Multi-threading hangs combine on Julia nightly

[bkamins]

Reproducer:

julia> using DataFrames, Test, Random

julia> const ≅ = isequal
isequal (generic function with 44 methods)

julia> y = Any[1, missing, missing, 2, 4]
5-element Vector{Any}:
 1
  missing
  missing
 2
 4

julia> x = 1:length(y)
1:5

julia>         df = DataFrame(x=x, y1=y, y2=reverse(y))
5×3 DataFrame
 Row │ x      y1       y2
     │ Int64  Any      Any
─────┼─────────────────────────
   1 │     1  1        4
   2 │     2  missing  2
   3 │     3  missing  missing
   4 │     4  2        missing
   5 │     5  4        1

julia>         gd = groupby(df, :x)
GroupedDataFrame with 5 groups based on key: x
First Group (1 row): x = 1
 Row │ x      y1   y2
     │ Int64  Any  Any
─────┼─────────────────
   1 │     1  1    4
⋮
Last Group (1 row): x = 5
 Row │ x      y1   y2
     │ Int64  Any  Any
─────┼─────────────────
   1 │     5  4    1

julia>               combine(gd, [:x, :y1] => ((x, y) -> (sleep((x == [5])/10); y[1])) => :y1,
                                 [:x, :y2] => ((x, y) -> (sleep((x == [5])/10); y[end])) => :y2)
5×3 DataFrame
 Row │ x      y1       y2
     │ Int64  Int64?   Int64?
─────┼─────────────────────────
   1 │     1        1        4
   2 │     2  missing        2
   3 │     3  missing  missing
   4 │     4        2  missing
   5 │     5        4        1

julia>               combine(gd, [:x, :y1] => ((x, y) -> (sleep((x == [5])/10); y[1])) => :y1,
                                 [:x, :y2] => ((x, y) -> (sleep((x == [5])/10); y[end])) => :y2)
5×3 DataFrame
 Row │ x      y1       y2
     │ Int64  Int64?   Int64?
─────┼─────────────────────────
   1 │     1        1        4
   2 │     2  missing        2
   3 │     3  missing  missing
   4 │     4        2  missing
   5 │     5        4        1

julia>               combine(gd, [:x, :y1] => ((x, y) -> (sleep((x == [5])/10); y[1])) => :y1,
                                 [:x, :y2] => ((x, y) -> (sleep((x == [5])/10); y[end])) => :y2)

And the operation does not terminate (note that I run the same operation three times). Most likely we have some race condition in handling of multi-threading that got exposed on Julia nightly.

For the issue to show up we need to pass two operations to combine. If one is passed things work OK.

[bkamins]

OK - I have narrowed it down. The issue is with sleep:

julia> @sync begin
           Threads.@spawn sleep(0.01)
       end
Task (done) @0x000001e7a9a71750

julia> @sync begin
           Threads.@spawn sleep(0.01)
       end
Task (done) @0x000001e7fa5c45e0

julia> @sync begin
           Threads.@spawn sleep(0.01)
       end # hangs

[bkamins]

Fixed in Base Julia