use a shared lock in read-only mode

boltdb/bolt#371 (comment)

bolt_unix.go

@@ -11,7 +11,7 @@ import (
 )
 
 // flock acquires an advisory lock on a file descriptor.
-func flock(f *os.File, timeout time.Duration) error {
+func flock(f *os.File, exclusive bool, timeout time.Duration) error {
 	var t time.Time
 	for {
 		// If we're beyond our timeout then return an error.
@@ -21,9 +21,13 @@ func flock(f *os.File, timeout time.Duration) error {
 		} else if timeout > 0 && time.Since(t) > timeout {
 			return ErrTimeout
 		}
+		flag := syscall.LOCK_SH
+		if exclusive {
+			flag = syscall.LOCK_EX
+		}
 
 		// Otherwise attempt to obtain an exclusive lock.
-		err := syscall.Flock(int(f.Fd()), syscall.LOCK_EX|syscall.LOCK_NB)
+		err := syscall.Flock(int(f.Fd()), flag|syscall.LOCK_NB)
 		if err == nil {
 			return nil
 		} else if err != syscall.EWOULDBLOCK {

bolt_windows.go

@@ -16,7 +16,7 @@ func fdatasync(db *DB) error {
 }
 
 // flock acquires an advisory lock on a file descriptor.
-func flock(f *os.File, _ time.Duration) error {
+func flock(f *os.File, _ bool, _ time.Duration) error {
 	return nil
 }
 

db.go

@@ -140,14 +140,8 @@ 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)
+	if options.ReadOnly {
 		flag = os.O_RDONLY
 		db.readOnly = true
 		// Ignore truncations.
@@ -156,21 +150,26 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) {
 
 	// Open data file and separate sync handler for metadata writes.
 	db.path = path
+	var err error
 	if db.file, err = os.OpenFile(db.path, flag|os.O_CREATE, mode); err != nil {
 		_ = db.close()
 		return nil, err
 	}
 
-	// No need to lock read-only file.
 	if !db.readOnly {
 		db.ops.Truncate = db.file.Truncate
-		// Lock file so that other processes using Bolt cannot use the database
-		// at the same time. This would cause corruption since the two processes
-		// would write meta pages and free pages separately.
-		if err := flock(db.file, options.Timeout); err != nil {
-			_ = db.close()
-			return nil, err
-		}
+	}
+
+	// Lock file so that other processes using Bolt in read-write mode cannot
+	// use the database  at the same time. This would cause corruption since
+	// the two processes would write meta pages and free pages separately.
+	// The database file is locked exclusively (only one process can grab the lock)
+	// if !options.ReadOnly.
+	// The database file is locked using the shared lock (more than one process may
+	// hold a lock at the same time) otherwise (options.ReadOnly is set).
+	if err := flock(db.file, !db.readOnly, options.Timeout); err != nil {
+		_ = db.close()
+		return nil, err
 	}
 
 	// Default values for test hooks
@@ -660,6 +659,10 @@ type Options struct {
 
 	// Sets the DB.NoGrowSync flag before memory mapping the file.
 	NoGrowSync bool
+
+	// Open database in read-only mode. Uses flock(..., LOCK_SH |LOCK_NB) to
+	// grab a shared lock (UNIX).
+	ReadOnly bool
 }
 
 // DefaultOptions represent the options used if nil options are passed into Open().

db_test.go

@@ -224,7 +224,9 @@ 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.
+// Ensure that a database can be opened in read-only mode by multiple processes
+// and that a database can not be opened in read-write mode and in read-only
+// mode at the same time.
 func TestOpen_ReadOnly(t *testing.T) {
 	var bucket = []byte(`bucket`)
 	var key = []byte(`key`)
@@ -243,14 +245,15 @@ func TestOpen_ReadOnly(t *testing.T) {
 	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)
+	// Open in read-only mode.
+	db0, err := bolt.Open(path, 0666, &bolt.Options{ReadOnly: true})
 	ok(t, err)
 	defer db0.Close()
-	// And again.
-	db1, err := bolt.Open(path, 0666, nil)
+	// Try opening in regular mode.
+	_, err = bolt.Open(path, 0666, &bolt.Options{Timeout: time.Millisecond * 100})
+	assert(t, err != nil, "")
+	// And again (in read-only mode).
+	db1, err := bolt.Open(path, 0666, &bolt.Options{ReadOnly: true})
 	ok(t, err)
 	defer db1.Close()
 	for _, db := range []*bolt.DB{db0, db1} {