levelDB源码分析-Status

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。

声明如下：

class Status {
public:
// Create a success status.
Status() : state_(NULL) { }						// 构造函数，默认状态为success
~Status() { delete[] state_; }						// 析构函数，释放状态字符串

// Copy the specified status.
Status(const Status& s);
void operator=(const Status& s);

// Return a success status.
static Status OK() { return Status(); }				// 返回一个success的状态

// 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; }

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

private:
// 为了节省空间Status并没有用std::string来存储错误信息，
// 而是将返回码(Code), 错误信息msg及长度打包存储于一个字符串数组中。
// 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 {											// 返回状态码
return (state_ == NULL) ? kOk : static_cast<Code>(state_[4]);
}

Status(Code code, const Slice& msg, const Slice& msg2);			// 内部构造函数
static const char* CopyState(const char* s);						//
};

实现如下：

inline Status::Status(const Status& s) {							// 拷贝构造函数
state_ = (s.state_ == NULL) ? NULL : CopyState(s.state_);
}
inline void Status::operator=(const Status& s) {						// 赋值运算符重载
// 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_);
}
}

const char* Status::CopyState(const char* state) {						// 复制状态字符串
uint32_t size;
memcpy(&size, state, sizeof(size));
uint32_t len = size + sizeof(size) + 1 ;
char* result = new char[len];
memcpy(result, state, len);
return result;
}

Status::Status(Code code, const Slice& msg, const Slice& msg2) {		// 内部构造函数
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 + 6];   // 没有结束符? --- 由于保留了size字段，采用memcpy指定size操作，可以没有结束符
memcpy(result, &size, sizeof(size));
result[4] = static_cast<char>(code); // 第5个字节存放code
//memcpy(result + 5, msg.data(), len1);
memcpy(result + sizeof(size) + 1, 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 {								// 返回状态字符串
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);				// 状态字符串
return result;
}
}

使用方法：

Status status_ = Status::Corruption("bad entry in block");
Status::Corruption("bad block contents") ;
Status::InvalidArgument((string)dbname_, "does not exist (create_if_missing is false)");
Status::Corruption("log record too small");
Status s = Status::IOError("Deleting DB during memtable compaction");
Status::OK() ;
status.ToString() ;