feat(GODT-2829): New Sync Service

Implementation of the new sync service that interleaves syncing jobs for
all active users.

It also includes improvements to the message downloader. The download
will now auto rate limit the parallel workers based on the server
responses.

Additionally each of the stages is now tested in isolation to ensure the
behavior matches the expectations.

Finally, this patch does not replace the existing IMAP sync. A follow up
patch is necessary to integrate the IMAP bits into the interfaces
required by these changes.

internal/services/syncservice/download_cache.go

@@ -0,0 +1,115 @@
+// 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 syncservice
+
+import (
+	"sync"
+
+	"github.com/ProtonMail/go-proton-api"
+)
+
+type DownloadCache struct {
+	messageLock    sync.RWMutex
+	messages       map[string]proton.Message
+	attachmentLock sync.RWMutex
+	attachments    map[string][]byte
+}
+
+func newDownloadCache() *DownloadCache {
+	return &DownloadCache{
+		messages:    make(map[string]proton.Message, 64),
+		attachments: make(map[string][]byte, 64),
+	}
+}
+
+func (s *DownloadCache) StoreMessage(message proton.Message) {
+	s.messageLock.Lock()
+	defer s.messageLock.Unlock()
+
+	s.messages[message.ID] = message
+}
+
+func (s *DownloadCache) StoreAttachment(id string, data []byte) {
+	s.attachmentLock.Lock()
+	defer s.attachmentLock.Unlock()
+
+	s.attachments[id] = data
+}
+
+func (s *DownloadCache) DeleteMessages(id ...string) {
+	s.messageLock.Lock()
+	defer s.messageLock.Unlock()
+
+	for _, id := range id {
+		delete(s.messages, id)
+	}
+}
+
+func (s *DownloadCache) DeleteAttachments(id ...string) {
+	s.attachmentLock.Lock()
+	defer s.attachmentLock.Unlock()
+
+	for _, id := range id {
+		delete(s.attachments, id)
+	}
+}
+
+func (s *DownloadCache) GetMessage(id string) (proton.Message, bool) {
+	s.messageLock.RLock()
+	defer s.messageLock.RUnlock()
+
+	v, ok := s.messages[id]
+
+	return v, ok
+}
+
+func (s *DownloadCache) GetAttachment(id string) ([]byte, bool) {
+	s.attachmentLock.RLock()
+	defer s.attachmentLock.RUnlock()
+
+	v, ok := s.attachments[id]
+
+	return v, ok
+}
+
+func (s *DownloadCache) Clear() {
+	s.messageLock.Lock()
+	s.messages = make(map[string]proton.Message, 64)
+	s.messageLock.Unlock()
+
+	s.attachmentLock.Lock()
+	s.attachments = make(map[string][]byte, 64)
+	s.attachmentLock.Unlock()
+}
+
+func (s *DownloadCache) Count() (int, int) {
+	var (
+		messageCount    int
+		attachmentCount int
+	)
+
+	s.messageLock.Lock()
+	messageCount = len(s.messages)
+	s.messageLock.Unlock()
+
+	s.attachmentLock.Lock()
+	attachmentCount = len(s.attachments)
+	s.attachmentLock.Unlock()
+
+	return messageCount, attachmentCount
+}