Preserve Exact Body Bytes

Some HTTP servers are not well-behaved and respond with invalid data: the response body may not be encoded according to the encoding specified in the HTTP headers, or there may be bytes that are invalid for the given encoding. The YAML and JSON serializers are not generally designed to handle these cases gracefully, and you may get errors when the cassette is serialized or deserialized. Also, the encoding may not be preserved when round-tripped through the serializer.

VCR provides a configuration option to deal with cases like these. The preserve_exact_body_bytes method accepts a block that VCR will use to determine if the body of the given request or response object should be base64 encoded in order to preserve the bytes exactly as-is. VCR does not do this by default, since base64-encoding the string removes the human readability of the cassette.

Alternately, if you want to force an entire cassette to preserve the exact body bytes, you can pass the :preserve_exact_body_bytes => true cassette option when inserting your cassette.

Preserve exact bytes for response body with invalid encoding

Given a file named "preserve.rb" with:

# encoding: utf-8
string = "abc \xFA"
puts "Valid encoding: #{string.valid_encoding?}"

$server = start_sinatra_app do
  get('/') { string }
end

require 'vcr'

VCR.configure do |c|
  c.cassette_library_dir = 'cassettes'
  c.hook_into :webmock
  c.preserve_exact_body_bytes do |http_message|
    http_message.body.encoding.name == 'ASCII-8BIT' ||
    !http_message.body.valid_encoding?
  end
end

def make_request(label)
  puts
  puts label
  VCR.use_cassette('example', :serialize_with => :json) do
    body = Net::HTTP.get_response('localhost', '/', $server.port).body
    puts "Body: #{body.inspect}"
  end
end

make_request("Recording:")
make_request("Playback:")

When I run ruby preserve.rb

Then the output should contain exactly:

Valid encoding: false

Recording:
Body: "abc \xFA"

Playback:
Body: "abc \xFA"

And the file "cassettes/example.json" should contain:

"body":{"encoding":"ASCII-8BIT","base64_string":"YWJjIPo=\n"}

Preserve exact bytes for cassette with :preserve_exact_body_bytes option

Given a file named "preserve.rb" with:

$server = start_sinatra_app do
  get('/') { "Hello World" }
end

require 'vcr'

VCR.configure do |c|
  c.cassette_library_dir = 'cassettes'
  c.hook_into :webmock
  c.default_cassette_options = { :serialize_with => :json }

  c.before_record do |i|
    # otherwise Ruby 2.0 will default to UTF-8:
    i.response.body.force_encoding('US-ASCII')
  end
end

VCR.use_cassette('preserve_bytes', :preserve_exact_body_bytes => true) do
  Net::HTTP.get_response('localhost', '/', $server.port)
end

VCR.use_cassette('dont_preserve_bytes') do
  Net::HTTP.get_response('localhost', '/', $server.port)
end

When I run ruby preserve.rb

Then the file "cassettes/preserve_bytes.json" should contain:

"body":{"encoding":"US-ASCII","base64_string":"SGVsbG8gV29ybGQ=\n"}

And the file "cassettes/dont_preserve_bytes.json" should contain:

"body":{"encoding":"US-ASCII","string":"Hello World"}