Rails - Jul 2005 - DoubleRenderError exception

The DoubleRenderError exception recently introduced is forcing me to go back and
rewrite my apps authentication handler.  Before I could do a redirect right in
the middle of a request. Whatever was happening after that went unnoticed, I
assumed it was simply ending the request after the redirect was called, but
apparently it was going on and rendering the page.

Why can''t a redirect simply halt execution of the action? Is there a
way to rescue the exception outside of the controller and gracefully exit?

-Jeff

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>Whatever was happening after that went unnoticed, I assumed it was simply 
> ending the request after the redirect was called, but apparently it was
going
> on and rendering the page.
If you do your authentication in a before_filter (which you probably
should, unless you have a specific reason not to), just return false
from the filter and execution will stop.  That''s how the login
generator does it...

If your authentication code doesn''t return false, exit, throw an
exception, or do something drastic like that, you should probably
alter it so it does.  I''ve seen too many php scripts that do
authentication by something like

if(!$_SESSION["logged_in"]) { header("Location: login.php");
}

When you hit that page and you''re not logged in, you do get
redirected.  If you modify your user-agent to ignore http redirects,
however, you can see the page''s content just fine, even if
you''re not
logged in.

Just something to be aware of :)

Tyler

Well it is not just my authentication handler, things have broken all over 
my application...

I''ll do stuff like this

def generic_action
  @data = @order.generic_data ||= GenericData.new
  if @request.post?
    @data.attributes = @params[''data'']
    redirect_to :action => next if @data.save
  end
end

Now I have to "return redirect_to" everywhere instead. It''s
just clutter. I
think if I were able to rescue the exception at some good place without 
modifying the base libraries, perhaps with a configuration option, I could 
keep my code nice and simple and not have to return out of the action.

Or, it could simply not render anything after the first one, instead of 
throwing the exception.  I can think of some instances when I would actually 
want to render multiple pages. It just seems like an unnecessary limitation. 
What purpose does it serve? Was it added to facilitate something else? 
I''d
like a simple way to revert back to the old behavior.

-Jeff

----- Original Message ----- 
From: "Tyler Kiley"
<tyler.kiley-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
To: <rails-1W37MKcQCpIf0INCOvqR/iCwEArCW2h5@public.gmane.org>
Sent: Wednesday, July 06, 2005 3:29 PM
Subject: Re: [Rails] DoubleRenderError exception

>Whatever was happening after that went unnoticed, I assumed it was simply
> ending the request after the redirect was called, but apparently it was 
> going
> on and rendering the page.
If you do your authentication in a before_filter (which you probably
should, unless you have a specific reason not to), just return false
from the filter and execution will stop.  That''s how the login
generator does it...

If your authentication code doesn''t return false, exit, throw an
exception, or do something drastic like that, you should probably
alter it so it does.  I''ve seen too many php scripts that do
authentication by something like

if(!$_SESSION["logged_in"]) { header("Location: login.php");
}

When you hit that page and you''re not logged in, you do get
redirected.  If you modify your user-agent to ignore http redirects,
however, you can see the page''s content just fine, even if
you''re not
logged in.

Just something to be aware of :)

Tyler
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> def generic_action
>   @data = @order.generic_data ||= GenericData.new
>   if @request.post?
>     @data.attributes = @params[''data'']
>     redirect_to :action => next if @data.save
>   end
> end
This should continue to work,  I have stuff like this all over my
application which has been running on edge rails for a while.
> Or, it could simply not render anything after the first one, instead of
> throwing the exception.  I can think of some instances when I would
actually
> want to render multiple pages. It just seems like an unnecessary
limitation.
> What purpose does it serve? Was it added to facilitate something else? 
I''d
> like a simple way to revert back to the old behavior.
This behaviour has never worked.  The only difference now is that
instead of failing silently, we tell you that something isn''t working.

render_text "aaa"
render_text "BBB"

has never rendered "aaaBBB"

-- 
Cheers

Koz

Some more commentary on this:

http://tech.rufy.com/entry/61


On Jul 6, 2005, at 5:29 PM, Michael Koziarski wrote:
>> def generic_action
>>   @data = @order.generic_data ||= GenericData.new
>>   if @request.post?
>>     @data.attributes = @params[''data'']
>>     redirect_to :action => next if @data.save
>>   end
>> end
>>
>
> This should continue to work,  I have stuff like this all over my
> application which has been running on edge rails for a while.
>
>
>> Or, it could simply not render anything after the first one,  
>> instead of
>> throwing the exception.  I can think of some instances when I  
>> would actually
>> want to render multiple pages. It just seems like an unnecessary  
>> limitation.
>> What purpose does it serve? Was it added to facilitate something  
>> else?  I''d
>> like a simple way to revert back to the old behavior.
>>
>
> This behaviour has never worked.  The only difference now is that
> instead of failing silently, we tell you that something isn''t
working.
>
> render_text "aaa"
> render_text "BBB"
>
> has never rendered "aaaBBB"
>
> -- 
> Cheers
>
> Koz
> _______________________________________________
> Rails mailing list
> Rails-1W37MKcQCpIf0INCOvqR/iCwEArCW2h5@public.gmane.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
>

May I suggest a redirect_to! method?

Here is some code I picked off the RAA, called "ruby-goto":

## goto.rb

STACK = []

class Label
   attr_accessor :name;
   attr_accessor :block;

   def initialize(name, block);
     @name  = name
     @block = block
   end

   def ==(sym)
     @name == sym
   end
end

class Goto < Exception;
   attr_accessor :label
   def initialize(label); @label = label; end
end

def label(sym, &block)
   STACK.last << Label.new(sym, block)
end

def frame_start
   STACK << []
end

def frame_end
   frame = STACK.pop
   idx   = 0

   begin
     for i in (idx...frame.size)
       frame[i].block.call if frame[i].block
     end
   rescue Goto => g
     idx = frame.index(g.label)
     retry
   end
end

def goto(label)
   raise Goto.new(label)
end

And from the goto_test.rb file:

## goto_test.rb

require ''goto''

def test
   frame_start

   label(:start) { goto :b }
   label(:a) { print "world!\n"; goto :c }
   label(:b) { print "hello "; goto :a }

   label(:c)

   frame_end
end


frame_start

label(:a) { test }
label(:b) { goto :a }

frame_end

Does this seem like a viable solution?

Duane Johnson
(canadaduane)




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Jeff,
> def generic_action
>   @data = @order.generic_data ||= GenericData.new
>   if @request.post?
>     @data.attributes = @params[''data'']
>     redirect_to :action => next if @data.save
>   end
> end
That will work.  I''m assuming that you''re running into
problems
because you have an explicit render call at the bottom of your
generic_action.  If you rely on rails'' implicit rendering,
you''re
fine, but if you call it yourself, you get the exception.

My only beef with the DoubleRenderException is that a redirect_to
counts as a render, which is kinda non-intuitive the first time you
run into it.  It took me a while to figure that out.