Wrapping Ruby global variables within a scope

Question

I'm evaluating scripts in Ruby, and I'd like each script to have it's own sandbox with a global variable called $window. The $window variable should point to something different depending on which sandbox the script is running in. A thread-local variable would work, but I'm not using threads. I'm using the Ruby C API, so that opens the possibilities a bit.

Right now, I run each script within a Binding, so they are somewhat sandboxed there. A binding can have closed local variables, but not globals. Here's the idea:

sandbox1 = window1.get_binding
sandbox2 = window2.get_binding
sandbox3 = window3.get_binding

sandbox1.eval('$window.foo') # calls 'foo' on window 1
sandbox2.eval('$window.foo') # calls 'foo' on window 2
sandbox3.eval('$window.foo') # calls 'foo' on window 3

Is there any way to close a global variable within a Binding? I found a possible solution and posted it below.

It sounds like you're worried about synchronizing the access of several threads to a global? — Eric Walker
– Eric Walker, Commented Dec 21, 2012 at 6:35
They're not in separate threads, although that would make it easy! They're different windows in a web browser. — Tim Mahoney
– Tim Mahoney, Commented Dec 21, 2012 at 16:09
What is the underlying goal you are trying to accomplish? Global variables are generally considered a bad practice. — Aaron Kurtzhals
– Aaron Kurtzhals, Commented Dec 21, 2012 at 16:28
I clarified my question. I want to have multiple sandboxes with global variables of the same name. — Tim Mahoney
– Tim Mahoney, Commented Dec 23, 2012 at 20:40

systho · Accepted Answer · 2012-12-21 10:54:10Z

1

What will be the different windows be tied to ?

If they're bound to a Thread, then the easiest way to solve your problem is to have a Thread local variable, if is bound to something else (let's say current time), then you might want to use a global hash.

I would personnally go for a dedicated class which will make it easier to refactor later (you will probably refactor this since globals are a code smell for poor dependency injection ) :

class Windows < Hash
end
WINDOWS = Windows.new
...
window_scope = WINDOWS[Time.now].binding

then you have a mutable constant which is actually a global collection.

answered Dec 21, 2012 at 10:54

systho

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Tim Mahoney Over a year ago

Sadly, they're not bound to threads, but using a global hash of some sort might work. I'll edit the main post with some clarification.

Tim Mahoney · Accepted Answer · 2012-12-24 18:44:49Z

0

I found a possible solution using the Ruby C API. Anything that's evaluated within a binding will be somewhere under the call frame of that binding. Using Ruby 1.8.7, I can do this in C:

public VALUE getCurrentWindow()
{
    // Get the current call frame from Ruby.
    FRAME* frame = ruby_frame;

    // Traverse up the call frame stack until we find a Window class.
    while (frame) {
        if (RTEST(rb_obj_is_kind_of(frame->self, rb_cWindow))
            return frame->self;

        frame = frame->prev;
    }

    // Couldn't find the window.
    return Qnil;
}

It seems to work, but we'll see if I run into problems.

answered Dec 24, 2012 at 18:44

Tim Mahoney

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