make sure getindex on DataFrameRows does not alias passed selector

[bkamins]

Fixes #3191

[bkamins]

This works around the issue that when making a view a copy of idx is not made. Unfortunately copy is not always doable so it is a bit tricky.

The problem with filter in Base Julia is as follows:

function filter(f, a::AbstractArray)
    (IndexStyle(a) != IndexLinear()) && return a[map(f, a)::AbstractArray{Bool}]

    j = 1
    idxs = Vector{Int}(undef, length(a))
    for idx in eachindex(a)
        @inbounds idxs[j] = idx
        ai = @inbounds a[idx]
        j = ifelse(f(ai), j+1, j)
    end
    resize!(idxs, j-1)
    res = a[idxs]
    empty!(idxs) # I do not know why this line is needed
    sizehint!(idxs, 0) # I do not know why this line is needed
    return res
end

[nalimilan]

Woops. This was introduced by https://github.com/JuliaLang/julia/pull/31929/files#r987877494. Sounds like a good fix at least in the short term. Though it's a bit annoying to have to make all getindex calls copy the input index unless that's really required by the AbstractArray interface. If not, we could define our own filter implementation.

[bkamins]

Though it's a bit annoying to have to make all getindex calls copy the input index

The point is that Base Julia assumes (correctly IMO) that getindex produces an independent object from idx. We internally, for performance reasons, create a view, so I think it is our responsibility to de-alias (note that for other objects than AbstractVector{Int} de-aliasing is ensured by SubDataFrame logic).

[bkamins]

@nalimilan - even if something is changed in Base Julia, I think we still need this fix for backward versions.

[bkamins]

Thank you!