CSHARP-1525 - Add DiscardBufferedData method to JsonReader

src/MongoDB.Bson/IO/JsonReader.cs

@@ -111,15 +111,24 @@ public JsonReader(TextReader textReader, JsonReaderSettings settings)
         {
         }
 
-        private JsonReader(JsonBuffer buffer, JsonReaderSettings settings)
+        internal JsonReader(JsonBuffer buffer, JsonReaderSettings settings)
             : base(settings)
         {
             _buffer = buffer;
             _jsonReaderSettings = settings; // already frozen by base class
             _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>

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