Fix #100

src/tools.jl

@@ -31,8 +31,8 @@ May be used to mark a region in parallel code to be executed by a single task on
 See [`try_enter!`](@ref) and [`reset!`](@ref).
 """
 mutable struct OnlyOneRegion
-    @atomic latch::Bool
-    OnlyOneRegion() = new(false)
+    @atomic task::Union{Task, Nothing}
+    OnlyOneRegion() = new(nothing)
 end
 
 """
@@ -62,24 +62,23 @@ end
 ```
 """
 function try_enter!(f, s::OnlyOneRegion)
-    latch = @atomic :monotonic s.latch
-    if latch
+    ct = current_task()
+    t = @atomic :monotonic s.task
+    if !isnothing(t) && ct != t
         return
     end
-    (_, success) = @atomicreplace s.latch false=>true
-    if !success
-        return
+    if ct == t || (@atomicreplace s.task nothing=>ct).success
+        f()
     end
-    f()
     return
 end
 
 """
 Reset the `OnlyOneRegion` (so that it can be used again).
 """
 function reset!(s::OnlyOneRegion)
-    @atomicreplace s.latch true=>false
-    nothing
+    @atomic s.task = nothing
+    return
 end
 
 """

test/runtests.jl

@@ -454,6 +454,22 @@ end
         catch ErrorException
             @test false
         end
+
+        x = 0
+        y = 0
+        try
+            @tasks for i in 1:10
+                @set ntasks = 2
+
+                y += 1 # not safe (race condition)
+                @only_one begin
+                    x += 1 # parallel-safe because only a single task will execute this
+                end
+            end
+            @test x == 5 # a single task should have incremented x 5 times
+        catch ErrorException
+            @test false
+        end
     end
 
     @testset "@only_one + @one_by_one" begin