Jason’s postings and stuff

There are a ton of possible upload progress solutions for Web applications that need them. I had been using the upload progress that came with Merb, but I had modify mup.js to make Ajax calls using mootools instead of Prototype. Ultimately the progress status wasn’t updating consistently, so I sought a pure mootools compatible solution.

That brought me to the FancyUpload upload progress solution by Harald Kirschner. It’s Adobe Flash based, meaning upload status can be determined on the client side using Flash itself. It’s easy to use and fairly sexy, but Flash comes with its own bundle of deep hurting.

I discovered that Flash 8/Win does something extremely retarded. It sends an initial POST with a Content-Length of 0. Sane multipart MIME parsers simply toss it out as the garbage it is. Unfortunately, that means Rails via CGI.rb and Merb are raising exceptions and Flash is giving up. I don’t agree with the solution to directly monkey patch CGI.rb; I don’t know enough about multipart MIME parsing to feel secure running a monkey patched CGI.rb on a production system.

Moreover, I am using Merb, and its multipart MIME parser is completely different than CGI.rb, meaning I’d have to come up with my own monkey patch without potentially compromising security. No, thanks.

Instead, I am taking advantage of Apache 2.2, which I am already using for mod_proxy_balancer between several Mongrel instances, to fix Flash 8’s broken HTTP POST. If you’ve ever wondered how to do some extremely strange stuff using Apache’s powerful rewrite rules, examine my solution to Flash 8’s suckiness below.

BrowserMatch ^Shockwave Flash$ swf=true
RewriteCond %{HTTP:Content-Length} ^0$
RewriteCond %{ENV:swf} ^true$
RewriteRule . %{REQUEST_URI}?flash8win=1 [QSA]

RewriteCond %{QUERY_STRING} flash8win
RewriteRule . - [E=strip_content_type:true]
RequestHeader unset Content-Type env=strip_content_type

First, we check to see if it’s a Flash client. Flash 8 wasn’t reporting a version string in any of my packet captures, so I simply used the string ‘Shockwave Flash’. Apache’s BrowserMatch allows you to set an environment variable which I do if the client is Flash.

Next, I perform a series of checks to see if the current request is a broken one created by Flash 8/Win. If the Content-Length is 0, we have a winner. The query string is overloaded with a string signaling Flash 8/Win has been detected.

Finally, I perform a match and if my query string indicates I found Flash 8/Win, I set an environment variable which RequestHeader can match upon. The Content-Type header is then removed. Without a Content-Type header indicating a multipart POST, the MIME parser is skipped and CGI.rb and Merb’s parser have nothing to do and I can ignore the empty request entirely.

def handle_upload
  if ! params[:flash8win].blank?
    render :text => '', :status => 200
    return
  end
  .. other code ..
end

Clearly, you can do some amazing things with Apache2 and its rewrite rules.

It’s useful to note that under Merb 0.3.4, a 500 error is returned when the MIME parser encounters something worthy of raising an exception. Flash 8/Win doesn’t care if it gets a 500 response to its initial Content-Length: 0 request. It did care when it received a 404 Not Found, which is what Merb 0.2.0 and Mongrel using CGI.rb return. If I simply upgraded to Merb 0.3.4, I would never have written any rewrites rules, but had tons of mysterious 500 errors in my merb.4000.log.

Yay Flash.