Add multithreading thread safety unit test

test/runtests.jl

@@ -72,6 +72,7 @@ include("helpers.jl")
     include("rounding.jl")
     include("parse.jl")
     include("plotting.jl")
+    include("thread-safety.jl")
 
     # Note: Run the build tests last to ensure that re-compiling the time zones files
     # doesn't interfere with other tests.

test/thread-safety.jl

@@ -0,0 +1,51 @@
+# Test that TimeZones.jl can be safely used in a multithreaded environment.
+# Note that the number of threads being used cannot be changed dynamically, so
+# this test file spawns a new julia process running with multiple threads.
+
+using Test
+
+const program = """
+using TimeZones
+using Test
+
+@assert Threads.nthreads() > 1 "This system does not support multiple threads, so the thread-safety tests cannot be run."
+
+function create_zdt(year, month, day, tz_name)
+    ZonedDateTime(DateTime(year, month, day), TimeZone(tz_name))
+end
+function cycle_zdts()
+    return [
+        try
+            create_zdt(year, month, day, tz_name)
+        catch e
+            e isa Union{ArgumentError,NonExistentTimeError} || rethrow()
+            nothing
+        end
+        for year in 2000:2020
+        for month in 1:5
+        for day in 10:15
+        for tz_name in timezone_names()
+    ]
+end
+
+const outputs = Channel(Inf)
+@sync begin
+    for _ in 1:15
+        Threads.@spawn begin
+            put!(outputs, cycle_zdts())
+        end
+    end
+end
+close(outputs)
+
+const tzs = collect(outputs)
+
+# Test that every Task produced the same result
+allsame(x) = all(y -> y == first(x), x)
+@test allsame(tzs)
+"""
+
+@info "Running Thread Safety tests"
+@testset "Multithreaded TimeZone construction" begin
+    run(`$(Base.julia_cmd()) -t8 --proj -E $(program)`)
+end