result.txt

{:here=>"write0", :length=>124}
{:read=>
  {:timekey=>24998551,
   :tagkey=>"hoge.pos.moge.hogehogehogehoge",
   :keys=>{:moge=>111},
   :id=>"d0d71b3d-b680-4549-b805-a08825e0f71d",
   :s=>1024,
   :c=>1499913050,
   :m=>1499913110}}
{:here=>"write1", :length=>102}
{:read=>
  {:timekey=>24998551,
   :tagkey=>"hoge.pos",
   :keys=>{:moge=>111},
   :id=>"0f98e5f1-e7cb-41e6-a95e-99c1085647c9",
   :s=>20480,
   :c=>1499913050,
   :m=>1499913110}}
{:here=>"write0", :length=>124}
{:read=>
  {:timekey=>24998551,
   :tagkey=>"hoge.pos.moge.hogehogehogehoge",
   :keys=>{:moge=>111},
   :id=>"dd82f6fb-eeb4-47b9-86e9-7aff59a7e1ec",
   :s=>1024,
   :c=>1499913050,
   :m=>1499913110}}
{:here=>"write1", :length=>102}
{:read=>
  {:timekey=>24998551,
   :tagkey=>"hoge.pos",
   :keys=>{:moge=>111},
   :id=>"547caaa2-1684-4e84-8729-dc58a1a98426",
   :s=>20480,
   :c=>1499913050,
   :m=>1499913110}}

truncate-after-write-works-well.rb

require 'securerandom'
require 'pp'
require 'msgpack'

def msgpack_factory
  MessagePack::Factory.new
end

def msgpack_packer(*args)
  msgpack_factory.packer(*args)
end

def msgpack_unpacker(*args)
  msgpack_factory.unpacker(*args)
end

def write_data(data, truncate: true)
  File.open("testing0", 'wb+', 0644) do |file|
    file.set_encoding(Encoding::ASCII_8BIT)
    file.sync = true
    file.binmode
    file.seek(0, IO::SEEK_SET)
    file.write(data)
    file.truncate(data.bytesize) if truncate
  end
end

def read
  File.open("testing0", "rb+") do |file|
    file.set_encoding(Encoding::ASCII_8BIT)
    file.sync = true
    file.binmode
    bindata = file.read
    data = msgpack_unpacker(symbolize_keys: true).feed(bindata).read rescue {}
    pp(read: data)
  end
end

def write0
  data = {
    timekey: Time.now.to_i / 60,
    tagkey: 'hoge.pos.moge.hogehogehogehoge',
    keys: {'moge' => 111},
    id: SecureRandom.uuid,
    s: 1024,
    c: (Time.now.to_i - 60),
    m: Time.now.to_i,
  }
  bin = msgpack_packer.pack(data).to_s
  pp(here: 'write0', length: bin.bytesize)
  write_data(bin)
end

def write1(truncate: true)
  data = {
    timekey: Time.now.to_i / 60,
    tagkey: 'hoge.pos',
    keys: {'moge' => 111},
    id: SecureRandom.uuid,
    s: 20480,
    c: (Time.now.to_i - 60),
    m: Time.now.to_i,
  }
  bin = msgpack_packer.pack(data).to_s
  pp(here: 'write1', length: bin.bytesize)
  write_data(bin, truncate: truncate)
end

File.unlink("testing0")
write0 # longer than 1
read
write1
read

File.unlink("testing0")
write0 # longer than 1
read
write1(truncate: false)
read