Why does Ruby open-uri's open return a StringIO in my unit test, but a FileIO in my controller?

Question

I inherited a Rails 2.2.2 app that stores user-uploaded images on Amazon S3. The attachment_fu-based Photo model offers a rotate method that uses <

Answer 1

The open-uri library uses a constant to set the 10KB size limit for StringIO objects.

> OpenURI::Buffer::StringMax
=> 10240 

You can change this setting to 0 to prevent open-uri from ever creating a StringIO object. Instead, this will force it to always generate a temp file.

Just throw this in an initializer:

# Don't allow downloaded files to be created as StringIO. Force a tempfile to be created.
require 'open-uri'
OpenURI::Buffer.send :remove_const, 'StringMax' if OpenURI::Buffer.const_defined?('StringMax')
OpenURI::Buffer.const_set 'StringMax', 0

You can't just set the constant directly. You need to actually remove the constant and then set it again (as above), otherwise you'll get a warning:

warning: already initialized constant StringMax

UPDATED 12/18/2012: Rails 3 doesn't require OpenURI by default, so you need to add require 'open-uri' at the top of the initializer. I updated the code above to reflect that change.