Add JsonParser setting to ignore unknown field values

Note that the default behavior is still to throw an exception; you
need to opt into ignoring unknown fields.

Fixes #2838.

csharp/src/Google.Protobuf.Test/JsonParserTest.cs

@@ -926,6 +926,27 @@ namespace Google.Protobuf
             Assert.Throws<InvalidProtocolBufferException>(() => TestAllTypes.Parser.ParseJson(json));
         }
 
+        [Test]
+        public void UnknownField_NotIgnored()
+        {
+            string json = "{ \"unknownField\": 10, \"singleString\": \"x\" }";
+            Assert.Throws<InvalidProtocolBufferException>(() => TestAllTypes.Parser.ParseJson(json));
+        }
+
+        [Test]
+        [TestCase("5")]
+        [TestCase("\"text\"")]
+        [TestCase("[0, 1, 2]")]
+        [TestCase("{ \"a\": { \"b\": 10 } }")]
+        public void UnknownField_Ignored(string value)
+        {
+            var parser = new JsonParser(JsonParser.Settings.Default.WithIgnoreUnknownFields(true));
+            string json = "{ \"unknownField\": " + value + ", \"singleString\": \"x\" }";
+            var actual = parser.Parse<TestAllTypes>(json);
+            var expected = new TestAllTypes { SingleString = "x" };
+            Assert.AreEqual(expected, actual);
+        }
+
         /// <summary>
         /// Various tests use strings which have quotes round them for parsing or as the result
         /// of formatting, but without those quotes being specified in the tests (for the sake of readability).

csharp/src/Google.Protobuf.Test/JsonTokenizerTest.cs

@@ -349,6 +349,22 @@ namespace Google.Protobuf
             Assert.AreEqual(JsonToken.EndDocument, tokenizer.Next());
             Assert.Throws<InvalidOperationException>(() => tokenizer.Next());
         }
+
+        [Test]
+        [TestCase("{ 'skip': 0, 'next': 1")]
+        [TestCase("{ 'skip': [0, 1, 2], 'next': 1")]
+        [TestCase("{ 'skip': 'x', 'next': 1")]
+        [TestCase("{ 'skip': ['x', 'y'], 'next': 1")]
+        [TestCase("{ 'skip': {'a': 0}, 'next': 1")]
+        [TestCase("{ 'skip': {'a': [0, {'b':[]}]}, 'next': 1")]
+        public void SkipValue(string json)
+        {
+            var tokenizer = JsonTokenizer.FromTextReader(new StringReader(json.Replace('\'', '"')));
+            Assert.AreEqual(JsonToken.StartObject, tokenizer.Next());
+            Assert.AreEqual("skip", tokenizer.Next().StringValue);
+            tokenizer.SkipValue();
+            Assert.AreEqual("next", tokenizer.Next().StringValue);
+        }
        
         /// <summary>
         /// Asserts that the specified JSON is tokenized into the given sequence of tokens.

csharp/src/Google.Protobuf/JsonParser.cs

@@ -203,10 +203,14 @@ namespace Google.Protobuf
                 }
                 else
                 {
-                    // TODO: Is this what we want to do? If not, we'll need to skip the value,
-                    // which may be an object or array. (We might want to put code in the tokenizer
-                    // to do that.)
-                    throw new InvalidProtocolBufferException("Unknown field: " + name);
+                    if (settings.IgnoreUnknownFields)
+                    {
+                        tokenizer.SkipValue();
+                    }
+                    else
+                    {
+                        throw new InvalidProtocolBufferException("Unknown field: " + name);
+                    }
                 }
             }
         }
@@ -996,6 +1000,19 @@ namespace Google.Protobuf
             /// </summary>
             public TypeRegistry TypeRegistry { get; }
 
+            /// <summary>
+            /// Whether the parser should ignore unknown fields (<c>true</c>) or throw an exception when
+            /// they are encountered (<c>false</c>).
+            /// </summary>
+            public bool IgnoreUnknownFields { get; }
+
+            private Settings(int recursionLimit, TypeRegistry typeRegistry, bool ignoreUnknownFields)
+            {
+                RecursionLimit = recursionLimit;
+                TypeRegistry = ProtoPreconditions.CheckNotNull(typeRegistry, nameof(typeRegistry));
+                IgnoreUnknownFields = ignoreUnknownFields;
+            }
+
             /// <summary>
             /// Creates a new <see cref="Settings"/> object with the specified recursion limit.
             /// </summary>
@@ -1009,11 +1026,17 @@ namespace Google.Protobuf
             /// </summary>
             /// <param name="recursionLimit">The maximum depth of messages to parse</param>
             /// <param name="typeRegistry">The type registry used to parse <see cref="Any"/> messages</param>
-            public Settings(int recursionLimit, TypeRegistry typeRegistry)
+            public Settings(int recursionLimit, TypeRegistry typeRegistry) : this(recursionLimit, typeRegistry, false)
             {
-                RecursionLimit = recursionLimit;
-                TypeRegistry = ProtoPreconditions.CheckNotNull(typeRegistry, nameof(typeRegistry));
             }
+
+            /// <summary>
+            /// Creates a new <see cref="Settings"/> object set to either ignore unknown fields, or throw an exception
+            /// when unknown fields are encountered.
+            /// </summary>
+            /// <param name="ignoreUnknownFields"><c>true</c> if unknown fields should be ignored when parsing; <c>false</c> to throw an exception.</param>
+            public Settings WithIgnoreUnknownFields(bool ignoreUnknownFields) =>
+                new Settings(RecursionLimit, TypeRegistry, ignoreUnknownFields);
         }
     }
 }

csharp/src/Google.Protobuf/JsonTokenizer.cs

@@ -137,6 +137,34 @@ namespace Google.Protobuf
         /// <exception cref="InvalidJsonException">The input text does not comply with RFC 7159</exception>
         protected abstract JsonToken NextImpl();
 
+        /// <summary>
+        /// Skips the value we're about to read. This must only be called immediately after reading a property name.
+        /// If the value is an object or an array, the complete object/array is skipped.
+        /// </summary>
+        internal void SkipValue()
+        {
+            // We'll assume that Next() makes sure that the end objects and end arrays are all valid.
+            // All we care about is the total nesting depth we need to close.
+            int depth = 0;
+
+            // do/while rather than while loop so that we read at least one token.
+            do
+            {
+                var token = Next();
+                switch (token.Type)
+                {
+                    case JsonToken.TokenType.EndArray:
+                    case JsonToken.TokenType.EndObject:
+                        depth--;
+                        break;
+                    case JsonToken.TokenType.StartArray:
+                    case JsonToken.TokenType.StartObject:
+                        depth++;
+                        break;
+                }
+            } while (depth != 0);
+        }
+
         /// <summary>
         /// Tokenizer which first exhausts a list of tokens, then consults another tokenizer.
         /// </summary>