chore(GODT-2848): Simplify User Event Service

Rather than having the services subscribe to each individual event type,
have only one subscriber for the whole event object. This spreads the
logic to the service through the `EventHandler` type.

This is important for the next stage where the IMAP service will be
required to apply events from the past.

internal/services/userevents/subscription.go

@@ -0,0 +1,106 @@
+// 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 userevents
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/ProtonMail/go-proton-api"
+)
+
+type Subscription = EventSubscriber
+
+func NewEventSubscriber(name string) *EventChanneledSubscriber {
+	return newSubscriber(name)
+}
+
+type EventHandler struct {
+	RefreshHandler   RefreshEventHandler
+	AddressHandler   AddressEventHandler
+	UserHandler      UserEventHandler
+	LabelHandler     LabelEventHandler
+	MessageHandler   MessageEventHandler
+	UsedSpaceHandler UserUsedSpaceEventHandler
+}
+
+func (e EventHandler) OnEvent(ctx context.Context, event proton.Event) error {
+	if event.Refresh&proton.RefreshMail != 0 && e.RefreshHandler != nil {
+		return e.RefreshHandler.HandleRefreshEvent(ctx, event.Refresh)
+	}
+
+	// Start with user events.
+	if event.User != nil && e.UserHandler != nil {
+		if err := e.UserHandler.HandleUserEvent(ctx, event.User); err != nil {
+			return fmt.Errorf("failed to apply user event: %w", err)
+		}
+	}
+
+	// Next Address events
+	if len(event.Addresses) != 0 && e.AddressHandler != nil {
+		if err := e.AddressHandler.HandleAddressEvents(ctx, event.Addresses); err != nil {
+			return fmt.Errorf("failed to apply address events: %w", err)
+		}
+	}
+
+	// Next label events
+	if len(event.Labels) != 0 && e.LabelHandler != nil {
+		if err := e.LabelHandler.HandleLabelEvents(ctx, event.Labels); err != nil {
+			return fmt.Errorf("failed to apply label events: %w", err)
+		}
+	}
+
+	// Next message events
+	if len(event.Messages) != 0 && e.MessageHandler != nil {
+		if err := e.MessageHandler.HandleMessageEvents(ctx, event.Messages); err != nil {
+			return fmt.Errorf("failed to apply message events: %w", err)
+		}
+	}
+
+	// Finally user used space events
+	if event.UsedSpace != nil && e.UsedSpaceHandler != nil {
+		if err := e.UsedSpaceHandler.HandleUsedSpaceEvent(ctx, *event.UsedSpace); err != nil {
+			return fmt.Errorf("failed to apply message events: %w", err)
+		}
+	}
+
+	return nil
+}
+
+type RefreshEventHandler interface {
+	HandleRefreshEvent(ctx context.Context, flag proton.RefreshFlag) error
+}
+type UserEventHandler interface {
+	HandleUserEvent(ctx context.Context, user *proton.User) error
+}
+
+type UserUsedSpaceEventHandler interface {
+	HandleUsedSpaceEvent(ctx context.Context, newSpace int) error
+}
+
+type AddressEventHandler interface {
+	HandleAddressEvents(ctx context.Context, events []proton.AddressEvent) error
+}
+
+type LabelEventHandler interface {
+	HandleLabelEvents(ctx context.Context, events []proton.LabelEvent) error
+}
+
+type MessageEventHandler interface {
+	HandleMessageEvents(ctx context.Context, events []proton.MessageEvent) error
+}