Add advisory file locking.

This commit adds advisory locking via flock() to the database file. This ensures that two separate
processes cannot both open the same data file which would cause corruption.

Fixes #110.

db.go

@@ -79,6 +79,14 @@ func Open(path string, mode os.FileMode) (*DB, error) {
 		return nil, err
 	}
 
+	// 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 := syscall.Flock(int(db.file.Fd()), syscall.LOCK_EX); err != nil {
+		_ = db.close()
+		return nil, err
+	}
+
 	// Default values for test hooks
 	db.ops.writeAt = db.file.WriteAt
 
@@ -267,6 +275,10 @@ func (db *DB) close() error {
 
 	// Close file handles.
 	if db.file != nil {
+		// Unlock the file.
+		_ = syscall.Flock(int(db.file.Fd()), syscall.LOCK_UN)
+
+		// Close the file descriptor.
 		if err := db.file.Close(); err != nil {
 			return fmt.Errorf("db file close: %s", err)
 		}