Sync shared code from aspnetcore

src/libraries/Common/tests/Tests/System/Net/aspnetcore/Http2/HPackDecoderTest.cs

@@ -151,6 +151,25 @@ public void DecodesIndexedHeaderField_DynamicTable()
             Assert.Equal(_headerValueString, _handler.DecodedHeaders[_headerNameString]);
         }
 
+        [Fact]
+        public void DecodesIndexedHeaderField_DynamicTable_ReferencedEntryRemovedOnInsertion()
+        {
+            // Pre-populate the dynamic table so we'll have something to reference.
+            // This entry will have index 62 (0x3E).
+            _dynamicTable.Insert(_headerNameBytes, _headerValueBytes);
+            Assert.Equal(1, _dynamicTable.Count);
+
+            Assert.InRange(_dynamicTable.MaxSize, 1, _literalHeaderNameBytes.Length); // Assert that our string will be too big
+
+            byte[] encoded = (new byte[] { 0x40 | 0x3E }) // Indexing enabled (0x40) | dynamic table (62 = 0x3E) as a 6-integer, 
+                .Concat(_literalHeaderName) // A header value that's too large to fit in the dynamic table
+                .ToArray();
+
+            _decoder.Decode(encoded, endHeaders: true, handler: _handler);
+            Assert.Equal(0, _dynamicTable.Count); // The large entry caused the table to be wiped
+            Assert.Equal(_literalHeaderNameString, _handler.DecodedHeaders[_headerNameString]); // but we got the header anyway
+        }
+
         [Fact]
         public void DecodesIndexedHeaderField_OutOfRange_Error()
         {