Safe File Access with os Root

The order service stores each customer's invoice as a file in one directory, and a function returns a file by name. A customer supplies the name, and the program below supplies one that nobody planned for.

Output:

1001.txt: "order 1001: 2500 cents\n" <nil>
../secrets.txt: "payment key: k-77\n" <nil>

The function works as written, and it has just returned a file from outside its directory. How can a program require that a set of file operations stay inside one directory, with no way for a name to break that rule?

Path Traversal

A file name that contains `..

.....

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