core mechanism relies on the atomic nature of O_EXCL, which ensures only one process can create a given file, eliminating the need for Redis or external dependencies. Early implementations contained subtle race conditions in the claim-expiry and reclaim logic, where unlinking stale lock files introduced windows for duplicate execution, confirmed by deterministic stress tests reproducing failures 20 out of 20 times. The final solution eliminated all unlink, rename, and flock calls entirely, replacing mutable claim files with append-only generational files so expired claims are never deleted or overwritten. Even after the write path was hardened, eleven additional bugs were found exclusively in the read side, particularly around claim-age calculation, highlighting that deciding whether a lock is valid is significantly harder than acquiring one.

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