Disallow negative values for nthreads

Author: nalimilan

src/groupeddataframe/splitapplycombine.jl

@@ -26,6 +26,9 @@ function _combine_prepare(gd::GroupedDataFrame,
     if !ungroup && !keepkeys
         throw(ArgumentError("keepkeys=false when ungroup=false is not allowed"))
     end
+    if nthreads <= 0
+        throw(ArgumentError("nthreads must be equal to or greater than 1 (got $nthreads)"))
+    end
 
     cs_vec = []
     for p in cs

test/grouping.jl

@@ -3299,4 +3299,12 @@ end
     end
 end
 
+@testset "invalid nthreads" begin
+    gdf = groupby(DataFrame(x=1:10, y=1:10), :y)
+    @test_throws ArgumentError select(gdf, :x => sum, nthreads=0)
+    @test_throws ArgumentError transform(gdf, :x => sum, nthreads=0)
+    @test_throws ArgumentError select!(gdf, :x => sum, nthreads=0)
+    @test_throws ArgumentError transform!(gdf, :x => sum, nthreads=0)
+end
+
 end # module