Avoid holding onto the buffer when parsing unknown length-delimited fields

protobuf/CHANGELOG.md

@@ -1,5 +1,10 @@
 ## 3.0.1-dev
 
+* Avoid holding onto the input buffer when parsing unknown length-delimited
+  fields. ([#863])
+
+[#863]: https://github.com/google/protobuf.dart/pull/863
+
 ## 3.0.0
 
 * Require Dart `2.19`.

protobuf/lib/src/protobuf/coded_buffer_reader.dart

@@ -133,7 +133,13 @@ class CodedBufferReader {
   }
 
   bool readBool() => _readRawVarint32(true) != 0;
-  List<int> readBytes() {
+
+  /// Read a length-delimited field as bytes. Note that the returned
+  /// [Uint8List] will be a view of the [CodedBufferReader]'s buffer. When
+  /// storing the returned value directly (instead of e.g. parsing it as a
+  /// UTF-8 string and copying) make sure to copy it to avoid holding on to the
+  /// whole message.
+  Uint8List readBytes() {
     final length = readInt32();
     _checkLimit(length);
     return Uint8List.view(

protobuf/lib/src/protobuf/unknown_field_set.dart

@@ -65,7 +65,8 @@ class UnknownFieldSet {
         mergeFixed64Field(number, input.readFixed64());
         return true;
       case WIRETYPE_LENGTH_DELIMITED:
-        mergeLengthDelimitedField(number, input.readBytes());
+        mergeLengthDelimitedField(
+            number, Uint8List.fromList(input.readBytes()));
         return true;
       case WIRETYPE_START_GROUP:
         final subGroup = input.readUnknownFieldSetGroup(number);

protoc_plugin/test/unknown_field_set_test.dart

@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+import 'dart:typed_data';
+
 import 'package:protobuf/protobuf.dart';
 import 'package:test/test.dart';
 
@@ -326,4 +328,29 @@ void main() {
     final m2 = TestAllExtensions()..unknownFields;
     expect(m.hashCode, m2.hashCode);
   });
+
+  test('Copy length delimited fields', () {
+    // Length-delimited fields should be copied before adding to the unknown
+    // field set to avoid aliasing.
+    final originalBytes = [1, 2, 3, 4, 5, 6];
+    final bytes = Uint8List.fromList([
+      10, // tag = 1, type = length delimited
+      originalBytes.length,
+      ...originalBytes
+    ]);
+
+    final parsed = UnknownFieldSet()
+      ..mergeFromCodedBufferReader(CodedBufferReader(bytes));
+
+    expect(parsed.getField(1)?.lengthDelimited, [originalBytes]);
+
+    // Modify the message. Input buffer should not be updated.
+    final newBytes = [9, 8, 7, 6, 5, 4];
+    parsed.getField(1)!.lengthDelimited[0].setRange(0, 6, newBytes);
+    expect(bytes.sublist(2), originalBytes);
+
+    // Modify the input buffer. Message should not be updated.
+    bytes.setRange(2, 8, [10, 11, 12, 13, 14, 15]);
+    expect(parsed.getField(1)!.lengthDelimited[0], newBytes);
+  });
 }