From 93bda02b786f73acc8c109aa7f1616872980b735 Mon Sep 17 00:00:00 2001
From: Adam Williamson <awilliam@redhat.com>
Date: Wed, 4 Jan 2017 09:46:28 -0800
Subject: [PATCH] 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.
---
 bmwqemu.pm | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/bmwqemu.pm b/bmwqemu.pm
index 4f12b30..e07b5ae 100755
--- a/bmwqemu.pm
+++ b/bmwqemu.pm
@@ -19,6 +19,7 @@ use strict;
 use warnings;
 use Time::HiRes qw(sleep gettimeofday);
 use IO::Socket;
+use Fcntl ':flock';
 
 use Thread::Queue;
 use POSIX;
@@ -80,7 +81,7 @@ sub save_vars() {
     my $fn = "vars.json";
     unlink "vars.json" if -e "vars.json";
     open(my $fd, ">", $fn);
-    fcntl($fd, F_SETLKW, pack('ssqql', F_WRLCK, 0, 0, 0, $$)) or die "cannot lock vars.json: $!\n";
+    flock($fd, LOCK_EX) or die "cannot lock vars.json: $!\n";
     truncate($fd, 0) or die "cannot truncate vars.json: $!\n";
 
     # make sure the JSON is sorted