LevelDB源码剖析之Status | 赖明星

levelDB中的所有操作的返回值都用一个名为Status的类来存储, Status有什么特别的呢？

Status 介绍

leveldb::Status表示levelDB的一个返回状态，通常的错误处理（如:errno）是返回一个错误号，然后根据错误号可以获得出错的描述信息。leveldb将错误号和错误信息封装成Status类，来统一进行处理。

Status 声明

leveldb::Status 的声明如下：

class Status { public:  // Create a success status.  Status() : state_(NULL) { }  ~Status() { delete[] state_; }  // Copy the specified status.  Status(const Status& s);  void operator=(const Status& s);  // 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_ == NULL); }  // 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; }  // Return a string representation of this status suitable for printing.  // Returns the string "OK" for success.  std::string ToString() const; private:  // 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_;  enum Code {    kOk = 0,    kNotFound = 1,    kCorruption = 2,    kNotSupported = 3,    kInvalidArgument = 4,    kIOError = 5  };  Code code() const {    return (state_ == NULL) ? kOk : static_cast<Code>(state_[4]);  }  Status(Code code, const Slice& msg, const Slice& msg2);  static const char* CopyState(const char* s);};inline Status::Status(const Status& s) {  state_ = (s.state_ == NULL) ? NULL : CopyState(s.state_);}inline void Status::operator=(const Status& s) {  // The following condition catches both aliasing (when this == &s),  // and the common case where both s and *this are ok.  if (state_ != s.state_) {    delete[] state_;    state_ = (s.state_ == NULL) ? NULL : CopyState(s.state_);  }}}  // namespace leveldb

Status 定义

leveldb::Status 的定义如下：

const char* Status::CopyState(const char* state) {  uint32_t size;  memcpy(&size, state, sizeof(size));  char* result = new char[size + 5];  memcpy(result, state, size + 5);  return result;}Status::Status(Code code, const Slice& msg, const Slice& msg2) {  assert(code != kOk);  const uint32_t len1 = msg.size();  const uint32_t len2 = msg2.size();  const uint32_t size = len1 + (len2 ? (2 + len2) : 0);  char* result = new char[size + 5];  memcpy(result, &size, sizeof(size));  result[4] = static_cast<char>(code);  memcpy(result + 5, msg.data(), len1);  if (len2) {    result[5 + len1] = ':';    result[6 + len1] = ' ';    memcpy(result + 7 + len1, msg2.data(), len2);  }  state_ = result;}std::string Status::ToString() const {  if (state_ == NULL) {    return "OK";  } else {    char tmp[30];    const char* type;    switch (code()) {      case kOk:        type = "OK";        break;      case kNotFound:        type = "NotFound: ";        break;      case kCorruption:        type = "Corruption: ";        break;      case kNotSupported:        type = "Not implemented: ";        break;      case kInvalidArgument:        type = "Invalid argument: ";        break;      case kIOError:        type = "IO error: ";        break;      default:        snprintf(tmp, sizeof(tmp), "Unknown code(%d): ",                 static_cast<int>(code()));        type = tmp;        break;    }    std::string result(type);    uint32_t length;    memcpy(&length, state_, sizeof(length));    result.append(state_ + 5, length);    return result;  }}

Status 解析

可以看到Status就只有一个成员变量const char *state_，是一个指向字符串的指针，对该字符串进行了封装：

// 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..]  == messageconst char* state_;

对Status 定义了一堆的静态成员函数，通过调用这些静态成员函数来构造Status的对象，Status 的使用方法如下：

Status WriteBatch::Iterate(Handler* handler) const {  Slice input(rep_);  if (input.size() < kHeader) {    return Status::Corruption("malformed WriteBatch (too small)");  }  input.remove_prefix(kHeader);  Slice key, value;  int found = 0;  while (!input.empty()) {    found++;    char tag = input[0];    input.remove_prefix(1);    switch (tag) {      case kTypeValue:        if (GetLengthPrefixedSlice(&input, &key) &&            GetLengthPrefixedSlice(&input, &value)) {          handler->Put(key, value);        } else {          return Status::Corruption("bad WriteBatch Put");        }        break;      case kTypeDeletion:        if (GetLengthPrefixedSlice(&input, &key)) {          handler->Delete(key);        } else {          return Status::Corruption("bad WriteBatch Delete");        }        break;      default:        return Status::Corruption("unknown WriteBatch tag");    }  }  if (found != WriteBatchInternal::Count(this)) {    return Status::Corruption("WriteBatch has wrong count");  } else {    return Status::OK();  }}

Status中通过静态成员函数构造对象，通过ToString()成员函数来获取错误消息。

leveldb::Status status = leveldb::DB::Open(options, "/tmp/testdb", &db);if (!status.ok()){    cerr << status.ToString() << endl;}

从Status中学到如下知识

1. 使用enum 来对错误信息编码
2. 将消息长度，消息类型和消息一起编码到一个char *数组，使用memcpy复制数组，加快程序的运行速度
3. sprintf不安全，请用snprintf
4. Slice::compare 比较两个已知长度的字符串，依然使用memcmp
5. 将赋值构造函数和拷贝构造函数的共同部分定义成一个辅助函数
6. 通过静态成员函数来返回对象