feat(GODT-2803): Add Event Poll Waiter

Wait for the current event poll to finish publishing events after a
request to pause the event loop. This is required to change the gluon
cache directory.

internal/services/userevents/event_poll_waiter.go

@@ -0,0 +1,58 @@
+// 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"
+	"time"
+)
+
+// EventPollWaiter is meant to be used to wait for the event loop to finish processing the current events after
+// being paused.
+type EventPollWaiter struct {
+	ch chan struct{}
+}
+
+func newEventPollWaiter() *EventPollWaiter {
+	return &EventPollWaiter{ch: make(chan struct{})}
+}
+
+func (e *EventPollWaiter) WaitPollFinished(ctx context.Context) error {
+	select {
+	case <-ctx.Done():
+		return ctx.Err()
+	case <-e.ch:
+		return nil
+	}
+}
+
+func (e *EventPollWaiter) WaitPollFinishedWithDeadline(ctx context.Context, t time.Time) error {
+	ctx, cancel := context.WithDeadline(ctx, t)
+	defer cancel()
+
+	select {
+	case <-ctx.Done():
+		return ctx.Err()
+	case <-e.ch:
+		return nil
+	}
+}
+
+func (e *EventPollWaiter) close() {
+	close(e.ch)
+}