Ruby tempfile anomalous behavior - ruby-on-rails

Question

This is my pry session output:
[1] pry(SomeTask)> epub
=> #<File:/somepath/tmp/x.epub>
[2] pry(SomeTask)> epub.size
=> 134
[3] pry(SomeTask)> File.size("/somepath/tmp/x.epub")
=> 44299
[4] pry(SomeTask)> epub.class
=> Tempfile
I see that File.size yields a different result than the size method of the Tempfile instance.
How is this possible?

Answer 1

The devil is in the details. From the docs for Tempfile#size (emphasis mine):
size()
Returns the size of the temporary file. As a side effect, the IO buffer is flushed before determining the size.
What's happening is that you're using File.size to read the size of the file before the buffer has been flushed—i.e. before all of the bytes have been written to the file—and then you're using Tempfile#size, which flushes that buffer before it calculates the size:
tmp = Tempfile.new('foo')
tmp.write('a' * 1000)
File.size(tmp)
# => 0
tmp.size
# => 1000
But see what happens when you call tmp.size before File.size(tmp):
tmp = Tempfile.new('bar')
tmp.write('a' * 1000)
tmp.size
# => 1000
File.size(tmp)
# => 1000
You can get the behavior you want out of File.size by manually flushing the buffer:
tmp = Tempfile.new('baz')
tmp.write('a' * 1000)
tmp.flush
File.size(tmp)
# => 1000

Answer 2

I'm using Pry version 0.10.1 on Ruby 2.2.2 and can't duplicate that situation:
[1] (pry) main: 0> foo = Tempfile.new('foo')
#<File:/var/folders/yb/whn8dwns6rl92jswry5cz87dsgk2n1/T/foo20150819-83612-1tpkqm4>
[2] (pry) main: 0> File.size(foo.path)
=> 0
[3] (pry) main: 0> foo.size
=> 0
After initialization, the file size is 0 bytes.
[4] (pry) main: 0> foo.write('a')
=> 1
[5] (pry) main: 0> File.size(foo.path)
=> 0
After writing one character to foo, the data has been buffered and not flushed to disk as I'd expect.
[6] (pry) main: 0> foo.size
=> 1
[7] (pry) main: 0> File.size(foo.path)
=> 1
foo.size flushes the buffer then returns the size of the file, which matches what File.size says it is.
When dealing with temporary files created by Tempfile, we don't care or want to know what their size is. They're temporary and will disappear (eventually) and are treated like buffers. If you need a file that is more permanent, then create and write to a normal file.