[#316] locode/boltdb: Add option to enable read-only mode

Add ReadOnly function that returns Option that enables read-only mode in DB.
RO mode can be used by processes that won't modify the DB in order to not
acquire write flock.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/util/locode/db/boltdb/calls.go

@@ -13,6 +13,8 @@ import (
 // Open opens underlying BoltDB instance.
 //
 // Timeout of BoltDB opening is 3s (only for Linux or Darwin).
+//
+// Opens BoltDB in read-only mode if DB is read-only.
 func (db *DB) Open() error {
 	// copy-paste from metabase:
 	// consider universal Open/Close for BoltDB wrappers
@@ -98,6 +100,7 @@ func recordFromValue(data []byte) (*locodedb.Record, error) {
 // The records are stored in internal binary JSON format.
 //
 // Must not be called before successful Open call.
+// Must not be called in read-only mode: behavior is undefined.
 func (db *DB) Put(key locodedb.Key, rec locodedb.Record) error {
 	return db.bolt.Update(func(tx *bbolt.Tx) error {
 		countryKey, err := countryBucketKey(key.CountryCode())

pkg/util/locode/db/boltdb/opts.go

@@ -24,3 +24,13 @@ func defaultOpts() *options {
 		},
 	}
 }
+
+// ReadOnly enables read-only mode of the DB.
+//
+// Do not call DB.Put method on instances with
+// this option: the behavior is undefined.
+func ReadOnly() Option {
+	return func(o *options) {
+		o.boltOpts.ReadOnly = true
+	}
+}