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archive.go
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archive.go
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// The archive utilities manage the internal format of a snapshot, which is a
// tar file with the following contents:
//
// meta.json - JSON-encoded snapshot metadata from Raft
// state.bin - Encoded snapshot data from Raft
// SHA256SUMS - SHA-256 sums of the above two files
//
// The integrity information is automatically created and checked, and a failure
// there just looks like an error to the caller.
package snapshot
import (
"archive/tar"
"bufio"
"bytes"
"crypto/sha256"
"encoding/json"
"fmt"
"hash"
"io"
"time"
"github.com/hashicorp/raft"
)
// hashList manages a list of filenames and their hashes.
type hashList struct {
hashes map[string]hash.Hash
}
// newHashList returns a new hashList.
func newHashList() *hashList {
return &hashList{
hashes: make(map[string]hash.Hash),
}
}
// Add creates a new hash for the given file.
func (hl *hashList) Add(file string) hash.Hash {
if existing, ok := hl.hashes[file]; ok {
return existing
}
h := sha256.New()
hl.hashes[file] = h
return h
}
// Encode takes the current sum of all the hashes and saves the hash list as a
// SHA256SUMS-style text file.
func (hl *hashList) Encode(w io.Writer) error {
for file, h := range hl.hashes {
if _, err := fmt.Fprintf(w, "%x %s\n", h.Sum([]byte{}), file); err != nil {
return err
}
}
return nil
}
// DecodeAndVerify reads a SHA256SUMS-style text file and checks the results
// against the current sums for all the hashes.
func (hl *hashList) DecodeAndVerify(r io.Reader) error {
// Read the file and make sure everything in there has a matching hash.
seen := make(map[string]struct{})
s := bufio.NewScanner(r)
for s.Scan() {
sha := make([]byte, sha256.Size)
var file string
if _, err := fmt.Sscanf(s.Text(), "%x %s", &sha, &file); err != nil {
return err
}
h, ok := hl.hashes[file]
if !ok {
return fmt.Errorf("list missing hash for %q", file)
}
if !bytes.Equal(sha, h.Sum([]byte{})) {
return fmt.Errorf("hash check failed for %q", file)
}
seen[file] = struct{}{}
}
if err := s.Err(); err != nil {
return err
}
// Make sure everything we had a hash for was seen.
for file := range hl.hashes {
if _, ok := seen[file]; !ok {
return fmt.Errorf("file missing for %q", file)
}
}
return nil
}
// write takes a writer and creates an archive with the snapshot metadata,
// the snapshot itself, and adds some integrity checking information.
func write(out io.Writer, metadata *raft.SnapshotMeta, snap io.Reader) error {
// Start a new tarball.
now := time.Now()
archive := tar.NewWriter(out)
// Create a hash list that we will use to write a SHA256SUMS file into
// the archive.
hl := newHashList()
// Encode the snapshot metadata, which we need to feed back during a
// restore.
metaHash := hl.Add("meta.json")
var metaBuffer bytes.Buffer
enc := json.NewEncoder(&metaBuffer)
if err := enc.Encode(metadata); err != nil {
return fmt.Errorf("failed to encode snapshot metadata: %v", err)
}
if err := archive.WriteHeader(&tar.Header{
Name: "meta.json",
Mode: 0600,
Size: int64(metaBuffer.Len()),
ModTime: now,
}); err != nil {
return fmt.Errorf("failed to write snapshot metadata header: %v", err)
}
if _, err := io.Copy(archive, io.TeeReader(&metaBuffer, metaHash)); err != nil {
return fmt.Errorf("failed to write snapshot metadata: %v", err)
}
// Copy the snapshot data given the size from the metadata.
snapHash := hl.Add("state.bin")
if err := archive.WriteHeader(&tar.Header{
Name: "state.bin",
Mode: 0600,
Size: metadata.Size,
ModTime: now,
}); err != nil {
return fmt.Errorf("failed to write snapshot data header: %v", err)
}
if _, err := io.CopyN(archive, io.TeeReader(snap, snapHash), metadata.Size); err != nil {
return fmt.Errorf("failed to write snapshot metadata: %v", err)
}
// Create a SHA256SUMS file that we can use to verify on restore.
var shaBuffer bytes.Buffer
if err := hl.Encode(&shaBuffer); err != nil {
return fmt.Errorf("failed to encode snapshot hashes: %v", err)
}
if err := archive.WriteHeader(&tar.Header{
Name: "SHA256SUMS",
Mode: 0600,
Size: int64(shaBuffer.Len()),
ModTime: now,
}); err != nil {
return fmt.Errorf("failed to write snapshot hashes header: %v", err)
}
if _, err := io.Copy(archive, &shaBuffer); err != nil {
return fmt.Errorf("failed to write snapshot metadata: %v", err)
}
// Finalize the archive.
if err := archive.Close(); err != nil {
return fmt.Errorf("failed to finalize snapshot: %v", err)
}
return nil
}
// read takes a reader and extracts the snapshot metadata and the snapshot
// itself, and also checks the integrity of the data. You must arrange to call
// Close() on the returned object or else you will leak a temporary file.
func read(in io.Reader, metadata *raft.SnapshotMeta, snap io.Writer) error {
// Start a new tar reader.
archive := tar.NewReader(in)
// Create a hash list that we will use to compare with the SHA256SUMS
// file in the archive.
hl := newHashList()
// Populate the hashes for all the files we expect to see. The check at
// the end will make sure these are all present in the SHA256SUMS file
// and that the hashes match.
metaHash := hl.Add("meta.json")
snapHash := hl.Add("state.bin")
// Look through the archive for the pieces we care about.
var shaBuffer bytes.Buffer
for {
hdr, err := archive.Next()
if err == io.EOF {
break
}
if err != nil {
return fmt.Errorf("failed reading snapshot: %v", err)
}
switch hdr.Name {
case "meta.json":
// Previously we used json.Decode to decode the archive stream. There are
// edgecases in which it doesn't read all the bytes from the stream, even
// though the json object is still being parsed properly. Since we
// simultaneously feeded everything to metaHash, our hash ended up being
// different than what we calculated when creating the snapshot. Which in
// turn made the snapshot verification fail. By explicitly reading the
// whole thing first we ensure that we calculate the correct hash
// independent of how json.Decode works internally.
buf, err := io.ReadAll(io.TeeReader(archive, metaHash))
if err != nil {
return fmt.Errorf("failed to read snapshot metadata: %v", err)
}
if err := json.Unmarshal(buf, &metadata); err != nil {
return fmt.Errorf("failed to decode snapshot metadata: %v", err)
}
case "state.bin":
if _, err := io.Copy(io.MultiWriter(snap, snapHash), archive); err != nil {
return fmt.Errorf("failed to read or write snapshot data: %v", err)
}
case "SHA256SUMS":
if _, err := io.Copy(&shaBuffer, archive); err != nil {
return fmt.Errorf("failed to read snapshot hashes: %v", err)
}
default:
return fmt.Errorf("unexpected file %q in snapshot", hdr.Name)
}
}
// Verify all the hashes.
if err := hl.DecodeAndVerify(&shaBuffer); err != nil {
return fmt.Errorf("failed checking integrity of snapshot: %v", err)
}
return nil
}