From edac2419f93a0cbca8e0b0995877b0e9bc2b16d5 Mon Sep 17 00:00:00 2001
From: Leander Beernaert <leander.beernaert@proton.ch>
Date: Fri, 5 May 2023 11:23:17 +0200
Subject: [PATCH] feat(GODT-2585): Add CPC utility

Add Channel based RPC utilities.
---
 cpc/cpc.go      | 138 ++++++++++++++++++++++++++++++++++++++++++++++++
 cpc/cpc_test.go |  63 ++++++++++++++++++++++
 2 files changed, 201 insertions(+)
 create mode 100644 cpc/cpc.go
 create mode 100644 cpc/cpc_test.go

diff --git a/cpc/cpc.go b/cpc/cpc.go
new file mode 100644
index 00000000..31185382
--- /dev/null
+++ b/cpc/cpc.go
@@ -0,0 +1,138 @@
+// 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 ErrRequestHasNoReply = errors.New("request has no reply channel")
+var ErrExpectedReply = errors.New("request does not have reply channel")
+
+// Utilities to implement Chanel Procedure Calls. Similar in concept to RPC, but with between go-routines.
+
+type RequestReply struct {
+	Value any
+	Error error
+}
+
+type Request struct {
+	Value any
+	Reply chan RequestReply
+}
+
+func NewRequest(value any) *Request {
+	return &Request{
+		Value: value,
+		Reply: make(chan RequestReply),
+	}
+}
+
+func NewRequestWithoutReply(value any) *Request {
+	return &Request{
+		Value: value,
+		Reply: nil,
+	}
+}
+
+func (r *Request) Close() {
+	if r.Reply != nil {
+		panic("request reply has not been sent")
+	}
+}
+
+func (r *Request) SendReply(ctx context.Context, value any, err error) {
+	if r.Reply == nil {
+		panic("request has no reply")
+	}
+
+	defer func() {
+		close(r.Reply)
+		r.Reply = nil
+	}()
+
+	select {
+	case <-ctx.Done():
+	case r.Reply <- RequestReply{
+		Value: value,
+		Error: err,
+	}:
+	}
+}
+
+type CPC struct {
+	request chan *Request
+}
+
+func NewCPC() *CPC {
+	return &CPC{
+		request: make(chan *Request),
+	}
+}
+
+// Receive is meant to be called by the code that is supposed to handle the requests that arrive.
+func (c *CPC) Receive(ctx context.Context, f func(context.Context, *Request)) {
+	for request := range c.request {
+		f(ctx, request)
+		request.Close()
+	}
+}
+
+func (c *CPC) Close() {
+	close(c.request)
+}
+
+// SendNoReply sends a request which doesn't expect a reply.
+func (c *CPC) SendNoReply(ctx context.Context, value any) error {
+	return c.executeNoReplyImpl(ctx, NewRequestWithoutReply(value))
+}
+
+// SendWithReply sends a request which expects a reply.
+func (c *CPC) SendWithReply(ctx context.Context, value any) (any, error) {
+	return c.executeReplyImpl(ctx, NewRequest(value))
+}
+
+func (c *CPC) executeNoReplyImpl(ctx context.Context, request *Request) error {
+	select {
+	case <-ctx.Done():
+		return ctx.Err()
+	case c.request <- request:
+	}
+
+	return nil
+}
+
+func (c *CPC) executeReplyImpl(ctx context.Context, request *Request) (any, error) {
+	if request.Reply == nil {
+		return nil, ErrExpectedReply
+	}
+
+	select {
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	case c.request <- request:
+	}
+
+	select {
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	case reply := <-request.Reply:
+		return reply.Value, reply.Error
+	}
+}
diff --git a/cpc/cpc_test.go b/cpc/cpc_test.go
new file mode 100644
index 00000000..8fc70406
--- /dev/null
+++ b/cpc/cpc_test.go
@@ -0,0 +1,63 @@
+// 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.SendReply(ctx, replyValue, nil)
+			case quitRequest:
+				cpc.Close()
+			default:
+				panic("unknown request")
+			}
+		})
+	}()
+
+	r, err := cpc.SendWithReply(context.Background(), sendIntRequest{})
+	require.NoError(t, err)
+	require.Equal(t, r, replyValue)
+
+	require.NoError(t, cpc.SendNoReply(context.Background(), quitRequest{}))
+
+	wg.Wait()
+}