Note that rust uses fcntl(F_SETLKW), not flock(). From the man page:

If a process closes any file descriptor referring to a file, then all of the process's locks on that file are released, regardless of the file descriptor(s) on which the locks were obtained. This is bad: it means that a process can lose its locks on a file such as /etc/passwd or /etc/mtab when for some reason a library function decides to open, read, and close the same file.
The threads in a process share locks. In other words, a multithreaded program can't use record locking to ensure that threads don't simultaneously access the same region of a file.

In other words, it's completely unsafe to use in a concurrent program. That's probably why that not production quality at all comment is in there.

If you want a flock() in your node, see this npm module.

add flock API to fs module #122

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