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2360

Pull #686

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Use `flock` not `fcntl` locking for portability (POO #13822)

The structure used by `fcntl` to define the type, location, and
length of the desired lock uses integer types - `off_t` and
`pid_t` - that are platform dependent. When writing C you can
just create a structure using those types, but when writing perl,
there doesn't seem to be any terribly easy way to find out what
`off_t` and `pid_t` actually are, in `pack()` terms. There's a
fairly good discussion here:

https://www.perlmonks.org/bare/index.pl?node_id=920008

The code we currently have will work only on platforms where
`off_t` happens to be a signed quad and `pid_t` happens to be
a signed long. This seems to be the case for Fedora and SUSE
64-bit arches, but certainly isn't the case for Fedora 32-bit
arches (where `off_t` is a signed long instead), and it may
differ in other scenarios too.

So I suggest we just use perl `flock` to lock the file before
writing instead. I've looked into it quite a lot, and AFAICS,
none of the differences between `fcntl` and `flock` should be a
problem for us. I don't think `vars.json` is ever likely to be
on an NFS share (`flock` doesn't work over NFS), we don't need
to lock a specific area of the file, and the fact that flock is
an advisory file descriptor lock rather than a filesystem lock
record should not be a problem, as AFAICS, this is the *only*
code we ever use to write to `vars.json`, and it will certainly
respect its own lock.
Pull Request #686: Use `flock` not `fcntl` locking for portability (POO #13822)

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