LevelDB的BlockHandle和Footer和读取block
主要作用就是记录一个block在sstable文件中的偏移量和大小
代码：table/format.h和table/format.cc
class BlockHandle {
public:
// Maximum encoding length of a BlockHandle
enum { kMaxEncodedLength = 10 + 10 };
BlockHandle();
// The offset of the block in the file.
uint64_t offset() const { return offset_; }
void set_offset(uint64_t offset) { offset_ = offset; }
// The size of the stored block
uint64_t size() const { return size_; }
void set_size(uint64_t size) { size_ = size; }
// 将size和offset编码到string里面
void EncodeTo(std::string* dst) const;
// 从Slice里面解码出size和offset
Status DecodeFrom(Slice* input);
private:
uint64_t offset_;
uint64_t size_;
};
void BlockHandle::EncodeTo(std::string* dst) const {
// Sanity check that all fields have been set
assert(offset_ != ~static_cast<uint64_t>(0));
assert(size_ != ~static_cast<uint64_t>(0));
PutVarint64(dst, offset_);
PutVarint64(dst, size_);
}
Status BlockHandle::DecodeFrom(Slice* input) {
if (GetVarint64(input, &offset_) && GetVarint64(input, &size_)) {
return Status::OK();
} else {
return Status::Corruption("bad block handle");
}
}
class Footer {
public:
// Encoded length of a Footer. Note that the serialization of a
// Footer will always occupy exactly this many bytes. It consists
// of two block handles and a magic number.
enum { kEncodedLength = 2 * BlockHandle::kMaxEncodedLength + 8 };
Footer() = default;
// The block handle for the metaindex block of the table
const BlockHandle& metaindex_handle() const { return metaindex_handle_; }
void set_metaindex_handle(const BlockHandle& h) { metaindex_handle_ = h; }
// The block handle for the index block of the table
const BlockHandle& index_handle() const { return index_handle_; }
void set_index_handle(const BlockHandle& h) { index_handle_ = h; }
void EncodeTo(std::string* dst) const;
Status DecodeFrom(Slice* input);
private:
BlockHandle metaindex_handle_;
BlockHandle index_handle_;
};
void Footer::EncodeTo(std::string* dst) const {
const size_t original_size = dst->size();
metaindex_handle_.EncodeTo(dst);
index_handle_.EncodeTo(dst);
dst->resize(2 * BlockHandle::kMaxEncodedLength); // Padding
PutFixed32(dst, static_cast<uint32_t>(kTableMagicNumber & 0xffffffffu));
PutFixed32(dst, static_cast<uint32_t>(kTableMagicNumber >> 32));
assert(dst->size() == original_size + kEncodedLength);
(void)original_size; // Disable unused variable warning.
}
Status Footer::DecodeFrom(Slice* input) {
const char* magic_ptr = input->data() + kEncodedLength - 8;
const uint32_t magic_lo = DecodeFixed32(magic_ptr);
const uint32_t magic_hi = DecodeFixed32(magic_ptr + 4);
const uint64_t magic = ((static_cast<uint64_t>(magic_hi) << 32) |
(static_cast<uint64_t>(magic_lo)));
if (magic != kTableMagicNumber) {
return Status::Corruption("not an sstable (bad magic number)");
}
Status result = metaindex_handle_.DecodeFrom(input);
if (result.ok()) {
result = index_handle_.DecodeFrom(input);
}
if (result.ok()) {
// We skip over any leftover data (just padding for now) in "input"
const char* end = magic_ptr + 8;
*input = Slice(end, input->data() + input->size() - end);
}
return result;
}
下面就是读取block的代码
struct BlockContents {
// 保存block的内容
Slice data; // Actual contents of data
bool cachable; // True iff data can be cached
bool heap_allocated; // True iff caller should delete[] data.data()
};
// Read the block identified by "handle" from "file". On failure
// return non-OK. On success fill *result and return OK.
Status ReadBlock(RandomAccessFile* file, const ReadOptions& options,
const BlockHandle& handle, BlockContents* result);
Status ReadBlock(RandomAccessFile* file, const ReadOptions& options,
const BlockHandle& handle, BlockContents* result) {
result->data = Slice();
result->cachable = false;
result->heap_allocated = false;
// Read the block contents as well as the type/crc footer.
// See table_builder.cc for the code that built this structure.
// 获取block->size()
size_t n = static_cast<size_t>(handle.size());
// block.size()+5(就是是否压缩和crc校验码，这两个没有算在size里面)
char* buf = new char[n + kBlockTrailerSize];
Slice contents;
// 读取，主要是将block的数据读到buf里面，然后用buf构建content
Status s = file->Read(handle.offset(), n + kBlockTrailerSize, &contents, buf);
if (!s.ok()) {
delete[] buf;
return s;
}
if (contents.size() != n + kBlockTrailerSize) {
delete[] buf;
return Status::Corruption("truncated block read");
}
// Check the crc of the type and the block contents
const char* data = contents.data(); // Pointer to where Read put the data
if (options.verify_checksums) {
// 校验crc
const uint32_t crc = crc32c::Unmask(DecodeFixed32(data + n + 1));
const uint32_t actual = crc32c::Value(data, n + 1);
if (actual != crc) {
delete[] buf;
s = Status::Corruption("block checksum mismatch");
return s;
}
}
switch (data[n]) {
case kNoCompression:
// 没有使用压缩，将数据导出到BlockContent
if (data != buf) {
// File implementation gave us pointer to some other data.
// Use it directly under the assumption that it will be live
// while the file is open.
delete[] buf;
result->data = Slice(data, n);
result->heap_allocated = false;
result->cachable = false; // Do not double-cache
} else {
result->data = Slice(buf, n);
result->heap_allocated = true;
result->cachable = true;
}
// Ok
break;
case kSnappyCompression: {
// 使用了压缩，就得先解压
size_t ulength = 0;
if (!port::Snappy_GetUncompressedLength(data, n, &ulength)) {
delete[] buf;
return Status::Corruption("corrupted compressed block contents");
}
char* ubuf = new char[ulength];
if (!port::Snappy_Uncompress(data, n, ubuf)) {
delete[] buf;
delete[] ubuf;
return Status::Corruption("corrupted compressed block contents");
}
delete[] buf;
result->data = Slice(ubuf, ulength);
result->heap_allocated = true;
result->cachable = true;
break;
}
default:
delete[] buf;
return Status::Corruption("bad block type");
}
return Status::OK();
}