From 57106a345960d31c4f6df74912c1f0b7e2fa95b5 Mon Sep 17 00:00:00 2001
From: Jiaming Yuan <jm.yuan@outlook.com>
Date: Wed, 18 Sep 2019 03:31:46 -0400
Subject: [PATCH] Fix parsing empty json object. (#4868)

* Fix parsing empty json object.

* Better error message.
---
 src/common/json.cc            | 17 +++++++++++++++--
 tests/cpp/common/test_json.cc | 14 ++++++++++++++
 2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/src/common/json.cc b/src/common/json.cc
index 396140a06..5c14db47b 100644
--- a/src/common/json.cc
+++ b/src/common/json.cc
@@ -384,8 +384,18 @@ void JsonReader::Error(std::string msg) const {
   auto end = cursor_.Pos() + kExtend >= raw_str_.size() ?
              raw_str_.size() : cursor_.Pos() + kExtend;
 
+  std::string const& raw_portion = raw_str_.substr(beg, end - beg);
+  std::string portion;
+  for (auto c : raw_portion) {
+    if (c == '\n') {
+      portion += "\\n";
+    } else {
+      portion += c;
+    }
+  }
+
   msg += "    ";
-  msg += raw_str_.substr(beg, end - beg);
+  msg += portion;
   msg += '\n';
 
   msg += "    ";
@@ -488,9 +498,12 @@ Json JsonReader::ParseArray() {
 }
 
 Json JsonReader::ParseObject() {
-  char ch = GetChar('{');
+  GetChar('{');
 
   std::map<std::string, Json> data;
+  SkipSpaces();
+  char ch = PeekNextChar();
+
   if (ch == '}') return Json(std::move(data));
 
   while (true) {
diff --git a/tests/cpp/common/test_json.cc b/tests/cpp/common/test_json.cc
index 1ae31236b..94e87bcac 100644
--- a/tests/cpp/common/test_json.cc
+++ b/tests/cpp/common/test_json.cc
@@ -214,6 +214,20 @@ TEST(Json, Null) {
   ASSERT_TRUE(IsA<Null>(json["key"]));
 }
 
+TEST(Json, EmptyObject) {
+  std::string str = R"json(
+{
+  "rank": 1,
+  "statistic": {
+
+  }
+}
+)json";
+  std::stringstream iss(str);
+  auto json = Json::Load(StringView{str.c_str(), str.size()});
+  ASSERT_TRUE(IsA<Object>(json["statistic"]));
+}
+
 TEST(Json, EmptyArray) {
   std::string str = R"json(
 {