open read-only databases in read-only mode

bolt_unix.go

@@ -45,7 +45,7 @@ func mmap(db *DB, sz int) error {
 	// Truncate and fsync to ensure file size metadata is flushed.
 	// https://github.com/boltdb/bolt/issues/284
 	if !db.NoGrowSync {
-		if err := db.file.Truncate(int64(sz)); err != nil {
+		if err := db.ops.Truncate(int64(sz)); err != nil {
 			return fmt.Errorf("file resize error: %s", err)
 		}
 		if err := db.file.Sync(); err != nil {

bolt_windows.go

@@ -29,7 +29,7 @@ func funlock(f *os.File) error {
 // Based on: https://github.com/edsrzf/mmap-go
 func mmap(db *DB, sz int) error {
 	// Truncate the database to the size of the mmap.
-	if err := db.file.Truncate(int64(sz)); err != nil {
+	if err := db.ops.Truncate(int64(sz)); err != nil {
 		return fmt.Errorf("truncate: %s", err)
 	}
 

db.go

@@ -102,8 +102,11 @@ type DB struct {
 	statlock sync.RWMutex // Protects stats access.
 
 	ops struct {
-		writeAt func(b []byte, off int64) (n int, err error)
+		writeAt  func(b []byte, off int64) (n int, err error)
+		Truncate func(size int64) error
 	}
+
+	readOnly bool // Read only mode. Update()/Begin(true) would return ErrDatabaseReadOnly immediately.
 }
 
 // Path returns the path to currently open database file.
@@ -137,21 +140,37 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) {
 	db.MaxBatchSize = DefaultMaxBatchSize
 	db.MaxBatchDelay = DefaultMaxBatchDelay
 
+	// Get file stats.
+	s, err := os.Stat(path)
+	flag := os.O_RDWR
+	if err != nil && !os.IsNotExist(err) {
+		return nil, err
+	} else if err == nil && (s.Mode().Perm()&0222) == 0 {
+		// remove www from mode as well.
+		mode ^= (mode & 0222)
+		flag = os.O_RDONLY
+		db.readOnly = true
+		// Ignore truncations.
+		db.ops.Truncate = func(int64) error { return nil }
+	}
+
 	// Open data file and separate sync handler for metadata writes.
 	db.path = path
-
+	if db.file, err = os.OpenFile(db.path, flag|os.O_CREATE, mode); err != nil {
-	var err error
-	if db.file, err = os.OpenFile(db.path, os.O_RDWR|os.O_CREATE, mode); err != nil {
 		_ = db.close()
 		return nil, err
 	}
 
-	// Lock file so that other processes using Bolt cannot use the database
+	// No need to lock read-only file.
-	// at the same time. This would cause corruption since the two processes
+	if !db.readOnly {
-	// would write meta pages and free pages separately.
+		db.ops.Truncate = db.file.Truncate
-	if err := flock(db.file, options.Timeout); err != nil {
+		// Lock file so that other processes using Bolt cannot use the database
-		_ = db.close()
+		// at the same time. This would cause corruption since the two processes
-		return nil, err
+		// would write meta pages and free pages separately.
+		if err := flock(db.file, options.Timeout); err != nil {
+			_ = db.close()
+			return nil, err
+		}
 	}
 
 	// Default values for test hooks
@@ -359,8 +378,11 @@ func (db *DB) close() error {
 
 	// Close file handles.
 	if db.file != nil {
-		// Unlock the file.
+		// No need to unlock read-only file.
-		_ = funlock(db.file)
+		if !db.readOnly {
+			// Unlock the file.
+			_ = funlock(db.file)
+		}
 
 		// Close the file descriptor.
 		if err := db.file.Close(); err != nil {
@@ -426,6 +448,9 @@ func (db *DB) beginTx() (*Tx, error) {
 }
 
 func (db *DB) beginRWTx() (*Tx, error) {
+	if db.readOnly {
+		return nil, ErrDatabaseReadOnly
+	}
 	// Obtain writer lock. This is released by the transaction when it closes.
 	// This enforces only one writer transaction at a time.
 	db.rwlock.Lock()
@@ -622,6 +647,10 @@ func (db *DB) allocate(count int) (*page, error) {
 	return p, nil
 }
 
+func (db *DB) IsReadOnly() bool {
+	return db.readOnly
+}
+
 // Options represents the options that can be set when opening a database.
 type Options struct {
 	// Timeout is the amount of time to wait to obtain a file lock.

db_test.go

@@ -224,6 +224,61 @@ func TestDB_Open_FileTooSmall(t *testing.T) {
 	equals(t, errors.New("file size too small"), err)
 }
 
+// Ensure that a database can be opened in read-only mode.
+func TestOpen_ReadOnly(t *testing.T) {
+	var bucket = []byte(`bucket`)
+	var key = []byte(`key`)
+	var value = []byte(`value`)
+	path := tempfile()
+	defer os.Remove(path)
+	db, err := bolt.Open(path, 0666, nil)
+	ok(t, db.Update(func(tx *bolt.Tx) error {
+		b, err := tx.CreateBucket(bucket)
+		if err != nil {
+			return err
+		}
+		return b.Put(key, value)
+	}))
+	assert(t, db != nil, "")
+	assert(t, !db.IsReadOnly(), "")
+	ok(t, err)
+	ok(t, db.Close())
+	// Make it read-only.
+	ok(t, os.Chmod(path, 0444))
+	// Open again.
+	db0, err := bolt.Open(path, 0666, nil)
+	ok(t, err)
+	defer db0.Close()
+	// And again.
+	db1, err := bolt.Open(path, 0666, nil)
+	ok(t, err)
+	defer db1.Close()
+	for _, db := range []*bolt.DB{db0, db1} {
+		// Verify is is in read only mode indeed.
+		assert(t, db.IsReadOnly(), "")
+		assert(t,
+			bolt.ErrDatabaseReadOnly == db.Update(func(*bolt.Tx) error {
+				panic(`should never get here`)
+			}),
+			"")
+		_, err = db.Begin(true)
+		assert(t, bolt.ErrDatabaseReadOnly == err, "")
+		// Verify the data.
+		ok(t, db.View(func(tx *bolt.Tx) error {
+			b := tx.Bucket(bucket)
+			if b == nil {
+				return fmt.Errorf("expected bucket `%s`", string(bucket))
+			}
+			got := string(b.Get(key))
+			expected := string(value)
+			if got != expected {
+				return fmt.Errorf("expected `%s`, got `%s`", expected, got)
+			}
+			return nil
+		}))
+	}
+}
+
 // TODO(benbjohnson): Test corruption at every byte of the first two pages.
 
 // Ensure that a database cannot open a transaction when it's not open.

errors.go

@@ -36,6 +36,10 @@ var (
 	// ErrTxClosed is returned when committing or rolling back a transaction
 	// that has already been committed or rolled back.
 	ErrTxClosed = errors.New("tx closed")
+
+	// ErrDatabaseReadOnly is returned when a mutating transaction is started on a
+	// read-only database.
+	ErrDatabaseReadOnly = errors.New("database is in read-only mode")
 )
 
 // These errors can occur when putting or deleting a value or a bucket.