Reject trailing commas in JSON arrays and objects

Per spec.

c++/src/capnp/compat/json-test.c++

@@ -415,10 +415,12 @@ KJ_TEST("basic json decoding") {
     KJ_EXPECT_THROW_MESSAGE("Unexpected input", json.decodeRaw("[}]", root));
     KJ_EXPECT_THROW_MESSAGE("Unexpected input", json.decodeRaw("[1, , ]", root));
     KJ_EXPECT_THROW_MESSAGE("Unexpected input", json.decodeRaw("[,]", root));
+    KJ_EXPECT_THROW_MESSAGE("Unexpected input", json.decodeRaw("[true,]", root));
     KJ_EXPECT_THROW_MESSAGE("Unexpected input", json.decodeRaw("[, 1]", root));
     KJ_EXPECT_THROW_MESSAGE("Unexpected input", json.decodeRaw("[1\"\"]", root));
     KJ_EXPECT_THROW_MESSAGE("Unexpected input", json.decodeRaw("[1,, \"\"]", root));
     KJ_EXPECT_THROW_MESSAGE("Unexpected input", json.decodeRaw("{\"a\"1: 0}", root));
+    KJ_EXPECT_THROW_MESSAGE("Unexpected input", json.decodeRaw(R"({"some": null,})", root));
     KJ_EXPECT_THROW_MESSAGE("Input remains", json.decodeRaw("11a", root));
     KJ_EXPECT_THROW_MESSAGE("Invalid escape", json.decodeRaw(R"("\z")", root));
     KJ_EXPECT_THROW_MESSAGE("Invalid escape", json.decodeRaw(R"("\z")", root));

c++/src/capnp/compat/json.c++

@@ -432,6 +432,7 @@ public:
     // Cap'n Proto message.  This also applies to parseObject below.
     kj::Vector<Orphan<JsonValue>> values;
     auto orphanage = Orphanage::getForMessageContaining(output);
+    bool expectComma = false;
 
     consume('[');
     KJ_REQUIRE(++nestingDepth_ <= maxNestingDepth_, "JSON message nested too deeply.");
@@ -440,13 +441,17 @@ public:
     while (consumeWhitespace(), nextChar() != ']') {
       auto orphan = orphanage.newOrphan<JsonValue>();
       auto builder = orphan.get();
-      parseValue(builder);
-      values.add(kj::mv(orphan));
 
-      if (consumeWhitespace(), nextChar() != ']') {
-        // TODO(someday): The JSON spec forbids a trailing comma, but we allow it.
+      if (expectComma) {
+        consumeWhitespace();
         consume(',');
+        consumeWhitespace();
       }
+
+      parseValue(builder);
+      values.add(kj::mv(orphan));
+
+      expectComma = true;
     }
 
     output.initArray(values.size());
@@ -462,6 +467,7 @@ public:
   void parseObject(JsonValue::Builder& output) {
     kj::Vector<Orphan<JsonValue::Field>> fields;
     auto orphanage = Orphanage::getForMessageContaining(output);
+    bool expectComma = false;
 
     consume('{');
     KJ_REQUIRE(++nestingDepth_ <= maxNestingDepth_, "JSON message nested too deeply.");
@@ -471,6 +477,12 @@ public:
       auto orphan = orphanage.newOrphan<JsonValue::Field>();
       auto builder = orphan.get();
 
+      if (expectComma) {
+        consumeWhitespace();
+        consume(',');
+        consumeWhitespace();
+      }
+
       builder.setName(consumeQuotedString());
 
       consumeWhitespace();
@@ -482,10 +494,7 @@ public:
 
       fields.add(kj::mv(orphan));
 
-      if (consumeWhitespace(), nextChar() != '}') {
-        // TODO(someday): The JSON spec forbids a trailing comma, but we allow it.
-        consume(',');
-      }
+      expectComma = true;
     }
 
     output.initObject(fields.size());