Add meta page checksums.

This commit adds checksums to the meta pages on every write. When the
database loads, it verifies the checksums on the meta pages and returns
an error if either one is corrupt.

In the future, it should fallback to the previous meta page but for right
now it just hard fails. This is at least preferable to opening the database
and getting a random error or further corruption.

Fixes #25.

3 files changed, 55 insertions, 10 deletions

diff --git a/db.go b/db.go
index c43eb5c..ee3a24b 100644
--- a/db.go
+++ b/db.go
@@ -105,7 +105,7 @@ func Open(path string, mode os.FileMode) (*DB, error) {
 		if _, err := db.file.ReadAt(buf[:], 0); err == nil {
 			m := db.pageInBuffer(buf[:], 0).meta()
 			if err := m.validate(); err != nil {
-				return nil, fmt.Errorf("meta error: %s", err)
+				return nil, fmt.Errorf("meta0 error: %s", err)
 			}
 			db.pageSize = int(m.pageSize)
 		}
diff --git a/db_test.go b/db_test.go
index df1aad3..e5b8c1a 100644
--- a/db_test.go
+++ b/db_test.go
@@ -117,10 +117,43 @@ func TestDBCorruptMeta0(t *testing.T) {
 
 		// Open the database.
 		_, err = Open(path, 0666)
-		assert.Equal(t, err, errors.New("meta error: invalid database"))
+		assert.Equal(t, err, errors.New("meta0 error: invalid database"))
 	})
 }
 
+// Ensure that a corrupt meta page checksum causes the open to fail.
+func TestDBMetaChecksumError(t *testing.T) {
+	for i := 0; i < 2; i++ {
+		withTempPath(func(path string) {
+			db, err := Open(path, 0600)
+			pageSize := db.pageSize
+			db.Update(func(tx *Tx) error {
+				return tx.CreateBucket("widgets")
+			})
+			db.Update(func(tx *Tx) error {
+				return tx.CreateBucket("woojits")
+			})
+			db.Close()
+
+			// Change a single byte in the meta page.
+			f, _ := os.OpenFile(path, os.O_RDWR|os.O_CREATE, 0600)
+			f.WriteAt([]byte{1}, int64((i*pageSize)+(pageHeaderSize+12)))
+			f.Sync()
+			f.Close()
+
+			// Reopen the database.
+			_, err = Open(path, 0600)
+			if assert.Error(t, err) {
+				if i == 0 {
+					assert.Equal(t, "meta0 error: checksum error", err.Error())
+				} else {
+					assert.Equal(t, "meta1 error: checksum error", err.Error())
+				}
+			}
+		})
+	}
+}
+
 // Ensure that a database cannot open a transaction when it's not open.
 func TestDBTxErrDatabaseNotOpen(t *testing.T) {
 	var db DB
diff --git a/meta.go b/meta.go
index cc62637..abb2a93 100644
--- a/meta.go
+++ b/meta.go
@@ -2,6 +2,8 @@ package bolt
 
 import (
 	"errors"
+	"hash/fnv"
+	"unsafe"
 )
 
 const magic uint32 = 0xED0CDAED
@@ -13,6 +15,9 @@ var (
 	// ErrVersionMismatch is returned when the data file was created with a
 	// different version of Bolt.
 	ErrVersionMismatch = errors.New("version mismatch")
+
+	// ErrChecksum is returned when either meta page checksum does not match.
+	ErrChecksum = errors.New("checksum error")
 )
 
 type meta struct {
@@ -24,11 +29,14 @@ type meta struct {
 	freelist pgid
 	pgid     pgid
 	txid     txid
+	checksum uint64
 }
 
 // validate checks the marker bytes and version of the meta page to ensure it matches this binary.
 func (m *meta) validate() error {
-	if m.magic != magic {
+	if m.checksum != 0 && m.checksum != m.sum64() {
+		return ErrChecksum
+	} else if m.magic != magic {
 		return ErrInvalid
 	} else if m.version != version {
 		return ErrVersionMismatch
@@ -38,13 +46,7 @@ func (m *meta) validate() error {
 
 // copy copies one meta object to another.
 func (m *meta) copy(dest *meta) {
-	dest.magic = m.magic
-	dest.version = m.version
-	dest.pageSize = m.pageSize
-	dest.buckets = m.buckets
-	dest.freelist = m.freelist
-	dest.pgid = m.pgid
-	dest.txid = m.txid
+	*dest = *m
 }
 
 // write writes the meta onto a page.
@@ -53,5 +55,15 @@ func (m *meta) write(p *page) {
 	p.id = pgid(m.txid % 2)
 	p.flags |= metaPageFlag
 
+	// Calculate the checksum.
+	m.checksum = m.sum64()
+
 	m.copy(p.meta())
 }
+
+// generates the checksum for the meta.
+func (m *meta) sum64() uint64 {
+	var h = fnv.New64a()
+	_, _ = h.Write((*[unsafe.Offsetof(meta{}.checksum)]byte)(unsafe.Pointer(m))[:])
+	return h.Sum64()
+}