UndefRefError: Race Condition parsing TimeZone from multiple threads: needs coordination

[NHDaly]

The TIME_ZONE_CACHE is not currently thread safe, so parsing a TimeZone object via the following constructor is currently a race condition, and can lead to crashes and errors:

TimeZones.jl/src/types/timezone.jl

Line 1 in 906cea0

const TIME_ZONE_CACHE = Dict{String,Tuple{TimeZone,Class}}()

TimeZones.jl/src/types/timezone.jl

Lines 43 to 47 in 906cea0

	function TimeZone(str::AbstractString, mask::Class=Class(:DEFAULT))
	# Note: If the class `mask` does not match the time zone we'll still load the
	# information into the cache to ensure the result is consistent.
	tz, class = get!(TIME_ZONE_CACHE, str) do
	tz_path = joinpath(TZData.COMPILED_DIR, split(str, "/")...)

The cache either needs to be surrounded by a lock (though this would serialize all TimeZone constructors and probably be worse than having no cache at all), or replicated via thread-local caches.

I would probably suggest a thread-local cache? But interested in other ideas! :)

Maybe just something like this?:

const THREADLOCAL_TIME_ZONE_CACHES = Vector{Dict{String,Tuple{TimeZone,Class}}}()

function TimeZone(str::AbstractString, mask::Class=Class(:DEFAULT))
    tz, class = get!(THREADLOCAL_TIME_ZONE_CACHES[Threads.threadid()], str) do
    end
end

function __init__()
    append!(THREADLOCAL_TIME_ZONE_CACHES, [Dict{String,Tuple{TimeZone,Class}}() for _ in 1:Threads.nthreads()]
end

[KristofferC]

As a note, it is generally better to allocate the object on the thread that it will be used (see JuliaWeb/HTTP.jl#704 and https://github.com/JuliaLang/julia/blob/e4fcdf5b04fd9751ce48b0afc700330475b42443/stdlib/Random/src/RNGs.jl#L369-L385 for examples).

[omus]

Thanks for filing the issue and the suggestions. Feel free to make a PR, otherwise I'll try to get to this during the week.

[NHDaly]

K, this is definitely not done, but i started a PR here: #344

@omus maybe you can pick it up from there? ❤️ Please feel free to edit the PR directly.

🙏 @KristofferC thanks for the examples, that's good to see. 👍