From 2692e5355a40ef4893e4ada9c2d5731375a82056 Mon Sep 17 00:00:00 2001
From: Alistair Evans <alistair.evans@7layer.net>
Date: Fri, 25 Oct 2019 14:28:40 +0100
Subject: [PATCH] CSHARP-1525 - Add DiscardBufferedData method to JsonReader,
 so consumers of the reader can choose to clear the memory use of the reader
 when reading large JSON documents.

---
 src/MongoDB.Bson/IO/JsonReader.cs             | 13 ++++++--
 .../MongoDB.Bson.Tests/IO/JsonReaderTests.cs  | 33 +++++++++++++++++++
 2 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/src/MongoDB.Bson/IO/JsonReader.cs b/src/MongoDB.Bson/IO/JsonReader.cs
index f33ec492c3c..a5d64f106b6 100644
--- a/src/MongoDB.Bson/IO/JsonReader.cs
+++ b/src/MongoDB.Bson/IO/JsonReader.cs
@@ -111,7 +111,7 @@ public JsonReader(TextReader textReader, JsonReaderSettings settings)
         {
         }
 
-        private JsonReader(JsonBuffer buffer, JsonReaderSettings settings)
+        internal JsonReader(JsonBuffer buffer, JsonReaderSettings settings)
             : base(settings)
         {
             _buffer = buffer;
@@ -119,7 +119,16 @@ private JsonReader(JsonBuffer buffer, JsonReaderSettings settings)
             _context = new JsonReaderContext(null, ContextType.TopLevel);
         }
 
-        // public methods
+        // public methods        
+        /// <summary>
+        /// Resets the underlying buffer; use this if you are streaming very large BSON documents and want to reduce memory
+        /// use.
+        /// </summary>
+        public void DiscardBufferedData()
+        {
+          _buffer.ResetBuffer();
+        }
+
         /// <summary>
         /// Closes the reader.
         /// </summary>
diff --git a/tests/MongoDB.Bson.Tests/IO/JsonReaderTests.cs b/tests/MongoDB.Bson.Tests/IO/JsonReaderTests.cs
index 823ed0dc1d3..a751992cdf8 100644
--- a/tests/MongoDB.Bson.Tests/IO/JsonReaderTests.cs
+++ b/tests/MongoDB.Bson.Tests/IO/JsonReaderTests.cs
@@ -1600,6 +1600,39 @@ public void IsValidBinaryDataSubTypeString_should_return_true_when_value_is_vali
                 result.Should().BeTrue();
             }
         }
+
+        [Fact]
+        public void Can_discard_buffered_data()
+        {
+            var json = "[ " + string.Join(",", Enumerable.Repeat("{ \"a\" : { \"x\" : 1 } }", 1024)) + "]";
+            var textReader = new StreamReader(new MemoryStream(Encoding.UTF8.GetBytes(json)));
+
+            var buffer = new JsonBuffer(textReader);
+            using (var reader = new JsonReader(buffer, JsonReaderSettings.Defaults))
+            {
+                reader.ReadStartArray();
+
+                // Deserialize some.
+                for (int idx = 0; idx < 512; idx++)
+                {
+                    BsonSerializer.Deserialize<BsonDocument>(reader);
+                }
+
+                buffer.Position.Should().BeGreaterThan(512);
+                
+                reader.DiscardBufferedData();
+
+                buffer.Position.Should().Be(0);
+                
+                // Make sure we can continue deserializing after the discard.
+                for (int idx = 0; idx < 512; idx++)
+                {
+                    BsonSerializer.Deserialize<BsonDocument>(reader);
+                }
+
+                reader.ReadEndArray();
+            }
+        }
     }
 
     public static class JsonReaderReflector