feat(externalMagic): Introduce external magic number

badger/cmd/info.go

@@ -51,6 +51,7 @@ type flagOptions struct {
 	encryptionKey            string
 	checksumVerificationMode string
 	discard                  bool
+	externalMagicVersion     uint16
 }
 
 var (
@@ -82,6 +83,8 @@ func init() {
 		"[none, table, block, tableAndBlock] Specifies when the db should verify checksum for SST.")
 	infoCmd.Flags().BoolVar(&opt.discard, "discard", false,
 		"Parse and print DISCARD file from value logs.")
+	infoCmd.Flags().Uint16Var(&opt.externalMagicVersion, "external-magic", 0,
+		"External magic number")
 }
 
 var infoCmd = &cobra.Command{
@@ -104,7 +107,8 @@ func handleInfo(cmd *cobra.Command, args []string) error {
 		WithBlockCacheSize(100 << 20).
 		WithIndexCacheSize(200 << 20).
 		WithEncryptionKey([]byte(opt.encryptionKey)).
-		WithChecksumVerificationMode(cvMode)
+		WithChecksumVerificationMode(cvMode).
+		WithExternalMagic(opt.externalMagicVersion)
 
 	if opt.discard {
 		ds, err := badger.InitDiscardStats(bopt)
@@ -322,7 +326,7 @@ func printInfo(dir, valueDir string) error {
 			fp.Close()
 		}
 	}()
-	manifest, truncOffset, err := badger.ReplayManifestFile(fp)
+	manifest, truncOffset, err := badger.ReplayManifestFile(fp, opt.externalMagicVersion)
 	if err != nil {
 		return err
 	}

manifest.go

@@ -23,6 +23,7 @@ import (
 	"fmt"
 	"hash/crc32"
 	"io"
+	"math"
 	"os"
 	"path/filepath"
 	"sync"
@@ -79,6 +80,10 @@ type TableManifest struct {
 type manifestFile struct {
 	fp        *os.File
 	directory string
+
+	// The external magic number used by the application running badger.
+	externalMagic uint16
+
 	// We make this configurable so that unit tests can hit rewrite() code quickly
 	deletionsRewriteThreshold int
 
@@ -124,11 +129,12 @@ func openOrCreateManifestFile(opt Options) (
 	if opt.InMemory {
 		return &manifestFile{inMemory: true, manifest: createManifest()}, Manifest{}, nil
 	}
-	return helpOpenOrCreateManifestFile(opt.Dir, opt.ReadOnly, manifestDeletionsRewriteThreshold)
+	return helpOpenOrCreateManifestFile(opt.Dir, opt.ReadOnly, opt.ExternalMagicVersion,
+		manifestDeletionsRewriteThreshold)
 }
 
-func helpOpenOrCreateManifestFile(dir string, readOnly bool, deletionsThreshold int) (
-	*manifestFile, Manifest, error) {
+func helpOpenOrCreateManifestFile(dir string, readOnly bool, extMagic uint16,
+	deletionsThreshold int) (*manifestFile, Manifest, error) {
 
 	path := filepath.Join(dir, ManifestFilename)
 	var flags y.Flags
@@ -144,21 +150,22 @@ func helpOpenOrCreateManifestFile(dir string, readOnly bool, deletionsThreshold
 			return nil, Manifest{}, fmt.Errorf("no manifest found, required for read-only db")
 		}
 		m := createManifest()
-		fp, netCreations, err := helpRewrite(dir, &m)
+		fp, netCreations, err := helpRewrite(dir, &m, extMagic)
 		if err != nil {
 			return nil, Manifest{}, err
 		}
 		y.AssertTrue(netCreations == 0)
 		mf := &manifestFile{
 			fp:                        fp,
 			directory:                 dir,
+			externalMagic:             extMagic,
 			manifest:                  m.clone(),
 			deletionsRewriteThreshold: deletionsThreshold,
 		}
 		return mf, m, nil
 	}
 
-	manifest, truncOffset, err := ReplayManifestFile(fp)
+	manifest, truncOffset, err := ReplayManifestFile(fp, extMagic)
 	if err != nil {
 		_ = fp.Close()
 		return nil, Manifest{}, err
@@ -179,6 +186,7 @@ func helpOpenOrCreateManifestFile(dir string, readOnly bool, deletionsThreshold
 	mf := &manifestFile{
 		fp:                        fp,
 		directory:                 dir,
+		externalMagic:             extMagic,
 		manifest:                  manifest.clone(),
 		deletionsRewriteThreshold: deletionsThreshold,
 	}
@@ -237,20 +245,27 @@ func (mf *manifestFile) addChanges(changesParam []*pb.ManifestChange) error {
 // Has to be 4 bytes.  The value can never change, ever, anyway.
 var magicText = [4]byte{'B', 'd', 'g', 'r'}
 
-// The magic version number.
-const magicVersion = 8
+// The magic version number. It is allocated 2 bytes, so it's value must be <= math.MaxUint16
+const badgerMagicVersion = 8
 
-func helpRewrite(dir string, m *Manifest) (*os.File, int, error) {
+func helpRewrite(dir string, m *Manifest, extMagic uint16) (*os.File, int, error) {
 	rewritePath := filepath.Join(dir, manifestRewriteFilename)
 	// We explicitly sync.
 	fp, err := y.OpenTruncFile(rewritePath, false)
 	if err != nil {
 		return nil, 0, err
 	}
 
+	// magic bytes are structured as
+	// +---------------------+-------------------------+-----------------------+
+	// | magicText (4 bytes) | externalMagic (2 bytes) | badgerMagic (2 bytes) |
+	// +---------------------+-------------------------+-----------------------+
+
+	y.AssertTrue(badgerMagicVersion <= math.MaxUint16)
 	buf := make([]byte, 8)
 	copy(buf[0:4], magicText[:])
-	binary.BigEndian.PutUint32(buf[4:8], magicVersion)
+	binary.BigEndian.PutUint16(buf[4:6], extMagic)
+	binary.BigEndian.PutUint16(buf[6:8], badgerMagicVersion)
 
 	netCreations := len(m.Tables)
 	changes := m.asChanges()
@@ -305,7 +320,7 @@ func (mf *manifestFile) rewrite() error {
 	if err := mf.fp.Close(); err != nil {
 		return err
 	}
-	fp, netCreations, err := helpRewrite(mf.directory, &mf.manifest)
+	fp, netCreations, err := helpRewrite(mf.directory, &mf.manifest, mf.externalMagic)
 	if err != nil {
 		return err
 	}
@@ -345,7 +360,7 @@ var (
 // Also, returns the last offset after a completely read manifest entry -- the file must be
 // truncated at that point before further appends are made (if there is a partial entry after
 // that).  In normal conditions, truncOffset is the file size.
-func ReplayManifestFile(fp *os.File) (Manifest, int64, error) {
+func ReplayManifestFile(fp *os.File, extMagic uint16) (Manifest, int64, error) {
 	r := countingReader{wrapped: bufio.NewReader(fp)}
 
 	var magicBuf [8]byte
@@ -355,14 +370,22 @@ func ReplayManifestFile(fp *os.File) (Manifest, int64, error) {
 	if !bytes.Equal(magicBuf[0:4], magicText[:]) {
 		return Manifest{}, 0, errBadMagic
 	}
-	version := y.BytesToU32(magicBuf[4:8])
-	if version != magicVersion {
+
+	extVersion := y.BytesToU16(magicBuf[4:6])
+	version := y.BytesToU16(magicBuf[6:8])
+
+	if version != badgerMagicVersion {
 		return Manifest{}, 0,
 			//nolint:lll
 			fmt.Errorf("manifest has unsupported version: %d (we support %d).\n"+
 				"Please see https://github.com/dgraph-io/badger/blob/master/README.md#i-see-manifest-has-unsupported-version-x-we-support-y-error"+
 				" on how to fix this.",
-				version, magicVersion)
+				version, badgerMagicVersion)
+	}
+	if extVersion != extMagic {
+		return Manifest{}, 0,
+			fmt.Errorf("Cannot open DB because the external magic number doesn't match. "+
+				"Expected: %d, version present in manifest: %d\n", extMagic, extVersion)
 	}
 
 	stat, err := fp.Stat()

manifest_test.go

@@ -102,7 +102,7 @@ func TestManifestMagic(t *testing.T) {
 }
 
 func TestManifestVersion(t *testing.T) {
-	helpTestManifestFileCorruption(t, 4, "unsupported version")
+	helpTestManifestFileCorruption(t, 6, "unsupported version")
 }
 
 func TestManifestChecksum(t *testing.T) {
@@ -213,7 +213,7 @@ func TestManifestRewrite(t *testing.T) {
 	require.NoError(t, err)
 	defer removeDir(dir)
 	deletionsThreshold := 10
-	mf, m, err := helpOpenOrCreateManifestFile(dir, false, deletionsThreshold)
+	mf, m, err := helpOpenOrCreateManifestFile(dir, false, 0, deletionsThreshold)
 	defer func() {
 		if mf != nil {
 			mf.close()
@@ -239,7 +239,7 @@ func TestManifestRewrite(t *testing.T) {
 	err = mf.close()
 	require.NoError(t, err)
 	mf = nil
-	mf, m, err = helpOpenOrCreateManifestFile(dir, false, deletionsThreshold)
+	mf, m, err = helpOpenOrCreateManifestFile(dir, false, 0, deletionsThreshold)
 	require.NoError(t, err)
 	require.Equal(t, map[uint64]TableManifest{
 		uint64(deletionsThreshold * 3): {Level: 0},

options.go

@@ -115,6 +115,10 @@ type Options struct {
 	// NamespaceOffset specifies the offset from where the next 8 bytes contains the namespace.
 	NamespaceOffset int
 
+	// Magic version used by the application using badger to ensure that it doesn't open the DB
+	// with incompatible data format.
+	ExternalMagicVersion uint16
+
 	// Transaction start and commit timestamps are managed by end-user.
 	// This is only useful for databases built on top of Badger (like Dgraph).
 	// Not recommended for most users.
@@ -796,6 +800,13 @@ func (opt Options) WithNamespaceOffset(offset int) Options {
 	return opt
 }
 
+// WithExternalMagic returns a new Options value with ExternalMagicVersion set to the given value.
+// The DB would fail to start if either the internal or the external magic number fails validated.
+func (opt Options) WithExternalMagic(magic uint16) Options {
+	opt.ExternalMagicVersion = magic
+	return opt
+}
+
 func (opt Options) getFileFlags() int {
 	var flags int
 	// opt.SyncWrites would be using msync to sync. All writes go through mmap.

y/y.go

@@ -262,6 +262,18 @@ func (t *Throttle) Finish() error {
 	return t.finishErr
 }
 
+// U16ToBytes converts the given Uint16 to bytes
+func U16ToBytes(v uint16) []byte {
+	var uBuf [2]byte
+	binary.BigEndian.PutUint16(uBuf[:], v)
+	return uBuf[:]
+}
+
+// BytesToU16 converts the given byte slice to uint16
+func BytesToU16(b []byte) uint16 {
+	return binary.BigEndian.Uint16(b)
+}
+
 // U32ToBytes converts the given Uint32 to bytes
 func U32ToBytes(v uint32) []byte {
 	var uBuf [4]byte