@async doesn't seem to actually use a let-block for variables?

[quinnj]

The docs for @async indicate that async-ed variables should be captured in a let-block, but actual use seems to contradict that.

julia> function async_add(x, y)
           println("adding $x + $y")
           return x + y
       end
async_add (generic function with 1 method)

julia> function async_wrong_value()
           y = x = 1
           while true
               sleep(1)
               println("going to add $x + $y")
               @async async_add(x, y)
               x += 1
           end
           return
       end
async_wrong_value (generic function with 1 method)

julia> async_wrong_value()
going to add 1 + 1
adding 2 + 1
going to add 2 + 1
adding 3 + 1
going to add 3 + 1
adding 4 + 1
going to add 4 + 1
adding 5 + 1

vs.

julia> function async_add(x, y)
           println("adding $x + $y")
           return x + y
       end
async_add (generic function with 1 method)

julia> function async_right_value()
           y = x = 1
           while true
               sleep(1)
               println("going to add $x + $y")
               let x=x
                   @async async_add(x, y)
               end
               x += 1
           end
           return
       end
async_right_value (generic function with 1 method)

julia> async_right_value()
going to add 1 + 1
adding 1 + 1
going to add 2 + 1
adding 2 + 1
going to add 3 + 1
adding 3 + 1
going to add 4 + 1
adding 4 + 1

[amitmurthy]

Works as expected in 0.4. The bug has crept in 0.5.

[amitmurthy]

julia/base/task.jl

Line 363 in 4a805b3

expr = localize_vars(esc(:(()->($expr))), false)

and

julia/base/expr.jl

Lines 177 to 188 in 4a805b3

	# wrap an expression in "let a=a,b=b,..." for each var it references
	localize_vars(expr) = localize_vars(expr, true)
	function localize_vars(expr, esca)
	v = find_vars(expr)
	# requires a special feature of the front end that knows how to insert
	# the correct variables. the list of free variables cannot be computed
	# from a macro.
	if esca
	v = map(esc,v)
	end
	Expr(:localize, expr, v...)
	end

is where the localization is being performed.

[amitmurthy]

The last set of changes w.r.t. this behavior was in #14562

@JeffBezanson do you think changes in #14562 is related to this issue?

[amitmurthy]

This is a bug, not really a doc issue.

[bjarthur]

closed by #19594 i believe

[amitmurthy]

Yes. @async no longer implicitly localizes variables. http://docs.julialang.org/en/latest/stdlib/parallel.html#Base.@async