Fixing NRE when CacheTagHelper throws and exception (#5989)

Fixes #5988

src/Microsoft.AspNetCore.Mvc.TagHelpers/CacheTagHelper.cs

@@ -104,10 +104,11 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                                 // such that the tokens are inherited.
 
                                 result = ProcessContentAsync(output);
-                                content = await result;
 
                                 entry.SetOptions(options);
                                 entry.Value = result;
+
+                                content = await result;
                             }
                         }
                         catch

test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/CacheTagHelperTest.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Text.Encodings.Web;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Html;
@@ -647,6 +648,100 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
 
             Assert.Equal(2, calls);
         }
+        
+
+        [Fact]
+        public async Task ProcessAsync_ExceptionInProcessing_DoNotThrowInSubsequentRequests()
+        {
+            // Arrange
+            var id = "unique-id";
+            var childContent = "some-content";
+            var cache = new MemoryCache(new MemoryCacheOptions());
+
+            var counter = 0;
+
+            Task<TagHelperContent> GetChildContentAsync(bool useCachedResult, HtmlEncoder encoder)
+            {
+                counter++;
+                if (counter < 3)
+                {
+                    // throw on first two calls
+                    throw new Exception();
+                }
+                else
+                {
+                    // produce content on third call
+                    var tagHelperContent = new DefaultTagHelperContent();
+                    tagHelperContent.SetHtmlContent(childContent);
+                    return Task.FromResult<TagHelperContent>(tagHelperContent);
+                }
+            }
+
+            var tagHelperContext1 = GetTagHelperContext(id);
+            var tagHelperContext2 = GetTagHelperContext(id);
+            var tagHelperContext3 = GetTagHelperContext(id);
+            var tagHelperContext4 = GetTagHelperContext(id);
+
+            var tagHelperOutput1 = new TagHelperOutput("cache", new TagHelperAttributeList(), GetChildContentAsync);
+            var tagHelperOutput2 = new TagHelperOutput("cache", new TagHelperAttributeList(), GetChildContentAsync);
+            var tagHelperOutput3 = new TagHelperOutput("cache", new TagHelperAttributeList(), GetChildContentAsync);
+            var tagHelperOutput4 = new TagHelperOutput("cache", new TagHelperAttributeList(), GetChildContentAsync);
+
+            var cacheTagHelper = new CacheTagHelper(cache, new HtmlTestEncoder())
+            {
+                ViewContext = GetViewContext(),
+                Enabled = true,
+                ExpiresAfter = TimeSpan.FromHours(1.0)
+            };
+
+            // Act - 1
+
+            await Assert.ThrowsAsync<Exception>(() => cacheTagHelper.ProcessAsync(tagHelperContext1, tagHelperOutput1));
+
+            // Assert - 1
+
+            Assert.Equal(1, counter);
+            Assert.Empty(tagHelperOutput1.PreContent.GetContent());
+            Assert.Empty(tagHelperOutput1.PostContent.GetContent());
+            Assert.False(tagHelperOutput1.IsContentModified);
+            Assert.Empty(tagHelperOutput1.Content.GetContent());
+
+            // Act - 2
+
+            await Assert.ThrowsAsync<Exception>(() => cacheTagHelper.ProcessAsync(tagHelperContext2, tagHelperOutput2));
+
+            // Assert - 2
+
+            Assert.Equal(2, counter);
+            Assert.Empty(tagHelperOutput2.PreContent.GetContent());
+            Assert.Empty(tagHelperOutput2.PostContent.GetContent());
+            Assert.False(tagHelperOutput2.IsContentModified);
+            Assert.Empty(tagHelperOutput2.Content.GetContent());
+
+            // Act - 3
+
+            await cacheTagHelper.ProcessAsync(tagHelperContext3, tagHelperOutput3);
+
+            // Assert - 3
+
+            Assert.Equal(3, counter);
+            Assert.Empty(tagHelperOutput3.PreContent.GetContent());
+            Assert.Empty(tagHelperOutput3.PostContent.GetContent());
+            Assert.True(tagHelperOutput3.IsContentModified);
+            Assert.Equal(childContent, tagHelperOutput3.Content.GetContent());
+
+            // Act - 4
+
+            await cacheTagHelper.ProcessAsync(tagHelperContext4, tagHelperOutput4);
+
+            // Assert - 4
+
+            Assert.Equal(3, counter);
+            Assert.Empty(tagHelperOutput4.PreContent.GetContent());
+            Assert.Empty(tagHelperOutput4.PostContent.GetContent());
+            Assert.True(tagHelperOutput4.IsContentModified);
+            Assert.Equal(childContent, tagHelperOutput4.Content.GetContent());
+        }
 
         private static ViewContext GetViewContext()
         {