Fix race condition in ReloadConfig

Calling ReloadConfig was not thread safe in spite of mutex.  One thread
could overwrite the package level cfg pointer after it was mutated by
another thread.

config/config.go

@@ -45,8 +45,6 @@ var (
 		Help:      "Timestamp of the last successful configuration reload.",
 	})
 
-	cfg *ini.File
-
 	opts = ini.LoadOptions{
 		// Do not error on nonexistent file to allow empty string as filename input
 		Loose: true,
@@ -97,12 +95,13 @@ func (ch *MySqlConfigHandler) ReloadConfig(filename string, mysqldAddress string
 		}
 	}()
 
-	if cfg, err = ini.LoadSources(
+	cfg, err := ini.LoadSources(
 		opts,
 		[]byte("[client]\npassword = ${MYSQLD_EXPORTER_PASSWORD}\n"),
 		filename,
-	); err != nil {
-		return fmt.Errorf("failed to load %s: %w", filename, err)
+	)
+	if err != nil {
+		return fmt.Errorf("failed to load config from %s: %w", filename, err)
 	}
 
 	if host, port, err = net.SplitHostPort(mysqldAddress); err != nil {
@@ -135,12 +134,6 @@ func (ch *MySqlConfigHandler) ReloadConfig(filename string, mysqldAddress string
 			TlsInsecureSkipVerify: tlsInsecureSkipVerify,
 		}
 
-		// FIXME: this error check seems orphaned
-		if err != nil {
-			level.Error(logger).Log("msg", "failed to load config", "section", sectionName, "err", err)
-			continue
-		}
-
 		err = sec.StrictMapTo(mysqlcfg)
 		if err != nil {
 			level.Error(logger).Log("msg", "failed to parse config", "section", sectionName, "err", err)