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It sounds like it is time for you to update your computer science knowledge by learning about race conditions.
I need to ensure only one conucrrent usage [....] works just fine
It works just fine as far as you have noticed so far. It certainly doesn't "ensure" only one concurrent use; it more like usually prevents more than one concurrent use. (:
You code must perform the following steps:
And, in a modern computer system, CPU resources are shared, so each process that is serving a CGI request can be interrupted between any of those steps (or in the middle of steps) in order to let some other process do some work for a bit. Two CGI requests coming it at roughly the same time can thus perform those steps in the following order:
Note that they both see the status as "1" and both end up running concurrently. This is why operating systems provide locking mechanisms and why you often need to use such.