m01：把日志做成深模块

void Store::load() {
    LogReader reader;
    uint64_t end = 0;
    if (reader.open(path_)) {
        std::string raw;
        while (reader.next(raw)) {
            Record r;
            try {
                r = RecordParser::parse(raw);
            } catch (const std::exception& e) {
                throw std::runtime_error(path_ + ": bad record at offset " +
                    std::to_string(reader.offset() - raw.size()) + ": " + e.what());
            }
            handle(r);
        }
        end = reader.offset();
    } else if (errno != ENOENT) {
        throw std::runtime_error("cannot open " + path_ + ": " + std::strerror(errno));
    }
    if (!writer_.open(path_, O_WRONLY | O_CREAT | O_APPEND, 0644, end)) {
        throw std::runtime_error("cannot open " + path_ + " for writing: " + std::strerror(errno));
    }
}

三个类、一个 errno 判断、一组 open 标志、一个要从读端搬到写端的偏移，还有"先读后写"的顺序。

Store::Store(const std::string& path, const Options& options)
    : index_(options.index_reserve),
      log_(path, [this](const Record& r, uint64_t size) { handle(r, size); }) {}

// log_file.h
class LogFile {
public:
    // Opens the log at `path`, creating an empty one if it does not exist,
    // and passes every record already in it to `visit`, oldest first. ...
    LogFile(const std::string& path, const RecordVisitor& visit);
    uint64_t append(const Record& rec, const WriteOptions& options); // returns bytes used
    void rewrite(const std::function<void(const RecordSink&)>& writeRecords);
    uint64_t size() const;
};

格式、校验、重试、文件不存在、偏移、原子替换，全部在 log_file.cpp 里。Store 只看见记录和字节数。

// log_writer.cpp
w.putU32(0);  // checksum, filled in below
w.putU8(static_cast<uint8_t>(rec.type));
w.putU32(static_cast<uint32_t>(rec.key.size()));
...
// log_reader.cpp
ByteReader r(header, sizeof header);
r.skip(5);                       // crc (4) + type (1)
uint32_t keyLen = r.getU32();
...
// record_parser.cpp
uint32_t storedCrc = r.getU32();
uint8_t type = r.getU8();
...
// store.cpp（出现 5 次）
uint64_t size = kRecordHeaderSize + key.size() + value.size();

// log_file.cpp, anonymous namespace
constexpr size_t kHeaderSize = 13;
constexpr size_t kTypeOffset = 4;
constexpr size_t kKeyLenOffset = 5;
constexpr size_t kValueLenOffset = 9;

std::string encode(const Record& rec);   // used by append() and rewrite()
uint64_t replay(int fd, const std::string& path,
                const LogFile::RecordVisitor& visit);

// store.cpp
uint64_t size = log_.append(rec, wo);