test: stabilize load-sensitive release contracts

tests/AcDream.App.Tests/Streaming/LandblockBuildFactoryTests.cs

@@ -193,32 +193,57 @@ public sealed class LandblockBuildFactoryTests
     }
 
     [Fact]
-    public async Task Build_UsesTheSuppliedSharedReaderGate()
+    public void Build_UsesTheSuppliedSharedReaderGate()
     {
         var dat = CreateDat(out RecordingDatProxy proxy);
         proxy.Add(LandblockId, new LandBlock { Id = LandblockId });
         object gate = new();
         var factory = Factory(dat, gate);
         using var started = new ManualResetEventSlim();
+        LandblockBuild? result = null;
+        Exception? workerError = null;
+        var worker = new Thread(() =>
+        {
+            started.Set();
+            try
+            {
+                result = factory.Build(Request(LandblockStreamJobKind.LoadFar));
+            }
+            catch (Exception error)
+            {
+                workerError = error;
+            }
+        })
+        {
+            IsBackground = true,
+            Name = "LandblockBuildFactory shared-gate contract",
+        };
 
         Monitor.Enter(gate);
-        Task<LandblockBuild?> build;
+        bool startedInTime;
+        bool blockedOnGate;
+        bool readWhileBlocked;
         try
         {
-            build = Task.Run(() =>
-            {
-                started.Set();
-                return factory.Build(Request(LandblockStreamJobKind.LoadFar));
-            });
-            Assert.True(started.Wait(TimeSpan.FromSeconds(2)));
-            Assert.False(proxy.ReadObserved.Wait(TimeSpan.FromMilliseconds(100)));
+            worker.Start();
+            startedInTime = started.Wait(TimeSpan.FromSeconds(5));
+            blockedOnGate = startedInTime && SpinWait.SpinUntil(
+                () => (worker.ThreadState & ThreadState.WaitSleepJoin) != 0,
+                TimeSpan.FromSeconds(5));
+            readWhileBlocked = proxy.ReadObserved.IsSet;
         }
         finally
         {
             Monitor.Exit(gate);
         }
 
-        Assert.NotNull(await build);
+        bool joined = worker.Join(TimeSpan.FromSeconds(10));
+        Assert.True(startedInTime, "the dedicated build thread did not start");
+        Assert.True(blockedOnGate, "the build thread never blocked on the supplied gate");
+        Assert.False(readWhileBlocked, "the DAT read bypassed the supplied gate");
+        Assert.True(joined, "the build thread did not finish after the gate was released");
+        Assert.Null(workerError);
+        Assert.NotNull(result);
         Assert.True(proxy.ReadObserved.IsSet);
     }