GODT-1917: gRPC service should use random port.

WIP: bridge-gui wait and parse gRPC service config fie.

internal/frontend/grpc/config.go

@@ -0,0 +1,65 @@
+// Copyright (c) 2022 Proton AG
+//
+// This file is part of Proton Mail Bridge.Bridge.
+//
+// Proton Mail Bridge is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Proton Mail Bridge is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Proton Mail Bridge. If not, see <https://www.gnu.org/licenses/>.
+
+package grpc
+
+import (
+	"encoding/json"
+	"os"
+)
+
+// config is a structure containing the service configuration data that are exchanged by the gRPC server and client.
+type config struct {
+	Port  int    `json:"port"`
+	Cert  string `json:"cert"`  // coming soon
+	Token string `json:"token"` // coming soon
+}
+
+// save saves a gRPC service configuration to file.
+func (s *config) save(path string) error {
+	// Another process may be waiting for this file to be available. In order to prevent this process to open
+	// the file while we are writing in it, we write it with a temp file name, then rename it.
+	tempPath := path + "_"
+	if err := s._save(tempPath); err != nil {
+		return err
+	}
+
+	return os.Rename(tempPath, path)
+}
+
+func (s *config) _save(path string) error {
+	f, err := os.Create(path) //nolint:errcheck,gosec
+	if err != nil {
+		return err
+	}
+
+	defer func() { _ = f.Close() }()
+
+	return json.NewEncoder(f).Encode(s)
+}
+
+// load loads a gRPC service configuration from file.
+func (s *config) load(path string) error {
+	f, err := os.Open(path) //nolint:errcheck,gosec
+	if err != nil {
+		return err
+	}
+
+	defer func() { _ = f.Close() }()
+
+	return json.NewDecoder(f).Decode(s)
+}