asyncmap hides actual error stack trace

[marius311]

Up to current master, asyncmap and single-worker pmap don't show you the stack trace to the actual error:

julia> foo(i) = error()
foo (generic function with 1 method)

julia> asyncmap(foo, 1:2)
ERROR: 
Stacktrace:
 [1] (::Base.var"#873#875")(x::Task)
   @ Base ./asyncmap.jl:177
 [2] foreach(f::Base.var"#873#875", itr::Vector{Any})
   @ Base ./abstractarray.jl:2576
 [3] maptwice(wrapped_f::Function, chnl::Channel{Any}, worker_tasks::Vector{Any}, c::UnitRange{Int64})
   @ Base ./asyncmap.jl:177
 [4] wrap_n_exec_twice
   @ ./asyncmap.jl:153 [inlined]
 [5] async_usemap(f::typeof(foo), c::UnitRange{Int64}; ntasks::Int64, batch_size::Nothing)
   @ Base ./asyncmap.jl:103
 [6] #asyncmap#857
   @ ./asyncmap.jl:81 [inlined]
 [7] asyncmap(f::Function, c::UnitRange{Int64})
   @ Base ./asyncmap.jl:81
 [8] top-level scope
   @ REPL[2]:1

julia> using Distributed

julia> nworkers()
1

julia> pmap(foo, 1:2)
ERROR: 
Stacktrace:
  [1] (::Base.var"#873#875")(x::Task)
    @ Base ./asyncmap.jl:177
  [2] foreach(f::Base.var"#873#875", itr::Vector{Any})
    @ Base ./abstractarray.jl:2576
  [3] maptwice(wrapped_f::Function, chnl::Channel{Any}, worker_tasks::Vector{Any}, c::UnitRange{Int64})
    @ Base ./asyncmap.jl:177
  [4] wrap_n_exec_twice
    @ ./asyncmap.jl:153 [inlined]
  [5] #async_usemap#858
    @ ./asyncmap.jl:103 [inlined]
  [6] #asyncmap#857
    @ ./asyncmap.jl:81 [inlined]
  [7] pmap(f::Function, p::WorkerPool, c::UnitRange{Int64}; distributed::Bool, batch_size::Int64, on_error::Nothing, retry_delays::Vector{Any}, retry_check::Nothing)
    @ Distributed /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.7/Distributed/src/pmap.jl:126
  [8] pmap(f::Function, p::WorkerPool, c::UnitRange{Int64})
    @ Distributed /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.7/Distributed/src/pmap.jl:101
  [9] pmap(f::Function, c::UnitRange{Int64}; kwargs::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Distributed /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.7/Distributed/src/pmap.jl:156
 [10] pmap(f::Function, c::UnitRange{Int64})
    @ Distributed /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.7/Distributed/src/pmap.jl:156
 [11] top-level scope
    @ REPL[7]:1

The single-worker pmap is particularly painful (for me at least) since often I want to debug first with a single worker, but then exactly in that case I don't get a stack trace (the multi-worker case does show it). There's a monkeypatched solution given in #36709 (comment) but would be great to have this fixed in Julia.

[ericphanson]

Should be fixed by #42105