MM-46402: Use pointer to pointer to set pointer to nil (#20827)

Summary
We need a pointer to a pointer to set the original pointer to nil
The original task was not being set to nil (the local variable containing the pointer was being set to nil). The cancel function was being called even though the task had already been cancelled. This repeated cancellation was causing a panic because we were trying to close a channel that had already been closed the first time the task was cancelled.

Ticket Link
https://mattermost.atlassian.net/browse/MM-46402

app/server.go

@@ -2060,12 +2060,12 @@ func withMut(mut *sync.Mutex, f func()) {
 	f()
 }
 
-func cancelTask(mut *sync.Mutex, task *model.ScheduledTask) {
+func cancelTask(mut *sync.Mutex, taskPointer **model.ScheduledTask) {
 	mut.Lock()
 	defer mut.Unlock()
-	if task != nil {
-		task.Cancel()
-		task = nil
+	if *taskPointer != nil {
+		(*taskPointer).Cancel()
+		*taskPointer = nil
 	}
 }
 
@@ -2082,7 +2082,7 @@ func runDNDStatusExpireJob(a *App) {
 				a.ch.dndTask = model.CreateRecurringTaskFromNextIntervalTime("Unset DND Statuses", a.UpdateDNDStatusOfUsers, 5*time.Minute)
 			})
 		} else {
-			cancelTask(&a.ch.dndTaskMut, a.ch.dndTask)
+			cancelTask(&a.ch.dndTaskMut, &a.ch.dndTask)
 		}
 	})
 }
@@ -2100,7 +2100,7 @@ func runPostReminderJob(a *App) {
 				a.ch.postReminderTask = model.CreateRecurringTaskFromNextIntervalTime("Check Post reminders", a.CheckPostReminders, 5*time.Minute)
 			})
 		} else {
-			cancelTask(&a.ch.postReminderMut, a.ch.postReminderTask)
+			cancelTask(&a.ch.postReminderMut, &a.ch.postReminderTask)
 		}
 	})
 }

app/server_test.go

@@ -16,6 +16,7 @@ import (
 	"path"
 	"strconv"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 
@@ -565,3 +566,12 @@ func TestSentry(t *testing.T) {
 		}
 	})
 }
+
+func TestCancelTaskSetsTaskToNil(t *testing.T) {
+	var taskMut sync.Mutex
+	task := model.CreateRecurringTaskFromNextIntervalTime("a test task", func() {}, 5*time.Minute)
+	require.NotNil(t, task)
+	cancelTask(&taskMut, &task)
+	require.Nil(t, task)
+	require.NotPanics(t, func() { cancelTask(&taskMut, &task) })
+}