msgpack seems to work

Specify cacheFormat: 1 (SerializeFormat::MessagePack) in initializationOptions to write to .mpack cache files.

src/indexer.h

@@ -154,7 +154,8 @@ inline bool operator!=(const IndexFuncRef& a, const IndexFuncRef& b) {
 }
 
 inline void Reflect(Reader& visitor, IndexFuncRef& value) {
-  const char* str_value = visitor.GetCString();
+  std::string s = visitor.GetString();
+  const char* str_value = s.c_str();
   if (str_value[0] == '~') {
     value.is_implicit = true;
     ++str_value;

src/position.cc

@@ -148,8 +148,10 @@ bool Range::operator<(const Range& that) const {
 void Reflect(Reader& visitor, Position& value) {
   if (!visitor.IsString())
     value = Position();
-  else
-    value = Position(visitor.GetCString());
+  else {
+    std::string s = visitor.GetString();
+    value = Position(s.c_str());
+  }
 }
 void Reflect(Writer& visitor, Position& value) {
   std::string output = value.ToString();
@@ -160,8 +162,10 @@ void Reflect(Writer& visitor, Position& value) {
 void Reflect(Reader& visitor, Range& value) {
   if (!visitor.IsString())
     value = Range();
-  else
-    value = Range(visitor.GetCString());
+  else {
+    std::string s = visitor.GetString();
+    value = Range(s.c_str());
+  }
 }
 void Reflect(Writer& visitor, Range& value) {
   std::string output = value.ToString();

src/serializer.cc

@@ -6,6 +6,7 @@
 #include "indexer.h"
 
 #include <doctest/doctest.h>
+#include <loguru.hpp>
 
 namespace {
 bool gTestOutputMode = false;
@@ -252,7 +253,8 @@ std::unique_ptr<IndexFile> Deserialize(SerializeFormat format,
         Reflect(reader, *file);
         if (file->version != expected_version)
           return nullptr;
-      } catch (msgpack::insufficient_bytes&) {
+      } catch (msgpack::unpack_error& ex) {
+        LOG_S(ERROR) << "msgpack::unpack_err for '" << path << "' " << ex.what();
         return nullptr;
       }
       break;

src/serializer.h

@@ -27,8 +27,7 @@ class Reader {
   virtual int GetInt() = 0;
   virtual int64_t GetInt64() = 0;
   virtual uint64_t GetUint64() = 0;
-  virtual const char* GetCString() = 0;
-  virtual std::string GetString() { return GetCString(); }
+  virtual std::string GetString() = 0;
 
   virtual bool HasMember(const char* x) = 0;
   virtual std::unique_ptr<Reader> operator[](const char* x) = 0;
@@ -163,8 +162,12 @@ void Reflect(Reader& visitor, optional<T>& value) {
 }
 template <typename T>
 void Reflect(Writer& visitor, optional<T>& value) {
+  // We omit optional fields for JsonWriter to reduce output.
+  // But don't omit them for other serialization formats.
   if (value)
-    Reflect(visitor, value.value());
+    Reflect(visitor, *value);
+  else if (visitor.Format() != SerializeFormat::Json)
+    visitor.Null();
 }
 
 // std::variant (Writer only)
@@ -179,8 +182,6 @@ void Reflect(Writer& visitor, std::variant<T0, T1>& value) {
 // std::vector
 template <typename T>
 void Reflect(Reader& visitor, std::vector<T>& values) {
-  if (!visitor.IsArray())
-    return;
   visitor.IterArray([&](Reader& entry) {
     T entry_value;
     Reflect(entry, entry_value);
@@ -224,10 +225,10 @@ void ReflectMember(Writer& visitor, const char* name, std::vector<T>& values) {
 }
 template <typename T>
 void ReflectMember(Writer& visitor, const char* name, optional<T>& value) {
-  if (!value)
-    return;
+  if (value || visitor.Format() != SerializeFormat::Json) {
     visitor.Key(name);
     Reflect(visitor, value);
+  }
 }
 void ReflectMember(Writer& visitor, const char* name, std::string& value);
 

src/serializers/json.h

@@ -24,7 +24,7 @@ class JsonReader : public Reader {
   int GetInt() override { return m_->GetInt(); }
   int64_t GetInt64() override { return m_->GetInt64(); }
   uint64_t GetUint64() override { return m_->GetUint64(); }
-  const char* GetCString() override { return m_->GetString(); }
+  std::string GetString() override { return m_->GetString(); }
 
   bool HasMember(const char* x) override { return m_->HasMember(x); }
   std::unique_ptr<Reader> operator[](const char* x) override {

src/serializers/msgpack.h

@@ -25,7 +25,7 @@ class MessagePackReader : public Reader {
   int GetInt() override { return o_.as<int>(); }
   int64_t GetInt64() override { return o_.as<int64_t>(); }
   uint64_t GetUint64() override { return o_.as<uint64_t>(); }
-  const char* GetCString() override { return o_.as<char*>(); }
+  std::string GetString() override { return o_.as<std::string>(); }
 
   bool HasMember(const char* x) override { return true; }
   std::unique_ptr<Reader> operator[](const char* x) override {