From 56ce0fa5c1777ef0a3aec6d9b3495d4bdfe3b29d Mon Sep 17 00:00:00 2001
From: Riddle Hsu <riddlehsu@google.com>
Date: Tue, 15 Mar 2022 15:46:31 +0800
Subject: [PATCH] Preserve task when clearing top activities

There is no guarantee that clear-top won't finish all activities
in the task, such as the last activity is declared as default
launch mode and the intent flags don't have SINGLE_TOP, then it
will also be finished.

Otherwise when adding the new activity into the detached task,
it will cause NPE in onChildPositionChanged because its display
content is null.

Fixes: 224462770
Test: TaskTests#testPerformClearTop
Change-Id: Icfa0390a5881043bc55486347aa2854db100d171
---
 services/core/java/com/android/server/wm/Task.java |  4 ++++
 .../src/com/android/server/wm/TaskTests.java       | 14 ++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/services/core/java/com/android/server/wm/Task.java b/services/core/java/com/android/server/wm/Task.java
index cd7ebe327c919..00f50ec4975ad 100644
--- a/services/core/java/com/android/server/wm/Task.java
+++ b/services/core/java/com/android/server/wm/Task.java
@@ -1623,12 +1623,16 @@ class Task extends TaskFragment {
     }
 
     ActivityRecord performClearTop(ActivityRecord newR, int launchFlags) {
+        // The task should be preserved for putting new activity in case the last activity is
+        // finished if it is normal launch mode and not single top ("clear-task-top").
+        mReuseTask = true;
         mTaskSupervisor.beginDeferResume();
         final ActivityRecord result;
         try {
             result = clearTopActivities(newR, launchFlags);
         } finally {
             mTaskSupervisor.endDeferResume();
+            mReuseTask = false;
         }
         return result;
     }
diff --git a/services/tests/wmtests/src/com/android/server/wm/TaskTests.java b/services/tests/wmtests/src/com/android/server/wm/TaskTests.java
index 636c6bc77e0a2..9304761fc1c96 100644
--- a/services/tests/wmtests/src/com/android/server/wm/TaskTests.java
+++ b/services/tests/wmtests/src/com/android/server/wm/TaskTests.java
@@ -258,6 +258,20 @@ public class TaskTests extends WindowTestsBase {
         assertNull(task1.isInTask(activity2));
     }
 
+    @Test
+    public void testPerformClearTop() {
+        final Task task = createTask(mDisplayContent);
+        final ActivityRecord activity1 = new ActivityBuilder(mAtm).setTask(task).build();
+        final ActivityRecord activity2 = new ActivityBuilder(mAtm).setTask(task).build();
+        // Detach from process so the activities can be removed from hierarchy when finishing.
+        activity1.detachFromProcess();
+        activity2.detachFromProcess();
+        assertNull(task.performClearTop(activity1, 0 /* launchFlags */));
+        assertFalse(task.hasChild());
+        // In real case, the task should be preserved for adding new activity.
+        assertTrue(task.isAttached());
+    }
+
     @Test
     public void testRemoveChildForOverlayTask() {
         final Task task = createTask(mDisplayContent);