From c94eee5fc34cd77333d4b6445a9616a4e544a10d Mon Sep 17 00:00:00 2001
From: vyzo <vyzo@hackzen.org>
Date: Sun, 30 Jan 2022 12:47:30 +0200
Subject: [PATCH] on disk checkpoints

---
 blockstore/splitstore/checkpoint.go | 115 ++++++++++++++++++++++++++++
 1 file changed, 115 insertions(+)
 create mode 100644 blockstore/splitstore/checkpoint.go

diff --git a/blockstore/splitstore/checkpoint.go b/blockstore/splitstore/checkpoint.go
new file mode 100644
index 000000000..a0fa84730
--- /dev/null
+++ b/blockstore/splitstore/checkpoint.go
@@ -0,0 +1,115 @@
+package splitstore
+
+import (
+	"bufio"
+	"io"
+	"os"
+
+	"golang.org/x/xerrors"
+
+	cid "github.com/ipfs/go-cid"
+	mh "github.com/multiformats/go-multihash"
+)
+
+type Checkpoint struct {
+	file *os.File
+	buf  *bufio.Writer
+}
+
+func NewCheckpoint(path string) (*Checkpoint, error) {
+	file, err := os.OpenFile(path, os.O_CREATE|os.O_TRUNC|os.O_WRONLY|os.O_SYNC, 0644)
+	if err != nil {
+		return nil, xerrors.Errorf("error creating checkpoint: %w", err)
+	}
+	buf := bufio.NewWriter(file)
+
+	return &Checkpoint{
+		file: file,
+		buf:  buf,
+	}, nil
+}
+
+func OpenCheckpoint(path string) (*Checkpoint, cid.Cid, error) {
+	filein, err := os.Open(path)
+	if err != nil {
+		return nil, cid.Undef, xerrors.Errorf("error opening checkpoint for reading: %w", err)
+	}
+	defer filein.Close() //nolint:errcheck
+
+	bufin := bufio.NewReader(filein)
+	start, err := readRawCid(bufin, nil)
+	if err != nil && err != io.EOF {
+		return nil, cid.Undef, xerrors.Errorf("error reading cid from checkpoint: %w", err)
+	}
+
+	fileout, err := os.OpenFile(path, os.O_WRONLY|os.O_SYNC, 0644)
+	if err != nil {
+		return nil, cid.Undef, xerrors.Errorf("error opening checkpoint for writing: %w", err)
+	}
+	bufout := bufio.NewWriter(fileout)
+
+	return &Checkpoint{
+		file: fileout,
+		buf:  bufout,
+	}, start, nil
+}
+
+func (cp *Checkpoint) Set(c cid.Cid) error {
+	if _, err := cp.file.Seek(0, io.SeekStart); err != nil {
+		return xerrors.Errorf("error seeking beginning of checkpoint: %w", err)
+	}
+
+	if err := writeRawCid(cp.buf, c, true); err != nil {
+		return xerrors.Errorf("error writing cid to checkpoint: %w", err)
+	}
+
+	return nil
+}
+
+func (cp *Checkpoint) Close() (err error) {
+	if cp.file != nil {
+		err = cp.file.Close()
+		cp.file = nil
+		cp.buf = nil
+	}
+	return err
+}
+
+func readRawCid(buf *bufio.Reader, hbuf []byte) (cid.Cid, error) {
+	sz, err := buf.ReadByte()
+	if err != nil {
+		return cid.Undef, err // don't wrap EOF as it is not an error here
+	}
+
+	if hbuf == nil {
+		hbuf = make([]byte, int(sz))
+	} else {
+		hbuf = hbuf[:int(sz)]
+	}
+
+	if _, err := buf.Read(hbuf); err != nil {
+		return cid.Undef, xerrors.Errorf("error reading hash: %w", err) // wrap EOF, it's corrupt
+	}
+
+	hash, err := mh.Cast(hbuf)
+	if err != nil {
+		return cid.Undef, xerrors.Errorf("error casting multihash: %w", err)
+	}
+
+	return cid.NewCidV1(cid.Raw, hash), nil
+}
+
+func writeRawCid(buf *bufio.Writer, c cid.Cid, flush bool) error {
+	hash := c.Hash()
+	if err := buf.WriteByte(byte(len(hash))); err != nil {
+		return err
+	}
+	if _, err := buf.Write(hash); err != nil {
+		return err
+	}
+	if flush {
+		return buf.Flush()
+	}
+
+	return nil
+}