Perform unmap when failing to mlock or both meta pages corrupted

Signed-off-by: Benjamin Wang <wachao@vmware.com>

db.go

@@ -388,7 +388,7 @@ func (db *DB) hasSyncedFreelist() bool {
 
 // mmap opens the underlying memory-mapped file and initializes the meta references.
 // minsz is the minimum size that the new mmap can be.
-func (db *DB) mmap(minsz int) error {
+func (db *DB) mmap(minsz int) (err error) {
 	db.mmaplock.Lock()
 	defer db.mmaplock.Unlock()
 
@@ -423,17 +423,27 @@ func (db *DB) mmap(minsz int) error {
 	}
 
 	// Unmap existing data before continuing.
-	if err := db.munmap(); err != nil {
+	if err = db.munmap(); err != nil {
 		return err
 	}
 
 	// Memory-map the data file as a byte slice.
 	// gofail: var mapError string
 	// return errors.New(mapError)
-	if err := mmap(db, size); err != nil {
+	if err = mmap(db, size); err != nil {
 		return err
 	}
 
+	// Perform unmmap on any error to reset all data fields:
+	// dataref, data, datasz, meta0 and meta1.
+	defer func() {
+		if err != nil {
+			if unmapErr := db.munmap(); unmapErr != nil {
+				err = fmt.Errorf("%w; unmap failed: %v", err, unmapErr)
+			}
+		}
+	}()
+
 	if db.Mlock {
 		// Don't allow swapping of data file
 		if err := db.mlock(fileSize); err != nil {

tests/failpoint/db_failpoint_test.go

@@ -2,7 +2,6 @@ package failpoint
 
 import (
 	"fmt"
-	"go.etcd.io/bbolt/internal/btesting"
 	"path/filepath"
 	"testing"
 	"time"
@@ -10,6 +9,7 @@ import (
 	"github.com/stretchr/testify/require"
 
 	bolt "go.etcd.io/bbolt"
+	"go.etcd.io/bbolt/internal/btesting"
 	gofail "go.etcd.io/gofail/runtime"
 )