How to verify that IO.copy_stream was successful

Question

A great answer here explains how, in Ruby, to download a file without loading it into memory:

https://stackoverflow.com/a/29743394/4852737

require 'open-uri'
download = open('http://example.com/image.png')
IO.copy_stream(download, '~/image.png')

How would I verify that the IO.copy_stream call to download the file was actually successful — meaning that the downloaded file is the exact same file I intended to download, not a half downloaded corrupt file? The documentation says that IO.copy_stream returns the number of bytes that it copied, but how can I know the number of bytes expected when I have not downloaded the file yet?

Answer 1

OpenURI open returns an object which exposes the HTTP response's headers, so you can get the expected byte count from the Content-Length header and compare it to the return value of IO.copy_stream:

require 'open-uri'
download = open 'http://cdn.sstatic.net/stackoverflow/img/apple-touch-icon.png'
bytes_expected = download.meta['content-length']
bytes_copied = IO.copy_stream download, 'image.png'
if bytes_expected != bytes_copied
  raise "Expected #{bytes_expected} bytes but got #{bytes_copied}"
end

It would be surprising if open executed without error and this check still failed, but you never know.