PagedBufferedTextWriter does not flush synchronous data prior to writing data asynchronously

Fixes #5241

src/Microsoft.AspNetCore.Mvc.ViewFeatures/Internal/PagedBufferedTextWriter.cs

@@ -86,19 +86,22 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
             _charBuffer.Append(value);
         }
 
-        public override Task WriteAsync(char value)
+        public override async Task WriteAsync(char value)
         {
-            return _inner.WriteAsync(value);
+            await FlushAsync();
+            await _inner.WriteAsync(value);
         }
 
-        public override Task WriteAsync(char[] buffer, int index, int count)
+        public override async Task WriteAsync(char[] buffer, int index, int count)
         {
-            return _inner.WriteAsync(buffer, index, count);
+            await FlushAsync();
+            await _inner.WriteAsync(buffer, index, count);
         }
 
-        public override Task WriteAsync(string value)
+        public override async Task WriteAsync(string value)
         {
-            return _inner.WriteAsync(value);
+            await FlushAsync();
+            await _inner.WriteAsync(value);
         }
 
         protected override void Dispose(bool disposing)

test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/Internal/PagedBufferedTextWriterTest.cs

@@ -187,6 +187,63 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
             Assert.Equal<char>(Content, inner.ToString().ToCharArray());
         }
 
+        [Fact]
+        public async Task SynchronousWrites_FollowedByAsyncWriteString_WritesAllContent()
+        {
+            // Arrange
+            var pool = new TestArrayPool();
+            var inner = new StringWriter();
+
+            var writer = new PagedBufferedTextWriter(new TestArrayPool(), inner);
+
+            // Act
+            writer.Write('a');
+            writer.Write(new[] { 'b', 'c', 'd' });
+            writer.Write("ef");
+            await writer.WriteAsync("ghi");
+
+            // Assert
+            Assert.Equal("abcdefghi", inner.ToString());
+        }
+
+        [Fact]
+        public async Task SynchronousWrites_FollowedByAsyncWriteChar_WritesAllContent()
+        {
+            // Arrange
+            var pool = new TestArrayPool();
+            var inner = new StringWriter();
+
+            var writer = new PagedBufferedTextWriter(new TestArrayPool(), inner);
+
+            // Act
+            writer.Write('a');
+            writer.Write(new[] { 'b', 'c', 'd' });
+            writer.Write("ef");
+            await writer.WriteAsync('g');
+
+            // Assert
+            Assert.Equal("abcdefg", inner.ToString());
+        }
+
+        [Fact]
+        public async Task SynchronousWrites_FollowedByAsyncWriteCharArray_WritesAllContent()
+        {
+            // Arrange
+            var pool = new TestArrayPool();
+            var inner = new StringWriter();
+
+            var writer = new PagedBufferedTextWriter(new TestArrayPool(), inner);
+
+            // Act
+            writer.Write('a');
+            writer.Write(new[] { 'b', 'c', 'd' });
+            writer.Write("ef");
+            await writer.WriteAsync(new[] { 'g', 'h', 'i' });
+
+            // Assert
+            Assert.Equal("abcdefghi", inner.ToString());
+        }
+
         [Fact]
         public async Task FlushAsync_ReturnsPages()
         {