Use atomic tag counter (#118)

* Use task-local storage for the tag, instead of thread-local

* Use atomic tag counter

NEWS.md

@@ -1,5 +1,12 @@
 # History of Measurements.jl
 
+## v2.8.0 (2022-09-01)
+
+### New features
+
+* The counter of the tags of `Measurement` objects is now atomic
+  ([#118](https://github.com/JuliaPhysics/Measurements.jl/pull/118)).
+
 ## v2.7.2 (2022-05-21)
 
 ### Bug Fixes

src/Measurements.jl

@@ -77,11 +77,10 @@ end
 @generated empty_der1(x::Measurement{T}) where {T<:AbstractFloat} = Derivatives{T}()
 @generated empty_der2(x::T) where {T<:AbstractFloat} = Derivatives{x}()
 
-const tag_counters = UInt64[1]
-function __init__()
-    nthr = Base.Threads.nthreads()
-    resize!(tag_counters, nthr)[:] = range(UInt64(1), step=typemax(UInt64)÷nthr, length=nthr)
+# Start from 1, 0 is reserved to derived quantities
+const tag_counter = Threads.Atomic{UInt64}(1)
 
+function __init__()
     @require Unitful="1986cc42-f94f-5a68-af5c-568840ba703d" include("unitful.jl")
     @require QuadGK="1fd47b50-473d-5c70-9696-f719f8f3bcdc" include("quadgk.jl")
     @require SpecialFunctions="276daf66-3868-5448-9aa4-cd146d93841b" include("special-functions.jl")
@@ -95,7 +94,7 @@ function measurement(val::T, err::T) where {T<:AbstractFloat}
     if iszero(err)
         Measurement{T}(val, err, UInt64(0), newder)
     else
-        @inbounds tag = tag_counters[Base.Threads.threadid()] += 1
+        tag = Threads.atomic_add!(tag_counter, UInt64(1))
         return Measurement{T}(val, err, tag, Derivatives(newder, (val, err, tag)=>one(T)))
     end
 end