[JSON] fix bug that incorrectly allowed trailing commas after an empty container (#33158)

src/core/lib/json/json_reader.cc

@@ -476,6 +476,7 @@ JsonReader::Status JsonReader::Run() {
                 return Status::GRPC_JSON_PARSE_ERROR;
               }
               state_ = State::GRPC_JSON_STATE_VALUE_END;
+              container_just_begun_ = false;
               EndContainer();
               if (stack_.empty()) {
                 state_ = State::GRPC_JSON_STATE_END;

test/core/client_channel/lb_policy/xds_override_host_lb_config_parser_test.cc

@@ -193,7 +193,7 @@ TEST(XdsOverrideHostConfigParsingTest, ReportsChildPolicyShouldBeArray) {
       "  \"loadBalancingConfig\":[{\n"
       "    \"xds_override_host_experimental\":{\n"
       "      \"childPolicy\":{\n"
-      "        \"grpclb\":{},\n"
+      "        \"grpclb\":{}\n"
       "      }\n"
       "    }\n"
       "  }]\n"

test/core/json/json_test.cc

@@ -202,7 +202,7 @@ TEST(Json, Keywords) {
 void RunParseFailureTest(const char* input) {
   gpr_log(GPR_INFO, "parsing string \"%s\" - should fail", input);
   auto json = JsonParse(input);
-  EXPECT_FALSE(json.ok());
+  EXPECT_FALSE(json.ok()) << "input: \"" << input << "\"";
 }
 
 TEST(Json, InvalidInput) {
@@ -213,6 +213,10 @@ TEST(Json, InvalidInput) {
   RunParseFailureTest("fals");
   RunParseFailureTest("0,0 ");
   RunParseFailureTest("\"foo\",[]");
+  RunParseFailureTest("{\"field\": {},}");
+  RunParseFailureTest("[{},]");
+  RunParseFailureTest("{\"field\": [],}");
+  RunParseFailureTest("[[],]");
 }
 
 TEST(Json, UnterminatedString) { RunParseFailureTest("\"\\x"); }