From 18005c149684642b5fbc0979201e31a1708eecd8 Mon Sep 17 00:00:00 2001
From: Makoto Onuki <omakoto@google.com>
Date: Thu, 20 Jan 2022 07:54:21 -0800
Subject: [PATCH] Add a check to detect "duplicate app instances" bug

Test: Boot
Test: Added debug code to induce the WTF situation, booted and checked the log
Bug: 185177290

Change-Id: Ie95164f2794b461115f07d02c2363fbdc9a517b4
---
 core/java/android/app/Application.java | 50 ++++++++++++++++++++++++++
 core/java/android/app/StackTrace.java  |  4 +++
 2 files changed, 54 insertions(+)

diff --git a/core/java/android/app/Application.java b/core/java/android/app/Application.java
index 7f849ef00d7d9..9eb3e8fb0160c 100644
--- a/core/java/android/app/Application.java
+++ b/core/java/android/app/Application.java
@@ -29,9 +29,13 @@ import android.content.Intent;
 import android.content.res.Configuration;
 import android.os.Build;
 import android.os.Bundle;
+import android.util.ArrayMap;
 import android.util.Log;
+import android.util.Slog;
 import android.view.autofill.AutofillManager;
 
+import com.android.internal.annotations.GuardedBy;
+
 import java.util.ArrayList;
 
 /**
@@ -53,6 +57,10 @@ import java.util.ArrayList;
  */
 public class Application extends ContextWrapper implements ComponentCallbacks2 {
     private static final String TAG = "Application";
+
+    /** Whether to enable the check to detect "duplicate application instances". */
+    private static final boolean DEBUG_DUP_APP_INSTANCES = true;
+
     @UnsupportedAppUsage
     private ArrayList<ActivityLifecycleCallbacks> mActivityLifecycleCallbacks =
             new ArrayList<ActivityLifecycleCallbacks>();
@@ -66,6 +74,13 @@ public class Application extends ContextWrapper implements ComponentCallbacks2 {
     @UnsupportedAppUsage
     public LoadedApk mLoadedApk;
 
+    @GuardedBy("sInstances")
+    private static final ArrayMap<Class<?>, Application> sInstances =
+            DEBUG_DUP_APP_INSTANCES ? new ArrayMap<>(1) : null;
+
+    // Only set when DEBUG_DUP_APP_INSTANCES is true.
+    private StackTrace mConstructorStackTrace;
+
     public interface ActivityLifecycleCallbacks {
 
         /**
@@ -231,6 +246,41 @@ public class Application extends ContextWrapper implements ComponentCallbacks2 {
 
     public Application() {
         super(null);
+        if (DEBUG_DUP_APP_INSTANCES) {
+            checkDuplicateInstances();
+        }
+    }
+
+    private void checkDuplicateInstances() {
+        final Class<?> myClass = this.getClass();
+
+        // We only activate this check for custom application classes.
+        // Otherwise, it'd misfire if multiple apps share the same process, if all of them use
+        // the same Application class (on the same classloader).
+        if (myClass == Application.class) {
+            return;
+        }
+        synchronized (sInstances) {
+            final Application firstInstance = sInstances.get(myClass);
+            if (firstInstance == null) {
+                this.mConstructorStackTrace = new StackTrace("First ctor was called here");
+                sInstances.put(myClass, this);
+                return;
+            }
+            final StackTrace currentStackTrace = new StackTrace("Current ctor was called here",
+                    firstInstance.mConstructorStackTrace);
+            this.mConstructorStackTrace = currentStackTrace;
+            Slog.wtf(TAG, "Application ctor called twice for " + myClass
+                    + " first LoadedApk=" + firstInstance.getLoadedApkInfo(),
+                    currentStackTrace);
+        }
+    }
+
+    private String getLoadedApkInfo() {
+        if (mLoadedApk == null) {
+            return "null";
+        }
+        return mLoadedApk + "/pkg=" + mLoadedApk.mPackageName;
     }
 
     /**
diff --git a/core/java/android/app/StackTrace.java b/core/java/android/app/StackTrace.java
index ec058f88118bc..6a77abdefea9b 100644
--- a/core/java/android/app/StackTrace.java
+++ b/core/java/android/app/StackTrace.java
@@ -24,4 +24,8 @@ public class StackTrace extends Exception {
     public StackTrace(String message) {
         super(message);
     }
+
+    public StackTrace(String message, Throwable innerStackTrace) {
+        super(message, innerStackTrace);
+    }
 }