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_
