Space: what is using a disk, WizTree-style

Pick a disk, not a partition; the scan walks every filesystem it backs, as root when
the host allows, in a helper process that streams the tree back. A tree of folders
largest-first over a treemap of the selected folder, nested to the file and coloured by
kind, plus the thousand largest files. The rail no longer offers the hub-only pages
while a remote machine is being looked at.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/space/codec.go

@@ -0,0 +1,140 @@
+package space
+
+import (
+	"bufio"
+	"encoding/binary"
+	"errors"
+	"io"
+)
+
+// The tree crosses from the helper process to the daemon as a preorder stream of varints:
+// a few bytes per node, decoded without reflection, so a million nodes take well under a
+// second either way.
+
+const codecMagic = "HLSPACE1"
+
+func Encode(w io.Writer, root *Node) error {
+	bw := bufio.NewWriterSize(w, 1<<20)
+	if _, err := bw.WriteString(codecMagic); err != nil {
+		return err
+	}
+	var buf [binary.MaxVarintLen64]byte
+	putU := func(v uint64) error { n := binary.PutUvarint(buf[:], v); _, err := bw.Write(buf[:n]); return err }
+	putI := func(v int64) error { n := binary.PutVarint(buf[:], v); _, err := bw.Write(buf[:n]); return err }
+	var enc func(n *Node) error
+	enc = func(n *Node) error {
+		if err := putU(uint64(len(n.Name))); err != nil {
+			return err
+		}
+		if _, err := bw.WriteString(n.Name); err != nil {
+			return err
+		}
+		flags := uint64(0)
+		if n.Dir {
+			flags |= 1
+		}
+		if n.Mount {
+			flags |= 2
+		}
+		if n.Denied {
+			flags |= 4
+		}
+		for _, f := range []func() error{
+			func() error { return putU(flags) },
+			func() error { return putI(n.Size) },
+			func() error { return putI(n.Apparent) },
+			func() error { return putI(n.MTime) },
+			func() error { return putU(uint64(n.Files)) },
+			func() error { return putU(uint64(n.Dirs)) },
+			func() error { return putU(uint64(n.SmallN)) },
+			func() error { return putI(n.SmallSize) },
+			func() error { return putU(uint64(len(n.Children))) },
+		} {
+			if err := f(); err != nil {
+				return err
+			}
+		}
+		for _, c := range n.Children {
+			if err := enc(c); err != nil {
+				return err
+			}
+		}
+		return nil
+	}
+	if err := enc(root); err != nil {
+		return err
+	}
+	return bw.Flush()
+}
+
+func Decode(r io.Reader) (*Node, error) {
+	br := bufio.NewReaderSize(r, 1<<20)
+	magic := make([]byte, len(codecMagic))
+	if _, err := io.ReadFull(br, magic); err != nil || string(magic) != codecMagic {
+		return nil, errors.New("space: not a scan stream")
+	}
+	var dec func() (*Node, error)
+	dec = func() (*Node, error) {
+		l, err := binary.ReadUvarint(br)
+		if err != nil {
+			return nil, err
+		}
+		if l > 1<<16 {
+			return nil, errors.New("space: bad name length")
+		}
+		name := make([]byte, l)
+		if _, err := io.ReadFull(br, name); err != nil {
+			return nil, err
+		}
+		n := &Node{Name: string(name)}
+		flags, err := binary.ReadUvarint(br)
+		if err != nil {
+			return nil, err
+		}
+		n.Dir, n.Mount, n.Denied = flags&1 != 0, flags&2 != 0, flags&4 != 0
+		if n.Size, err = binary.ReadVarint(br); err != nil {
+			return nil, err
+		}
+		if n.Apparent, err = binary.ReadVarint(br); err != nil {
+			return nil, err
+		}
+		if n.MTime, err = binary.ReadVarint(br); err != nil {
+			return nil, err
+		}
+		u, err := binary.ReadUvarint(br)
+		if err != nil {
+			return nil, err
+		}
+		n.Files = uint32(u)
+		if u, err = binary.ReadUvarint(br); err != nil {
+			return nil, err
+		}
+		n.Dirs = uint32(u)
+		if u, err = binary.ReadUvarint(br); err != nil {
+			return nil, err
+		}
+		n.SmallN = uint32(u)
+		if n.SmallSize, err = binary.ReadVarint(br); err != nil {
+			return nil, err
+		}
+		cnt, err := binary.ReadUvarint(br)
+		if err != nil {
+			return nil, err
+		}
+		if cnt > 1<<24 {
+			return nil, errors.New("space: bad child count")
+		}
+		if cnt > 0 {
+			n.Children = make([]*Node, 0, cnt)
+			for i := uint64(0); i < cnt; i++ {
+				c, err := dec()
+				if err != nil {
+					return nil, err
+				}
+				n.Children = append(n.Children, c)
+			}
+		}
+		return n, nil
+	}
+	return dec()
+}