Implement ParallelScope.None (#169)

Consumer/Consumer.fsproj

@@ -12,6 +12,7 @@
         <Compile Include="RunSubProcess.fs" />
         <Compile Include="TestNonParallel.fs" />
         <Compile Include="TestParallel.fs" />
+        <Compile Include="TestParallelIndividualTest.fs" />
         <Compile Include="TestStdout.fs" />
         <Compile Include="TestParameterisedFixture.fs" />
         <Compile Include="TestSetUp.fs" />

Consumer/TestParallelIndividualTest.fs

@@ -0,0 +1,59 @@
+namespace Consumer
+
+open System
+open System.Collections.Concurrent
+open System.Threading
+open NUnit.Framework
+open FsUnitTyped
+
+// These tests are flaky if the bug https://github.com/Smaug123/unofficial-nunit-runner/issues/168 is unfixed.
+[<TestFixture>]
+module TestParallelIndividualTest =
+
+    type private Transitions =
+        | Started of int
+        | LockAcquired of int
+        | Exited of int
+
+    let locker = obj ()
+    let private sequence = ConcurrentQueue<Transitions> ()
+
+    [<Test>]
+    [<Parallelizable(ParallelScope.None)>]
+    let ``does not run in parallel`` () =
+        sequence.Enqueue (Transitions.Started 0)
+        let entered = Monitor.TryEnter (locker, TimeSpan.Zero)
+
+        if entered then
+            sequence.Enqueue (Transitions.LockAcquired 0)
+            Monitor.Exit locker
+            sequence.Enqueue (Transitions.Exited 0)
+        else
+            sequence.Enqueue (Transitions.Exited 0)
+            failwith "failed to acquire the lock"
+
+    [<Test>]
+    let ``unrestricted parallelism`` () =
+        sequence.Enqueue (Transitions.Started 1)
+        let entered = Monitor.TryEnter (locker, TimeSpan.Zero)
+
+        if entered then
+            sequence.Enqueue (Transitions.LockAcquired 1)
+            Monitor.Exit locker
+            sequence.Enqueue (Transitions.Exited 1)
+        else
+            sequence.Enqueue (Transitions.Exited 1)
+            failwith "failed to acquire the lock"
+
+    [<OneTimeTearDown>]
+    let ``It worked`` () =
+        let sequence = sequence |> Seq.toList
+
+        let allowed n =
+            [ Transitions.Started n ; Transitions.LockAcquired n ; Transitions.Exited n ]
+
+        if sequence <> allowed 0 @ allowed 1 && sequence <> allowed 1 @ allowed 0 then
+            let s = sequence |> Seq.map string<Transitions> |> String.concat "\n"
+            failwith $"Unexpected sequence!\n%s{s}"
+
+        ()