chore: Improve CPC code

* Remove distinction between values with and without reply.
* Hide types that don't need to be public.
* Don't allow direct access to the request's internal types.

pkg/cpc/cpc_test.go

@@ -0,0 +1,65 @@
+// Copyright (c) 2023 Proton AG
+//
+// This file is part of Proton Mail 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 cpc
+
+import (
+	"context"
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+type sendIntRequest struct{}
+
+type quitRequest struct{}
+
+func TestCPC_Receive(t *testing.T) {
+	const replyValue = 20
+
+	cpc := NewCPC()
+
+	wg := sync.WaitGroup{}
+
+	go func() {
+		defer wg.Done()
+
+		wg.Add(1)
+
+		cpc.Receive(context.Background(), func(ctx context.Context, request *Request) {
+			switch request.Value().(type) {
+			case sendIntRequest:
+				request.Reply(ctx, replyValue, nil)
+			case quitRequest:
+				request.Reply(ctx, nil, nil)
+			default:
+				panic("unknown request")
+			}
+		})
+	}()
+
+	r, err := cpc.Send(context.Background(), sendIntRequest{})
+	require.NoError(t, err)
+	require.Equal(t, r, replyValue)
+
+	_, err = cpc.Send(context.Background(), quitRequest{})
+	require.NoError(t, err)
+
+	cpc.Close()
+	wg.Wait()
+}