LevelDB的错误处理和Status类

LevelDB错误处理基本套路

LevelDB在需要判断是否出现异常的部分，都会产生一个Status类对象，例如打开数据库

leveldb::Status status = leveldb::DB::Open(options, "/tmp/testdb", &db);

了解Status类的设计思路，即了解了LevelDB错误处理的基本思路

Status类分析

类的变量

Status类包含一个枚举类型的错误码Code和描述详细错误信息的state_

enum Code {
    kOk = 0,
    kNotFound = 1,
    kCorruption = 2,
    kNotSupported = 3,
    kInvalidArgument = 4,
    kIOError = 5
  };  
  Code code() const {
    return (state_ == nullptr) ? kOk : static_cast<Code>(state_[4]);
  }

  Status(Code code, const Slice& msg, const Slice& msg2);
static const char* CopyState(const char* s);

  // OK status has a null state_.  Otherwise, state_ is a new[] array
  // of the following form:
  //    state_[0..3] == length of message
  //    state_[4]    == code
  //    state_[5..]  == message
  const char* state_;

Code类中我们知道了Status可以表示的所有错误类型

根据Code函数我们知道对于无异常的表示其实就是state_为空, 不然才需要使用错误编码

state_根据注释可知, 其组合前4个字节是错误信息的长度, 第5个字节是错误编码, 第6个字节往后剩下的是错误信息

Status的初始化

//两个msg的意义：应该是出错位置和出错原因
Status::Status(Code code, const Slice& msg, const Slice& msg2) {
  assert(code != kOk);
  const uint32_t len1 = static_cast<uint32_t>(msg.size());
  const uint32_t len2 = static_cast<uint32_t>(msg2.size());
  const uint32_t size = len1 + (len2 ? (2 + len2) : 0);//当存在异常时, 加2是在msg和msg2中间加入":_"
  char* result = new char[size + 5];//5 表示0-4的固定头部, 即信息长度和错误码
  std::memcpy(result, &size, sizeof(size));//0..3 length of message
  result[4] = static_cast<char>(code);//错误码
  std::memcpy(result + 5, msg.data(), len1);
  if (len2) {
    result[5 + len1] = ':';
    result[6 + len1] = ' ';
    std::memcpy(result + 7 + len1, msg2.data(), len2);
  }
  state_ = result;
}

应用Status初始化的案例: 是函数Status DBImpl::Recover(VersionEdit* edit, bool* save_manifest)中的一部分代码, 我们可以看到Status使用的基本方法

if (!env_->FileExists(CurrentFileName(dbname_))) {
    if (options_.create_if_missing) {
      Log(options_.info_log, "Creating DB %s since it was missing.",
          dbname_.c_str());
      s = NewDB();
      if (!s.ok()) {
        return s;
      }
    } else {
      return Status::InvalidArgument(
          dbname_, "does not exist (create_if_missing is false)");
    }
  } else {
    if (options_.error_if_exists) {
      return Status::InvalidArgument(dbname_,
                                     "exists (error_if_exists is true)");
    }
  }

CopyState类函数

这个函数在错误处理使用的不多, 摘出来主要是因为其非常优雅的解决了对保存有长度和内容的char*数据的复制

const char* Status::CopyState(const char* state) {
  uint32_t size;
  std::memcpy(&size, state, sizeof(size));//这里memcpy直接用来复制整型是我以前没想过的用法, 其是memcpy对是对第一个参数指针指向的位置复制第二个参中size个字节, 并不一定是char
  char* result = new char[size + 5];
  std::memcpy(result, state, size + 5);
  return result;
}

问题解决

Status类中使用枚举变量是否会浪费空间?

参考一下文章:

(1条消息) C++类中的枚举类型_qinhan728的专栏-CSDN博客

简单总结:

在类中使用枚举类不会占用变量空间

这样做的目的是让某些常量只在类中有效

源码

/usr/local/include/leveldb/status.h

// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.
//
// A Status encapsulates the result of an operation.  It may indicate success,
// or it may indicate an error with an associated error message.
//
// Multiple threads can invoke const methods on a Status without
// external synchronization, but if any of the threads may call a
// non-const method, all threads accessing the same Status must use
// external synchronization.

#ifndef STORAGE_LEVELDB_INCLUDE_STATUS_H_
#define STORAGE_LEVELDB_INCLUDE_STATUS_H_

#include <algorithm>
#include <string>

#include "leveldb/export.h"
#include "leveldb/slice.h"

namespace leveldb {

class LEVELDB_EXPORT Status {
public:
  // Create a success status.
  Status() noexcept : state_(nullptr) {}
  ~Status() { delete[] state_; }

  Status(const Status& rhs);
  Status& operator=(const Status& rhs);

  Status(Status&& rhs) noexcept : state_(rhs.state_) { rhs.state_ = nullptr; }
  Status& operator=(Status&& rhs) noexcept;

  // Return a success status.
  static Status OK() { return Status(); }

  // Return error status of an appropriate type.
  static Status NotFound(const Slice& msg, const Slice& msg2 = Slice()) {
    return Status(kNotFound, msg, msg2);
  }
  static Status Corruption(const Slice& msg, const Slice& msg2 = Slice()) {
    return Status(kCorruption, msg, msg2);
  }
  static Status NotSupported(const Slice& msg, const Slice& msg2 = Slice()) {
    return Status(kNotSupported, msg, msg2);
  }
  static Status InvalidArgument(const Slice& msg, const Slice& msg2 = Slice()) {
    return Status(kInvalidArgument, msg, msg2);
  }
  static Status IOError(const Slice& msg, const Slice& msg2 = Slice()) {
    return Status(kIOError, msg, msg2);
  }

  // Returns true iff the status indicates success.
  bool ok() const { return (state_ == nullptr); }

  // Returns true iff the status indicates a NotFound error.
  bool IsNotFound() const { return code() == kNotFound; }

  // Returns true iff the status indicates a Corruption error.
  bool IsCorruption() const { return code() == kCorruption; }

  // Returns true iff the status indicates an IOError.
  bool IsIOError() const { return code() == kIOError; }

  // Returns true iff the status indicates a NotSupportedError.
  bool IsNotSupportedError() const { return code() == kNotSupported; }

  // Returns true iff the status indicates an InvalidArgument.
  bool IsInvalidArgument() const { return code() == kInvalidArgument; }

  // Return a string representation of this status suitable for printing.
  // Returns the string "OK" for success.
  std::string ToString() const;

private:
  enum Code {
    kOk = 0,
    kNotFound = 1,
    kCorruption = 2,
    kNotSupported = 3,
    kInvalidArgument = 4,
    kIOError = 5
  };

  Code code() const {
    return (state_ == nullptr) ? kOk : static_cast<Code>(state_[4]);
  }

  Status(Code code, const Slice& msg, const Slice& msg2);
  static const char* CopyState(const char* s);

  // OK status has a null state_.  Otherwise, state_ is a new[] array
  // of the following form:
  //    state_[0..3] == length of message
  //    state_[4]    == code
  //    state_[5..]  == message
  const char* state_;
};

inline Status::Status(const Status& rhs) {
  state_ = (rhs.state_ == nullptr) ? nullptr : CopyState(rhs.state_);
}
inline Status& Status::operator=(const Status& rhs) {
  // The following condition catches both aliasing (when this == &rhs),
  // and the common case where both rhs and *this are ok.
  if (state_ != rhs.state_) {
    delete[] state_;
    state_ = (rhs.state_ == nullptr) ? nullptr : CopyState(rhs.state_);
  }
  return *this;
}
inline Status& Status::operator=(Status&& rhs) noexcept {
  std::swap(state_, rhs.state_);
  return *this;
}

}  // namespace leveldb

#endif  // STORAGE_LEVELDB_INCLUDE_STATUS_H_

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