// cloneFileSystem returns a new memory-backed file system whose root contains
// a copy of the directory dirname in the source file system srcFS. The copy
// is not recursive; directories under dirname are not copied.
//
// Changes to the resultant file system do not modify the source file system.
//
// For example, if srcFS contained:
//   - /bar
//   - /baz/0
//   - /foo/x
//   - /foo/y
//   - /foo/z/A
//   - /foo/z/B
// then calling cloneFileSystem(srcFS, "/foo") would result in a file system
// containing:
//   - /x
//   - /y
func cloneFileSystem(srcFS db.FileSystem, dirname string) (db.FileSystem, error) {
	if len(dirname) == 0 || dirname[len(dirname)-1] != os.PathSeparator {
		dirname += string(os.PathSeparator)
	}

	dstFS := memfs.New()
	list, err := srcFS.List(dirname)
	if err != nil {
		return nil, err
	}
	for _, name := range list {
		srcFile, err := srcFS.Open(dirname + name)
		if err != nil {
			return nil, err
		}
		stat, err := srcFile.Stat()
		if err != nil {
			return nil, err
		}
		if stat.IsDir() {
			err = srcFile.Close()
			if err != nil {
				return nil, err
			}
			continue
		}
		data := make([]byte, stat.Size())
		_, err = io.ReadFull(srcFile, data)
		if err != nil {
			return nil, err
		}
		err = srcFile.Close()
		if err != nil {
			return nil, err
		}
		dstFile, err := dstFS.Create(name)
		if err != nil {
			return nil, err
		}
		_, err = dstFile.Write(data)
		if err != nil {
			return nil, err
		}
		err = dstFile.Close()
		if err != nil {
			return nil, err
		}
	}
	return dstFS, nil
}

func setCurrentFile(dirname string, fs db.FileSystem, fileNum uint64) error {
	newFilename := dbFilename(dirname, fileTypeCurrent, fileNum)
	oldFilename := fmt.Sprintf("%s.%06d.dbtmp", newFilename, fileNum)
	fs.Remove(oldFilename)
	f, err := fs.Create(oldFilename)
	if err != nil {
		return err
	}
	if _, err := fmt.Fprintf(f, "MANIFEST-%06d\n", fileNum); err != nil {
		return err
	}
	if err := f.Close(); err != nil {
		return err
	}
	return fs.Rename(oldFilename, newFilename)
}

// writeLevel0Table writes a memtable to a level-0 on-disk table.
//
// If no error is returned, it adds the file number of that on-disk table to
// d.pendingOutputs. It is the caller's responsibility to remove that fileNum
// from that set when it has been applied to d.versions.
//
// d.mu must be held when calling this, but the mutex may be dropped and
// re-acquired during the course of this method.
func (d *DB) writeLevel0Table(fs db.FileSystem, mem *memdb.MemDB) (meta fileMetadata, err error) {
	// meta用于记录新创建的level0 db文件的元信息
	meta.fileNum = d.versions.nextFileNum()
	// filename：新db文件的文件名
	filename := dbFilename(d.dirname, fileTypeTable, meta.fileNum)

	d.pendingOutputs[meta.fileNum] = struct{}{}
	defer func(fileNum uint64) {
		// 如果异常退出(err不为nil)，则从d.pendingOutputs中删除新db文件的记录
		// 否则d.pendingOutputs是用来干什么的呢？
		if err != nil {
			delete(d.pendingOutputs, fileNum)
		}
	}(meta.fileNum)

	// Release the d.mu lock while doing I/O.
	// Note the unusual order: Unlock and then Lock.
	d.mu.Unlock()
	defer d.mu.Lock()

	var (
		file db.File
		tw   *table.Writer
		iter db.Iterator
	)
	defer func() {
		if iter != nil {
			err = firstError(err, iter.Close())
		}
		if tw != nil {
			err = firstError(err, tw.Close())
		}
		if file != nil {
			err = firstError(err, file.Close())
		}
		if err != nil {
			fs.Remove(filename)
			meta = fileMetadata{}
		}
	}()

	file, err = fs.Create(filename)
	if err != nil {
		return fileMetadata{}, err
	}

	// table为磁盘db文件封装写入方式
	tw = table.NewWriter(file, &db.Options{
		Comparer: d.icmp,
	})

	// Find返回一个迭代器，用于遍历mem（这里即是d.imm）中的数据
	// memtable是以skiplist来组织数据的，有序
	// 所以取到的第一个数据的key即为当前imm中最小的key
	iter = mem.Find(nil, nil)
	iter.Next()
	// meta.smallest记录新db文件中最小的内部key，在写入memtable时就已经将用户key封装成了内部key，那么为什么还要使用internalKey来做类型转换？
	// 下面的meta.largest就没有进行类型转换就调用clone()方法了。
	meta.smallest = internalKey(iter.Key()).clone()
	for {
		// 最后一次循环中的key即为最大的key，但为什么不封装成内部key呢？
		meta.largest = iter.Key()
		// 将key、value写到新db文件中
		if err1 := tw.Set(meta.largest, iter.Value(), nil); err1 != nil {
			return fileMetadata{}, err1
		}
		// 如果imm中的数据已经遍历完，全部存入新db文件，则break
		if !iter.Next() {
			break
		}
	}
	meta.largest = meta.largest.clone()

	if err1 := iter.Close(); err1 != nil {
		iter = nil
		return fileMetadata{}, err1
	}
	iter = nil

	if err1 := tw.Close(); err1 != nil {
		tw = nil
		return fileMetadata{}, err1
	}
	tw = nil

	// TODO: currently, closing a table.Writer closes its underlying file.
	// We have to re-open the file to Sync or Stat it, which seems stupid.
	file, err = fs.Open(filename)
	if err != nil {
		return fileMetadata{}, err
	}

//.........这里部分代码省略.........

// replayLogFile replays the edits in the named log file.
//
// d.mu must be held when calling this, but the mutex may be dropped and
// re-acquired during the course of this method.
func (d *DB) replayLogFile(ve *versionEdit, fs db.FileSystem, filename string) (maxSeqNum uint64, err error) {
	file, err := fs.Open(filename)
	if err != nil {
		return 0, err
	}
	defer file.Close()

	var (
		mem      *memdb.MemDB
		batchBuf = new(bytes.Buffer)
		ikey     = make(internalKey, 512)
		rr       = record.NewReader(file)
	)
	for {
		r, err := rr.Next()
		if err == io.EOF {
			break
		}
		if err != nil {
			return 0, err
		}
		_, err = io.Copy(batchBuf, r)
		if err != nil {
			return 0, err
		}

		if batchBuf.Len() < batchHeaderLen {
			return 0, fmt.Errorf("leveldb: corrupt log file %q", filename)
		}
		b := Batch{batchBuf.Bytes()}
		seqNum := b.seqNum()
		seqNum1 := seqNum + uint64(b.count())
		if maxSeqNum < seqNum1 {
			maxSeqNum = seqNum1
		}

		if mem == nil {
			mem = memdb.New(&d.icmpOpts)
		}

		t := b.iter()
		for ; seqNum != seqNum1; seqNum++ {
			kind, ukey, value, ok := t.next()
			if !ok {
				return 0, fmt.Errorf("leveldb: corrupt log file %q", filename)
			}
			// Convert seqNum, kind and key into an internalKey, and add that ikey/value
			// pair to mem.
			//
			// TODO: instead of copying to an intermediate buffer (ikey), is it worth
			// adding a SetTwoPartKey(db.TwoPartKey{key0, key1}, value, opts) method to
			// memdb.MemDB? What effect does that have on the db.Comparer interface?
			//
			// The C++ LevelDB code does not need an intermediate copy because its memdb
			// implementation is a private implementation detail, and copies each internal
			// key component from the Batch format straight to the skiplist buffer.
			//
			// Go's LevelDB considers the memdb functionality to be useful in its own
			// right, and so leveldb/memdb is a separate package that is usable without
			// having to import the top-level leveldb package. That extra abstraction
			// means that we need to copy to an intermediate buffer here, to reconstruct
			// the complete internal key to pass to the memdb.
			ikey = makeInternalKey(ikey, ukey, kind, seqNum)
			mem.Set(ikey, value, nil)
		}
		if len(t) != 0 {
			return 0, fmt.Errorf("leveldb: corrupt log file %q", filename)
		}

		// TODO: if mem is large enough, write it to a level-0 table and set mem = nil.

		batchBuf.Reset()
	}

	if mem != nil && !mem.Empty() {
		// 如果初始化后memtable中有数据，则存入磁盘level0层级文件中
		meta, err := d.writeLevel0Table(fs, mem)
		if err != nil {
			return 0, err
		}
		ve.newFiles = append(ve.newFiles, newFileEntry{level: 0, meta: meta})
		// Strictly speaking, it's too early to delete meta.fileNum from d.pendingOutputs,
		// but we are replaying the log file, which happens before Open returns, so there
		// is no possibility of deleteObsoleteFiles being called concurrently here.
		delete(d.pendingOutputs, meta.fileNum)
	}

	return maxSeqNum, nil
}

func (d *DB) writeLevel0Table(fs db.FileSystem, mem *memdb.MemDB) (meta fileMetadata, err error) {
	meta.fileNum = d.versions.nextFileNum()
	filename := dbFilename(d.dirname, fileTypeTable, meta.fileNum)
	// TODO: add meta.fileNum to a set of 'pending outputs' so that a
	// concurrent sweep of obsolete db files won't delete the fileNum file.
	// It is the caller's responsibility to remove that fileNum from the
	// set of pending outputs.

	var (
		file db.File
		tw   *table.Writer
		iter db.Iterator
	)
	defer func() {
		if iter != nil {
			err = firstError(err, iter.Close())
		}
		if tw != nil {
			err = firstError(err, tw.Close())
		}
		if file != nil {
			err = firstError(err, file.Close())
		}
		if err != nil {
			fs.Remove(filename)
			meta = fileMetadata{}
		}
	}()

	file, err = fs.Create(filename)
	if err != nil {
		return fileMetadata{}, err
	}
	tw = table.NewWriter(file, &db.Options{
		Comparer: d.icmp,
	})

	iter = mem.Find(nil, nil)
	iter.Next()
	meta.smallest = internalKey(iter.Key()).clone()
	for {
		meta.largest = iter.Key()
		if err1 := tw.Set(meta.largest, iter.Value(), nil); err1 != nil {
			return fileMetadata{}, err1
		}
		if !iter.Next() {
			break
		}
	}
	meta.largest = meta.largest.clone()

	if err1 := iter.Close(); err1 != nil {
		iter = nil
		return fileMetadata{}, err1
	}
	iter = nil

	if err1 := tw.Close(); err1 != nil {
		tw = nil
		return fileMetadata{}, err1
	}
	tw = nil

	// TODO: currently, closing a table.Writer closes its underlying file.
	// We have to re-open the file to Sync or Stat it, which seems stupid.
	file, err = fs.Open(filename)
	if err != nil {
		return fileMetadata{}, err
	}

	if err1 := file.Sync(); err1 != nil {
		return fileMetadata{}, err1
	}

	if stat, err1 := file.Stat(); err1 != nil {
		return fileMetadata{}, err1
	} else {
		size := stat.Size()
		if size < 0 {
			return fileMetadata{}, fmt.Errorf("leveldb: table file %q has negative size %d", filename, size)
		}
		meta.size = uint64(size)
	}

	// TODO: compaction stats.

	return meta, nil
}

// writeLevel0Table writes a memtable to a level-0 on-disk table.
//
// If no error is returned, it adds the file number of that on-disk table to
// d.pendingOutputs. It is the caller's responsibility to remove that fileNum
// from that set when it has been applied to d.versions.
//
// d.mu must be held when calling this, but the mutex may be dropped and
// re-acquired during the course of this method.
func (d *DB) writeLevel0Table(fs db.FileSystem, mem *memdb.MemDB) (meta fileMetadata, err error) {
	meta.fileNum = d.versions.nextFileNum()
	filename := dbFilename(d.dirname, fileTypeTable, meta.fileNum)
	d.pendingOutputs[meta.fileNum] = struct{}{}
	defer func(fileNum uint64) {
		if err != nil {
			delete(d.pendingOutputs, fileNum)
		}
	}(meta.fileNum)

	// Release the d.mu lock while doing I/O.
	// Note the unusual order: Unlock and then Lock.
	d.mu.Unlock()
	defer d.mu.Lock()

	var (
		file db.File
		tw   *table.Writer
		iter db.Iterator
	)
	defer func() {
		if iter != nil {
			err = firstError(err, iter.Close())
		}
		if tw != nil {
			err = firstError(err, tw.Close())
		}
		if file != nil {
			err = firstError(err, file.Close())
		}
		if err != nil {
			fs.Remove(filename)
			meta = fileMetadata{}
		}
	}()

	file, err = fs.Create(filename)
	if err != nil {
		return fileMetadata{}, err
	}
	tw = table.NewWriter(file, &db.Options{
		Comparer: d.icmp,
	})

	iter = mem.Find(nil, nil)
	iter.Next()
	meta.smallest = internalKey(iter.Key()).clone()
	for {
		meta.largest = iter.Key()
		if err1 := tw.Set(meta.largest, iter.Value(), nil); err1 != nil {
			return fileMetadata{}, err1
		}
		if !iter.Next() {
			break
		}
	}
	meta.largest = meta.largest.clone()

	if err1 := iter.Close(); err1 != nil {
		iter = nil
		return fileMetadata{}, err1
	}
	iter = nil

	if err1 := tw.Close(); err1 != nil {
		tw = nil
		return fileMetadata{}, err1
	}
	tw = nil

	// TODO: currently, closing a table.Writer closes its underlying file.
	// We have to re-open the file to Sync or Stat it, which seems stupid.
	file, err = fs.Open(filename)
	if err != nil {
		return fileMetadata{}, err
	}

	if err1 := file.Sync(); err1 != nil {
		return fileMetadata{}, err1
	}

	if stat, err1 := file.Stat(); err1 != nil {
		return fileMetadata{}, err1
	} else {
		size := stat.Size()
		if size < 0 {
			return fileMetadata{}, fmt.Errorf("leveldb: table file %q has negative size %d", filename, size)
		}
		meta.size = uint64(size)
	}

	// TODO: compaction stats.
//.........这里部分代码省略.........