with_scratch_directory is not concurrent/thread safe and not nestable

[tkf]

This implementation of with_scratch_directory

Scratch.jl/src/Scratch.jl

Lines 16 to 23 in 5677106

	function with_scratch_directory(f::Function, scratch_dir::String)
	try
	SCRATCH_DIR_OVERRIDE[] = scratch_dir
	f()
	finally
	SCRATCH_DIR_OVERRIDE[] = nothing
	end
	end

is

1. not concurrent safe (mixing it with @async can cause bugs),
2. not thread safe, and
3. not nestable.

The last point is about

julia> with_scratch_directory("a") do
           @show Scratch.SCRATCH_DIR_OVERRIDE[]
           with_scratch_directory("b") do
               @show Scratch.SCRATCH_DIR_OVERRIDE[]
           end
           @show Scratch.SCRATCH_DIR_OVERRIDE[]
       end
Scratch.SCRATCH_DIR_OVERRIDE[] = "a"
Scratch.SCRATCH_DIR_OVERRIDE[] = "b"
Scratch.SCRATCH_DIR_OVERRIDE[] = nothing

where the last line should be Scratch.SCRATCH_DIR_OVERRIDE[] = "a".

I think with_scratch_directory is a perfect example where using a context variable JuliaLang/julia#35833 is useful.

If you are curious, here is a branch of Scratch.jl that uses ContextVariablesX.jl; a proof-of-concept/backport of ContextVariables I proposed in the PR JuliaLang/julia#35833. We just need to change a few lines:
v1.0.1...tkf:contextvariablesx

[staticfloat]

Well we can't fix the concurrency issues without doing some kind of TLS, but we can fix 3 rather easily by saving the old value then restoring it.