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the return value of os.NewFile should be checked before files is changed. It can fail if the environment variables don't match the file descriptor table, e.g. LISTEN_FDS is much larger than the number of open files. The current logic would force consumers of the interface to check for this condition or run into mysterious panics in a completely unrelated code. If os.NewFile fails, just returning nil might be reasonable, even if it means close-on-exec is set on some file descriptors.