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.go

@@ -0,0 +1,129 @@
+// 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"
+	"errors"
+)
+
+var ErrInvalidReplyType = errors.New("reply type does not match")
+
+// Utilities to implement Chanel Procedure Calls. Similar in concept to RPC, but with between go-routines.
+
+// Request contains the data for a request as well as the means to reply to a request.
+type Request struct {
+	value any
+	reply chan reply
+}
+
+// Value returns the request value.
+func (r *Request) Value() any {
+	return r.value
+}
+
+// Reply should be used to send a reply to a given request.
+func (r *Request) Reply(ctx context.Context, value any, err error) {
+	defer close(r.reply)
+
+	select {
+	case <-ctx.Done():
+	case r.reply <- reply{
+		value: value,
+		error: err,
+	}:
+	}
+}
+
+// CPC Channel Procedure Call. A play on RPC, but with channels. Use this type to send requests and wait for replies
+// from a goroutine.
+type CPC struct {
+	request chan *Request
+}
+
+func NewCPC() *CPC {
+	return &CPC{
+		request: make(chan *Request),
+	}
+}
+
+// Receive invokes the function on all the request that arrive.
+func (c *CPC) Receive(ctx context.Context, f func(context.Context, *Request)) {
+	for request := range c.request {
+		f(ctx, request)
+	}
+}
+
+// ReceiveCh returns the channel on which all requests are sent.
+func (c *CPC) ReceiveCh() <-chan *Request {
+	return c.request
+}
+
+// Close closes the CPC channel and no further requests should be made.
+func (c *CPC) Close() {
+	close(c.request)
+}
+
+// Send sends a request which expects a reply.
+func (c *CPC) Send(ctx context.Context, value any) (any, error) {
+	return c.execute(ctx, newRequest(value))
+}
+
+// SendTyped is similar to CPC.Send, but ensure that reply is of the given Type T.
+func SendTyped[T any](ctx context.Context, c *CPC, value any) (T, error) {
+	val, err := c.execute(ctx, newRequest(value))
+	if err != nil {
+		var t T
+		return t, err
+	}
+
+	switch vt := val.(type) {
+	case T:
+		return vt, nil
+	default:
+		var t T
+		return t, ErrInvalidReplyType
+	}
+}
+
+type reply struct {
+	value any
+	error error
+}
+
+func (c *CPC) execute(ctx context.Context, request *Request) (any, error) {
+	select {
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	case c.request <- request:
+	}
+
+	select {
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	case r := <-request.reply:
+		return r.value, r.error
+	}
+}
+
+func newRequest(value any) *Request {
+	return &Request{
+		value: value,
+		reply: make(chan reply),
+	}
+}

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()
+}