Same problem here... all my views are rendered twice.

During my debugging of this issue I tried to output the return value of a content_tag that where given a block. It seems to only return the empty tag without any content:

header = content_tag :h1 do

"Foobar"

end

puts header

The above code should output "

Foobar

Ups... The comment field did not escape my html tags as I expected... trying again:

The above code should output "<h1>Foobar</h1>", but outputs "<h1></h1>"

As far as I can see (and please bear with me if I don't get it right - it's the first time I look into the ActionView source), the problem is that concat (_actionpack/lib/action_view/helpers/text_helper.rb:28_) relies on the return value of the output_buffer method. If the return value is either nil or false, concat will return the given string without doing anything (the call to concat could in this case technically be omitted). In all other situations it will push it to the existing @output_buffer.

This would only work if we could expect @output_buffer to be nil if concat where called from a helper method inside another helper method, or a helper method inside a controller. But @output_buffer will always be initialized (and stay so) if a view has begun to be rendered.

So there is two solutions as far as I can see:

1. Either don't call concat unless your helper method is called directly from a view.
2. Or replace the simple "if output_buffer" check inside the concat method with something that knows if it is called directly from a view

Previously this was done by a call to _block_is_within_action_view?_ which would:

def block_is_within_action_view?(block)
  eval("defined? _erbout", block.binding)
end

But since we don't pass the binding to the concat method anymore this cannot be done. I'm wonder if there is another to get the same info?

You're right on. I've been investigating workarounds that preserve the speedup and memory savings I got from this change, but we're between a rock and a hard place.

What was it that took up processor time and memory? Was it the check to verify if '_erbout' existsed on the binding or was it the push to 'ActionView::Base.erb_variable'? ... or both?

Passing Proc#binding ate a ton of memory. Having to slice _erbout instead of swapping in a new buffer ate a ton of memory and cpu time.