Add StatusOr for error handling in modern C++ style (#768)

src/redis_cmd.cc

@@ -118,7 +118,7 @@ class CommandNamespace : public Commander {
       } else {
         std::string token;
         auto s = config->GetNamespace(args_[2], &token);
-        if (s.IsNotFound()) {
+        if (s.Is<Status::NotFound>()) {
           *output = Redis::NilString();
         } else {
           *output = Redis::BulkString(token);

src/redis_connection.cc

@@ -429,7 +429,7 @@ void Connection::ExecuteCommands(std::deque<CommandTokens> *to_process_cmds) {
 
     // Break the execution loop when occurring the blocking command like BLPOP or BRPOP,
     // it will suspend the connection and wait for the wakeup signal.
-    if (s.IsBlockingCommand()) {
+    if (s.Is<Status::BlockingCmd>()) {
       break;
     }
     // Reply for MULTI

src/status.h

@@ -21,12 +21,17 @@
 #pragma once
 
 #include <string>
+#include <type_traits>
 #include <utility>
+#include <memory>
+#include <algorithm>
+#include <tuple>
+#include <glog/logging.h>
 
 class Status {
  public:
-  enum Code {
-    cOK,
+  enum Code : unsigned char {
+    cOK = 0,
     NotOK,
     NotFound,
 
@@ -61,20 +66,204 @@ class Status {
   };
 
   Status() : Status(cOK) {}
-  explicit Status(Code code, std::string msg = {}) : code_(code), msg_(std::move(msg)) {}
-  bool IsOK() { return code_ == cOK; }
-  bool IsNotFound() { return code_ == NotFound; }
-  bool IsImorting() { return code_ == SlotImport; }
-  bool IsBlockingCommand() { return code_ == BlockingCmd; }
-  std::string Msg() {
-    if (IsOK()) {
-      return "ok";
-    }
+  explicit Status(Code code, std::string msg = {})
+    : code_(code), msg_(std::move(msg)) {}
+
+  template <Code code>
+  bool Is() const { return code_ == code; }
+
+  bool IsOK() const { return Is<cOK>(); }
+  operator bool() const { return IsOK(); }
+
+  Code GetCode() const {
+    return code_;
+  }
+
+  std::string Msg() const& {
+    if (*this) return ok_msg();
     return msg_;
   }
+
+  std::string Msg() && {
+    if (*this) return ok_msg();
+    return std::move(msg_);
+  }
+
   static Status OK() { return {}; }
 
  private:
   Code code_;
   std::string msg_;
+
+  static constexpr const char* ok_msg() {
+    return "ok";
+  }
+
+  template <typename T>
+  friend struct StatusOr;
+};
+
+template <typename ...Ts>
+using first_element = typename std::tuple_element<0, std::tuple<Ts...>>::type;
+
+template <typename T>
+using remove_cvref_t = typename std::remove_cv<typename std::remove_reference<T>::type>::type;
+
+template <typename T>
+struct StatusOr;
+
+template <typename T>
+struct IsStatusOr : std::integral_constant<bool, false> {};
+
+template <typename T>
+struct IsStatusOr<StatusOr<T>> : std::integral_constant<bool, true> {};
+
+template <typename T>
+struct StatusOr {
+  static_assert(!std::is_same<T, Status>::value, "value_type cannot be Status");
+  static_assert(!std::is_same<T, Status::Code>::value, "value_type cannot be Status::Code");
+  static_assert(!IsStatusOr<T>::value, "value_type cannot be StatusOr");
+  static_assert(!std::is_reference<T>::value, "value_type cannot be reference");
+
+  using value_type = T;
+
+  // we use std::unique_ptr to make the error part as small as enough
+  using error_type = std::unique_ptr<std::string>;
+
+  using Code = Status::Code;
+
+  explicit StatusOr(Status s) : code_(s.code_) {
+    CHECK(!s);
+    new(value_or_error_) error_type(new std::string(std::move(s.msg_)));
+  }
+
+  StatusOr(Code code, std::string msg = {}) : code_(code) { // NOLINT
+    CHECK(code != Code::cOK);
+    new(value_or_error_) error_type(new std::string(std::move(msg)));
+  }
+
+  template <typename ...Ts,
+    typename std::enable_if<
+      (sizeof...(Ts) > 0 &&
+        !std::is_same<Status, remove_cvref_t<first_element<Ts...>>>::value &&
+        !std::is_same<Code, remove_cvref_t<first_element<Ts...>>>::value &&
+        !std::is_same<value_type, remove_cvref_t<first_element<Ts...>>>::value &&
+        !std::is_same<StatusOr, remove_cvref_t<first_element<Ts...>>>::value
+      ), int>::type = 0> // NOLINT
+  explicit StatusOr(Ts && ... args) : code_(Code::cOK) {
+    new(value_or_error_) value_type(std::forward<Ts>(args)...);
+  }
+
+  StatusOr(T&& value) : code_(Code::cOK) { // NOLINT
+    new(value_or_error_) value_type(std::move(value));
+  }
+
+  StatusOr(const T& value) : code_(Code::cOK) { // NOLINT
+    new(value_or_error_) value_type(value);
+  }
+
+  StatusOr(const StatusOr&) = delete;
+  StatusOr(StatusOr&& other) : code_(other.code_) {
+    if (code_ == Code::cOK) {
+      new(value_or_error_) value_type(std::move(other.getValue()));
+    } else {
+      new(value_or_error_) error_type(std::move(other.getError()));
+    }
+  }
+
+  Status& operator=(const Status&) = delete;
+
+  template <Code code>
+  bool Is() const { return code_ == code; }
+
+  bool IsOK() const { return Is<Code::cOK>(); }
+  operator bool() const { return IsOK(); }
+
+  Status ToStatus() const& {
+    if (*this) return Status::OK();
+    return Status(code_, *getError());
+  }
+
+  Status ToStatus() && {
+    if (*this) return Status::OK();
+    return Status(code_, std::move(*getError()));
+  }
+
+  Code GetCode() const {
+    return code_;
+  }
+
+  value_type& GetValue() & {
+    CHECK(*this);
+    return getValue();
+  }
+
+  value_type&& GetValue() && {
+    CHECK(*this);
+    return std::move(getValue());
+  }
+
+  const value_type& GetValue() const& {
+    CHECK(*this);
+    return getValue();
+  }
+
+  value_type& operator*() & {
+    return GetValue();
+  }
+
+  value_type&& operator*() && {
+    return std::move(GetValue());
+  }
+
+  const value_type& operator*() const& {
+    return GetValue();
+  }
+
+  value_type* operator->() {
+    return &GetValue();
+  }
+
+  const value_type* operator->() const {
+    return &GetValue();
+  }
+
+  std::string Msg() const& {
+    if (*this) return Status::ok_msg();
+    return *getError();
+  }
+
+  std::string Msg() && {
+    if (*this) return Status::ok_msg();
+    return std::move(*getError());
+  }
+
+  ~StatusOr() {
+    if (*this) {
+      getValue().~value_type();
+    } else {
+      getError().~error_type();
+    }
+  }
+
+ private:
+  Status::Code code_;
+  alignas(value_type) alignas(error_type) unsigned char value_or_error_
+    [sizeof(value_type) < sizeof(error_type) ? sizeof(error_type) : sizeof(value_type)];
+
+  value_type& getValue() {
+    return *reinterpret_cast<value_type*>(value_or_error_);
+  }
+
+  const value_type& getValue() const {
+    return *reinterpret_cast<const value_type*>(value_or_error_);
+  }
+
+  error_type& getError() {
+    return *reinterpret_cast<error_type*>(value_or_error_);
+  }
+
+  const error_type& getError() const {
+    return *reinterpret_cast<const error_type*>(value_or_error_);
+  }
 };

tests/cppunit/status_test.cc

@@ -0,0 +1,146 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ *
+ */
+
+#include <gtest/gtest.h>
+
+#include <memory>
+#include <status.h>
+
+TEST(StatusOr, Scalar) {
+  auto f = [](int x) -> StatusOr<int> {
+    if (x > 10) {
+        return {Status::NotOK, "x large than 10"};
+    }
+
+    return 2 * x + 5;
+  };
+
+  ASSERT_EQ(*f(1), 7);
+  ASSERT_EQ(*f(5), 15);
+  ASSERT_EQ(f(7).GetValue(), 19);
+  ASSERT_EQ(f(7).GetCode(), Status::cOK);
+  ASSERT_EQ(f(7).Msg(), "ok");
+  ASSERT_TRUE(f(6));
+  ASSERT_EQ(f(11).GetCode(), Status::NotOK);
+  ASSERT_EQ(f(11).Msg(), "x large than 10");
+  ASSERT_FALSE(f(12));
+
+  auto x = f(5);
+  ASSERT_EQ(*x, 15);
+  ASSERT_EQ(x.Msg(), "ok");
+  ASSERT_EQ(x.GetValue(), 15);
+  ASSERT_EQ(x.GetCode(), Status::cOK);
+
+  auto y = f(11);
+  ASSERT_EQ(y.Msg(), "x large than 10");
+  ASSERT_EQ(y.GetCode(), Status::NotOK);
+
+  auto g = [f](int y) -> StatusOr<int> {
+    if (y > 5 && y < 15) {
+        return {Status::NotOK, "y large than 5"};
+    }
+
+    auto res = f(y);
+    if (!res) return res;
+
+    return *res * 10;
+  };
+
+  ASSERT_EQ(*g(1), 70);
+  ASSERT_EQ(*g(5), 150);
+  ASSERT_EQ(g(1).GetValue(), 70);
+  ASSERT_EQ(g(1).GetCode(), Status::cOK);
+  ASSERT_EQ(g(1).Msg(), "ok");
+  ASSERT_EQ(g(6).GetCode(), Status::NotOK);
+  ASSERT_EQ(g(6).Msg(), "y large than 5");
+  ASSERT_EQ(g(20).GetCode(), Status::NotOK);
+  ASSERT_EQ(g(20).Msg(), "x large than 10");
+  ASSERT_EQ(g(11).GetCode(), Status::NotOK);
+  ASSERT_EQ(g(11).Msg(), "y large than 5");
+}
+
+TEST(StatusOr, String) {
+  auto f = [](std::string x) -> StatusOr<std::string> {
+    if (x.size() > 10) {
+        return {Status::NotOK, "string too long"};
+    }
+
+    return x + " hello";
+  };
+
+  auto g = [f](std::string x) -> StatusOr<std::string> {
+    if (x.size() < 5) {
+        return {Status::NotOK, "string too short"};
+    }
+
+    auto res = f(x);
+    if (!res) return res;
+
+    return "hi " + *res;
+  };
+
+  ASSERT_TRUE(f("1"));
+  ASSERT_FALSE(f("12345678901"));
+  ASSERT_FALSE(g("1"));
+
+  ASSERT_EQ(*f("twice"), "twice hello");
+  ASSERT_EQ(*g("twice"), "hi twice hello");
+  ASSERT_EQ(g("shrt").GetCode(), Status::NotOK);
+  ASSERT_EQ(g("shrt").Msg(), "string too short");
+  ASSERT_EQ(g("loooooooooooog").GetCode(), Status::NotOK);
+  ASSERT_EQ(g("loooooooooooog").Msg(), "string too long");
+
+  ASSERT_EQ(g("twice").ToStatus().GetCode(), Status::cOK);
+  ASSERT_EQ(g("").ToStatus().GetCode(), Status::NotOK);
+
+  auto x = g("twice");
+  ASSERT_EQ(x.ToStatus().GetCode(), Status::cOK);
+  auto y = g("");
+  ASSERT_EQ(y.ToStatus().GetCode(), Status::NotOK);
+}
+
+TEST(StatusOr, SharedPtr) {
+    struct A {
+      A(int *x) : x(x) { *x = 233; }
+      ~A() { *x = 0; }
+
+      int *x;
+    };
+
+    int val = 0;
+
+    {
+      StatusOr<std::shared_ptr<A>> x(new A(&val));
+
+      ASSERT_EQ(val, 233);
+      ASSERT_EQ(x->use_count(), 1);
+
+      {
+        StatusOr<std::shared_ptr<A>> y(*x);
+        ASSERT_EQ(val, 233);
+        ASSERT_EQ(x->use_count(), 2);
+      }
+
+      ASSERT_EQ(x->use_count(), 1);
+    }
+
+    ASSERT_EQ(val, 0);
+
+}