From 99c0cc9d5e9351b475235234a7a419796864ef7d Mon Sep 17 00:00:00 2001
From: Steve Sanderson <SteveSandersonMS@users.noreply.github.com>
Date: Tue, 6 Aug 2019 17:34:46 +0100
Subject: [PATCH] Ensure render batches aren't started from inside each other

---
 .../Components/src/Rendering/Renderer.cs      |  6 +++
 .../Components/test/RendererTest.cs           | 40 +++++++++++++++++++
 2 files changed, 46 insertions(+)

diff --git a/src/Components/Components/src/Rendering/Renderer.cs b/src/Components/Components/src/Rendering/Renderer.cs
index 048ba9a868ad..ed3d075f0aa0 100644
--- a/src/Components/Components/src/Rendering/Renderer.cs
+++ b/src/Components/Components/src/Rendering/Renderer.cs
@@ -409,6 +409,12 @@ protected virtual void ProcessPendingRender()
         private void ProcessRenderQueue()
         {
             EnsureSynchronizationContext();
+
+            if (_isBatchInProgress)
+            {
+                throw new InvalidOperationException("Cannot start a batch when one is already in progress.");
+            }
+
             _isBatchInProgress = true;
             var updateDisplayTask = Task.CompletedTask;
 
diff --git a/src/Components/Components/test/RendererTest.cs b/src/Components/Components/test/RendererTest.cs
index f895c47f10d6..8d3ce2357000 100644
--- a/src/Components/Components/test/RendererTest.cs
+++ b/src/Components/Components/test/RendererTest.cs
@@ -3395,6 +3395,27 @@ public void EventFieldInfoWorksWhenEventHandlerIdWasSuperseded()
             }
         }
 
+        [Fact]
+        public void CannotStartOverlappingBatches()
+        {
+            // Arrange
+            var renderer = new InvalidRecursiveRenderer();
+            var component = new CallbackOnRenderComponent(() =>
+            {
+                // The renderer disallows one batch to be started inside another, because that
+                // would violate all kinds of state tracking invariants. It's not something that
+                // would ever happen except if you subclass the renderer and do something unsupported
+                // that commences batches from inside each other.
+                renderer.ProcessPendingRender();
+            });
+            var componentId = renderer.AssignRootComponentId(component);
+
+            // Act/Assert
+            var ex = Assert.Throws<InvalidOperationException>(
+                () => renderer.RenderRootComponent(componentId));
+            Assert.Contains("Cannot start a batch when one is already in progress.", ex.Message);
+        }
+
         private class NoOpRenderer : Renderer
         {
             public NoOpRenderer() : base(new TestServiceProvider(), NullLoggerFactory.Instance)
@@ -4109,5 +4130,24 @@ protected override void BuildRenderTree(RenderTreeBuilder builder)
         private class DerivedEventArgs : EventArgs
         {
         }
+
+        class CallbackOnRenderComponent : AutoRenderComponent
+        {
+            private readonly Action _callback;
+
+            public CallbackOnRenderComponent(Action callback)
+            {
+                _callback = callback;
+            }
+
+            protected override void BuildRenderTree(RenderTreeBuilder builder)
+                => _callback();
+        }
+
+        class InvalidRecursiveRenderer : TestRenderer
+        {
+            public new void ProcessPendingRender()
+                => base.ProcessPendingRender();
+        }
     }
 }