Other: Fix user sync leaks/race conditions

This fixes various race conditions and leaks related to the user's sync
and API event stream. It was possible for a sync/stream to begin after a
user was already closed; this change prevents that by managing the
goroutines related to sync/stream within cancellable groups.

internal/async/context.go

@@ -0,0 +1,53 @@
+// Copyright (c) 2022 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 async
+
+import (
+	"context"
+	"sync"
+)
+
+// Abortable collects groups of functions that can be aborted by calling Abort.
+type Abortable struct {
+	abortFunc []context.CancelFunc
+	abortLock sync.RWMutex
+}
+
+func (a *Abortable) Do(ctx context.Context, fn func(context.Context)) {
+	fn(a.newCancelCtx(ctx))
+}
+
+func (a *Abortable) Abort() {
+	a.abortLock.RLock()
+	defer a.abortLock.RUnlock()
+
+	for _, fn := range a.abortFunc {
+		fn()
+	}
+}
+
+func (a *Abortable) newCancelCtx(ctx context.Context) context.Context {
+	a.abortLock.Lock()
+	defer a.abortLock.Unlock()
+
+	ctx, cancel := context.WithCancel(ctx)
+
+	a.abortFunc = append(a.abortFunc, cancel)
+
+	return ctx
+}