`asyncmap`: silently incorrect results with StructArrays (at least)

[aplavin]

MWE:

julia> using StructArrays

# asyncmap() with sleep() inside returns nothings:
julia> asyncmap(StructArray(a=[1,2,3])) do x
               sleep(1)
               "abc $x"
       end
3-element Vector{Nothing}:
 nothing
 nothing
 nothing

# fine without sleep():
julia> asyncmap(StructArray(a=[1,2,3])) do x
               "abc $x"
       end
3-element Vector{String}:
 "abc (a = 1,)"
 "abc (a = 2,)"
 "abc (a = 3,)"

# fine with map() instead of asyncmap():
julia> map(StructArray(a=[1,2,3])) do x
               sleep(1)
               "abc $x"
       end
3-element Vector{String}:
 "abc (a = 1,)"
 "abc (a = 2,)"
 "abc (a = 3,)"

The bug seems to be in asyncmap, it assumes too much about the collection. asyncmap works like this:

• create array of Refs in

  julia/base/asyncmap.jl

  Line 155 in 95f30e5

  asyncrun = map(wrapped_f, c...)

• loop over these Refs (in multiple tasks, but the multi-task aspect doesn't matter)
• mutate ref.x to store the resulting element

  julia/base/asyncmap.jl

  Line 94 in 95f30e5

  exec_func = (r,args) -> (r.x = f(args...))

In the MWE above with StructArrays, the asyncrun variable is naturally also a StructArray (it results from map). But extracting an element from a StructArray creates a new object, not a copy – no way around it. So, the r.x = ... assignment doesn't mutate anything in the resulting array that gets returned.

I guess, to be generic, the Channel should store indices in the original array instead of elements?