async can't modify out-of-scope simple variables

[staticfloat]

Let us imagine we have a simple variable, such as a number. We call an @async block that uses it and modifies it, but are then surprised that the value is not reflected in the outer scope. Example:

x = 0
@sync begin
    x += 10
    @async begin
        x += 20
    end
end
println("Unboxed: $x")

x = [0]
@sync begin
    x[1] += 10
    @async begin
        x[1] += 20
    end
end
println("Boxed: $x")

Result:

$ julia blah.jl 
Unboxed: 10
Boxed: [30]

Is there any way this can be fixed? I feel like this has something to do with the fact that x is immutable object.

[JeffBezanson]

All these constructs copy bindings, since they were designed for distributed memory where another processor's variables cannot (efficiently), and should not be updated this way.

Use @task directly, or a version of @async (found in task.jl) that does not call localize_vars.

closing due to many related issues: #2669 #5930 #3713 #6760

[staticfloat]

Great, that fixed my problem, Jeff!

For those looking at this in the future, I was able to get something that "just works" by changing my @async calls to @task, saving the resultant Task object and passing it to Base.sync_add() and Base.enq_work():

x = 0
@sync begin
    x += 10
    t = @task begin
        x += 20
    end
    Base.sync_add(t)
    Base.enq_work(t)
end

println("Unboxed: $x")

[JeffBezanson]

I think we should probably change @async not to call localize_vars, since it doesn't do anything with remote memory. However the other thing localize_vars does is copy in global variables, which is kind of useful.