test(host): red repro — apply-boundary cache clear races in-flight VT evaluation into ObjectDisposedException (02/S12)

2026-07-13 10:09:37 -04:00
parent 05c35c93d9
commit d67900e455
2 changed files with 59 additions and 1 deletions
@@ -380,6 +380,64 @@ public sealed class RoslynVirtualTagEvaluatorTests
        CompiledCacheCount(sut).ShouldBe(1);
    }

+    /// <summary>02/S12 concurrent stress repro (RED on current code): while one loop runs many compiled
+    /// (non-passthrough) evaluations of the same expression, a second loop hammers
+    /// <see cref="RoslynVirtualTagEvaluator.ClearCompiledScripts"/> — the apply-boundary clear. On the
+    /// unfixed tree a clear landing between a child's <c>GetOrCompile</c> and the run's disposed-guard
+    /// surfaces an ObjectDisposedException as a <c>Failure("script threw: Cannot access a disposed
+    /// object…")</c>; the fix retries once and every result stays Success. The whole test is WaitAsync-
+    /// bounded so a regression fails fast rather than hanging.</summary>
+    [Fact]
+    public async Task Evaluate_racing_ClearCompiledScripts_never_fails_with_disposed()
+    {
+        // Generous timeout so the wall-clock budget never fires and confounds the race assertion.
+        using var sut = new RoslynVirtualTagEvaluator(
+            NullLogger<RoslynVirtualTagEvaluator>.Instance, NoOpScriptRoot(), TimeSpan.FromSeconds(5));
+        const string expr = "return (int)ctx.GetTag(\"a\").Value + 1;";
+
+        const int loopCount = 12;
+        const int iterations = 400;
+        var failures = new System.Collections.Concurrent.ConcurrentBag<string>();
+        var remaining = loopCount; // number of concurrent evaluation loops still running
+
+        // Many concurrent evaluation loops put the thread pool under enough pressure that the inner
+        // Task.Run hop inside TimedScriptEvaluator is measurably delayed — widening the
+        // fetch→run-disposed-guard window so a clear reliably lands inside SOME in-flight evaluation.
+        // The clear loop clears on a short spin cadence (not back-to-back) so (a) most evaluations hit
+        // the warm cache — keeping compile cost bounded — and (b) at most one clear lands within a
+        // single evaluation, so the single retry the fix adds is sufficient (a back-to-back clear loop
+        // would both compile-swamp the RED run into a timeout and defeat the one-shot retry). Pre-fix a
+        // clear disposing the just-fetched evaluator surfaces as Failure("script threw: Cannot access a
+        // disposed object…"); post-fix the retry re-fetches into the current generation → all Success.
+        var evalLoops = Enumerable.Range(0, loopCount).Select(_ => Task.Run(() =>
+        {
+            for (var i = 0; i < iterations; i++)
+            {
+                var r = sut.Evaluate("vt-race", expr, new Dictionary<string, object?> { ["a"] = i });
+                if (!r.Success)
+                {
+                    failures.Add(r.Reason ?? "<null reason>");
+                }
+            }
+            Interlocked.Decrement(ref remaining);
+        }, TestContext.Current.CancellationToken)).ToArray();
+
+        var clearLoop = Task.Run(() =>
+        {
+            while (Volatile.Read(ref remaining) > 0)
+            {
+                sut.ClearCompiledScripts();
+                Thread.SpinWait(4000); // ~tens of µs between clears — frequent, but not back-to-back
+            }
+        }, TestContext.Current.CancellationToken);
+
+        await Task.WhenAll(evalLoops.Append(clearLoop))
+            .WaitAsync(TimeSpan.FromSeconds(45), TestContext.Current.CancellationToken);
+
+        failures.ShouldBeEmpty(
+            $"every evaluation must succeed despite racing clears; observed: {string.Join(" | ", failures)}");
+    }
+
    /// <summary>Reads the count of the private compiled-script cache via reflection. The cache is a
    /// <c>CompiledScriptCache&lt;,&gt;</c> which exposes a <c>Count</c> property (not <c>ICollection</c>).</summary>
    private static int CompiledCacheCount(RoslynVirtualTagEvaluator sut)