add PreLoadFreelist to support loading free pages in readonly mode

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

cmd/bbolt/main.go

@@ -195,7 +195,10 @@ func (cmd *CheckCommand) Run(args ...string) error {
 	}
 
 	// Open database.
-	db, err := bolt.Open(path, 0666, &bolt.Options{ReadOnly: true})
+	db, err := bolt.Open(path, 0666, &bolt.Options{
+		ReadOnly:        true,
+		PreLoadFreelist: true,
+	})
 	if err != nil {
 		return err
 	}
@@ -644,7 +647,10 @@ func (cmd *PagesCommand) Run(args ...string) error {
 	}
 
 	// Open database.
-	db, err := bolt.Open(path, 0666, &bolt.Options{ReadOnly: true})
+	db, err := bolt.Open(path, 0666, &bolt.Options{
+		ReadOnly:        true,
+		PreLoadFreelist: true,
+	})
 	if err != nil {
 		return err
 	}

cmd/bbolt/main_test.go

@@ -255,6 +255,37 @@ func TestGetCommand_Run(t *testing.T) {
 	}
 }
 
+// Ensure the "pages" command neither panic, nor change the db file.
+func TestPagesCommand_Run(t *testing.T) {
+	db := btesting.MustCreateDB(t)
+
+	err := db.Update(func(tx *bolt.Tx) error {
+		for _, name := range []string{"foo", "bar"} {
+			b, err := tx.CreateBucket([]byte(name))
+			if err != nil {
+				return err
+			}
+			for i := 0; i < 3; i++ {
+				key := fmt.Sprintf("%s-%d", name, i)
+				val := fmt.Sprintf("val-%s-%d", name, i)
+				if err := b.Put([]byte(key), []byte(val)); err != nil {
+					return err
+				}
+			}
+		}
+		return nil
+	})
+	require.NoError(t, err)
+	db.Close()
+
+	defer requireDBNoChange(t, dbData(t, db.Path()), db.Path())
+
+	// Run the command.
+	m := NewMain()
+	err = m.Run("pages", db.Path())
+	require.NoError(t, err)
+}
+
 // Main represents a test wrapper for main.Main that records output.
 type Main struct {
 	*main.Main

db.go

@@ -95,6 +95,11 @@ type DB struct {
 	// https://github.com/boltdb/bolt/issues/284
 	NoGrowSync bool
 
+	// When `true`, bbolt will always load the free pages when opening the DB.
+	// When opening db in write mode, this flag will always automatically
+	// set to `true`.
+	PreLoadFreelist bool
+
 	// If you want to read the entire database fast, you can set MmapFlag to
 	// syscall.MAP_POPULATE on Linux 2.6.23+ for sequential read-ahead.
 	MmapFlags int
@@ -196,6 +201,7 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) {
 	db.NoGrowSync = options.NoGrowSync
 	db.MmapFlags = options.MmapFlags
 	db.NoFreelistSync = options.NoFreelistSync
+	db.PreLoadFreelist = options.PreLoadFreelist
 	db.FreelistType = options.FreelistType
 	db.Mlock = options.Mlock
 
@@ -208,6 +214,9 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) {
 	if options.ReadOnly {
 		flag = os.O_RDONLY
 		db.readOnly = true
+	} else {
+		// always load free pages in write mode
+		db.PreLoadFreelist = true
 	}
 
 	db.openFile = options.OpenFile
@@ -277,12 +286,14 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) {
 		return nil, err
 	}
 
+	if db.PreLoadFreelist {
+		db.loadFreelist()
+	}
+
 	if db.readOnly {
 		return db, nil
 	}
 
-	db.loadFreelist()
-
 	// Flush freelist when transitioning from no sync to sync so
 	// NoFreelistSync unaware boltdb can open the db later.
 	if !db.NoFreelistSync && !db.hasSyncedFreelist() {
@@ -1163,6 +1174,11 @@ type Options struct {
 	// under normal operation, but requires a full database re-sync during recovery.
 	NoFreelistSync bool
 
+	// PreLoadFreelist sets whether to load the free pages when opening
+	// the db file. Note when opening db in write mode, bbolt will always
+	// load the free pages.
+	PreLoadFreelist bool
+
 	// FreelistType sets the backend freelist type. There are two options. Array which is simple but endures
 	// dramatic performance degradation if database is large and framentation in freelist is common.
 	// The alternative one is using hashmap, it is faster in almost all circumstances

tx.go

@@ -652,6 +652,9 @@ func (tx *Tx) Page(id int) (*PageInfo, error) {
 		return nil, nil
 	}
 
+	// Force loading free list if opened in ReadOnly mode.
+	tx.db.loadFreelist()
+
 	// Build the page info.
 	p := tx.db.page(pgid(id))
 	info := &PageInfo{