web2py trunk ver 1281, modified base on stable Version 1.67.2 (2009-09-28 16:29:33)

beaware, no browser can handle file over 2GB,
related article http://www.motobit.com/help/scptutl/pa98.htm

In Short

1. set GET value X-Progress-ID with yourUUID, for example

   <form action="http://127.0.0.1:8000/exsample/upload/post?X-Progress-ID=4c1dd2cced4f7c961a0919d58120957e">
   

2. retrieve upload total length from Cache with key "X-Progress-ID:"+yourUUID+":length"

   cache.ram('X-Progress-ID:'+'4c1dd2cced4f7c961a0919d58120957e'+':length', lambda: 0, None)
   

3. retrieve current uploaded length from Cache with key "X-Progress-ID:"+yourUUID+":uploaded"

   cache.ram('X-Progress-ID:'+'4c1dd2cced4f7c961a0919d58120957e'+':uploaded', lambda: 0, None)
   

REALLY LONG STORY

the implement is base on Django snippts,

http://www.djangosnippets.org/snippets/678/
http://www.djangosnippets.org/snippets/679/

you can take a look for reference

Current Implementation

here is the related part for caching the current upload progress at gluon/main.py

def copystream_progress(request, chunk_size= 10**5):
    """
    copies request.env.wsgi_input into request.body 
    and stores progress upload status in cache.ram
    X-Progress-ID:length and X-Progress-ID:uploaded
    """
    if not request.env.content_length:
        return None
    source = request.env.wsgi_input
    size = int(request.env.content_length)
    dest = tempfile.TemporaryFile()
    if not 'X-Progress-ID' in request.get_vars:
        copystream(source, dest, size, chunk_size)
        return dest
    cache_key = 'X-Progress-ID:'+request.get_vars['X-Progress-ID']
    cache = Cache(request)
    cache.ram(cache_key+':length', lambda: size, 0)
    cache.ram(cache_key+':uploaded', lambda: 0, 0)
    while size > 0:
        if size < chunk_size:
            data = source.read(size)
            cache.ram.increment(cache_key+':uploaded', size)
        else:
            data = source.read(chunk_size)
            cache.ram.increment(cache_key+':uploaded', chunk_size)
        length = len(data)
        if length > size:
            (data, length) = (data[:size], size)
        size -= length
        if length == 0:
            break
        dest.write(data)
        if length < chunk_size:
            break
    dest.seek(0)
    cache.ram(cache_key+':length', None)
    cache.ram(cache_key+':uploaded', None)
    return dest

you can see it retrieve request.get_vars['X-Progress-ID'] as cache key and just store information in Cache,
so, you have to pass a GET with key X-Progress-ID to make this work,
then retrieve what you want from Cache.

Why not use a hidden form field to pass this X-Progress-ID?
don't be silly, you can't get any POST vars before you read all of it lol

EXAMPLE

I am gonna bring in the example, here is the form view post.html

{{extend 'layout.html'}}

<script type="text/javascript">
// Generate 32 char random uuid 
function gen_uuid() {
    var uuid = ""
    for (var i=0; i < 32; i++) {
        uuid += Math.floor(Math.random() * 16).toString(16); 
    }
    return uuid
}

// Add upload progress for multipart forms.
$(function() {
    $('form[enctype=multipart/form-data]').submit(function(){ 
        // Prevent multiple submits
        if ($.data(this, 'submitted')) return false;

        var freq = 1000; // freqency of update in ms
        var uuid = gen_uuid(); // id for this upload so we can fetch progress info.
        var progress_url = '/{{=request.application}}/{{=request.controller}}/{{=request.function}}.json'; // ajax view serving progress info

        // Append X-Progress-ID uuid form action
        this.action += (this.action.indexOf('?') == -1 ? '?' : '&') + 'X-Progress-ID=' + uuid;

        var $progress = $('<div id="upload-progress" class="upload-progress"></div>').
            insertAfter($('input[type=submit]')).append('<div class="progress-container"><span class="progress-info">uploading 0%</span><div class="progress-bar"></div></div>');

        $('input[type=submit]').remove()
        // progress bar position
        /*
        $progress.css({
            position: ($.browser.msie && $.browser.version < 7 )? 'absolute' : 'fixed',  
            left: '50%', marginLeft: 0-($progress.width()/2), bottom: '20%'
        }).show();
        */

        $progress.find('.progress-bar').height('1em').width(0).css("background-color", "red");
        // Update progress bar
        function update_progress_info() {
            $progress.show();
            $.getJSON(progress_url, {'X-Progress-ID': uuid, 'random': Math.random()}, function(data, status){
                if (data) {
                    var progress = parseInt(data.uploaded) / parseInt(data.length);
                    var width = $progress.find('.progress-container').width()
                    var progress_width = width * progress;
                    $progress.find('.progress-bar').width(progress_width);
                    $progress.find('.progress-info').text('uploading ' + progress*100 + '%');
                }
                window.setTimeout(update_progress_info, freq);
            });
        };
        window.setTimeout(update_progress_info, freq);

        $.data(this, 'submitted', true); // mark form as submitted.
    });
});
</script>
{{=BEAUTIFY(response._vars)}}

most AJAX stealed from Django snippets http://www.djangosnippets.org/snippets/679/
the import part is

this.action += (this.action.indexOf('?') == -1 ? '?' : '&') + 'X-Progress-ID=' + uuid;

yap, feed URL with GET X-Progress-ID,
you can do it from your view to generate this or from client side JavaScript,
oops, there is another part also important,

var progress_url = '/{{=request.application}}/{{=request.controller}}/{{=request.function}}.json'; // ajax view serving progress info

yup, I cheat it, I get the form and read my progress from same controller function with JSON, you can make another controller function to feed this.

there is also a trick part,

$.getJSON(progress_url, {'X-Progress-ID': uuid, 'random': Math.random()},

what the heck am I doing? why I need a random value that I never used at all??
well, this is a IE issue
http://robertnyman.com/2007/04/04/weird-xmlhttprequest-error-in-ie-just-one-call-allowed/
IE won't let you continue request same URI, it block the request for a while,
I have tested the behavior on IE6/7/8 and they all have same issue,
so you have to make sure screw the URI to get JSON result, FOR IE.

next is the sample controller upload_progress_examples.py to generate the form, receive the form and read the progress

def post():
    if request.get_vars.has_key('X-Progress-ID'):

        cache_key = 'X-Progress-ID:'+request.get_vars['X-Progress-ID']

        length=cache.ram(cache_key+':length', lambda: 0, None)
        uploaded=cache.ram(cache_key+':uploaded', lambda: 0, None)

        return dict(length=length, uploaded=uploaded)

    form = FORM(TABLE(
        TR('File:', INPUT(_type='file', _name='file',
            requires=IS_NOT_EMPTY())),
        TR('', INPUT(_type='submit', _value='SUBMIT')),
        ))

    return dict(form=form)

because I cheat in my view, I have to deal with request.get_vars['X-Progress-ID'],
I retrieve Cache with None expire time in order to get current value without writing into in, tricky part,

but wait, where is my JSON view? ahh, here is it, post.json

{{
###
# response._vars contains the dictionary returned by the controller action
###
try:
   from gluon.serializers import json
   response.write(json(response._vars),escape=False)
   response.headers['Content-Type']='text/json'
except:
   raise HTTP(405,'no json')
}}

this create JSON output

I know current implementation is not safe, but it works,
it allows you to flush any value in GET, anyone can screw it,
the further improvement maybe a ticket function to issue a UUID that you can use for this specific task.

that's all, any comment is welcome :)