Do not overwrite tab_state0 during multi-instance migration

chrome/android/java/src/org/chromium/chrome/browser/tabmodel/TabbedModeTabPersistencePolicy.java

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package org.chromium.chrome.browser.tabmodel;
 
 import android.content.Context;
 import android.content.SharedPreferences;
 import android.os.AsyncTask;
 import android.os.StrictMode;
 import android.support.annotation.WorkerThread;
 import android.util.Pair;
 import android.util.SparseBooleanArray;
 
 import org.chromium.base.Callback;
 import org.chromium.base.ContextUtils;
 import org.chromium.base.Log;
 import org.chromium.base.PathUtils;
 import org.chromium.base.StreamUtil;
 import org.chromium.base.ThreadUtils;
 import org.chromium.base.VisibleForTesting;
+import org.chromium.base.library_loader.LibraryLoader;
+import org.chromium.base.metrics.RecordHistogram;
 import org.chromium.chrome.browser.TabState;
 import org.chromium.chrome.browser.compositor.layouts.content.TabContentManager;
 
 import java.io.BufferedInputStream;
 import java.io.DataInputStream;
 import java.io.File;
 import java.io.FileInputStream;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.concurrent.ExecutionException;
@@ skipping 177 matching lines @@
         }
         setLegacyFileMigrationPref();
     }
 
     /**
      * Upgrades users from an older version of Chrome when the state files for multi-instance
      * were each kept in separate subdirectories.
      */
     @WorkerThread
     private void performMultiInstanceMigration() {
-        // 1. Rename tab metadata file for tab directory "0".
+        // 0. Do not rename the old metadata file if the new metadata file already exists. This
+        //    should not happen, but if it does and the metadata file is overwritten then users
+        //    may lose tabs. See crbug.com/649384.
         File stateDir = getOrCreateStateDirectory();
-        File metadataFile = new File(stateDir, LEGACY_SAVED_STATE_FILE);
-        if (metadataFile.exists()) {
-            if (!metadataFile.renameTo(new File(stateDir, getStateFileName()))) {
-                Log.e(TAG, "Failed to rename file: " + metadataFile);
+        File newMetadataFile = new File(stateDir, getStateFileName());
+        File oldMetadataFile = new File(stateDir, LEGACY_SAVED_STATE_FILE);
+        if (newMetadataFile.exists()) {
+            Log.e(TAG, "New metadata file already exists");
+            if (LibraryLoader.isInitialized()) {
+                RecordHistogram.recordBooleanHistogram(
+                        "Android.MultiInstanceMigration.NewMetadataFileExists", true);
+            }
+        } else if (oldMetadataFile.exists()) {
+            // 1. Rename tab metadata file for tab directory "0".
+            if (!oldMetadataFile.renameTo(newMetadataFile)) {
+                Log.e(TAG, "Failed to rename file: " + oldMetadataFile);
+
+                if (LibraryLoader.isInitialized()) {
+                    RecordHistogram.recordBooleanHistogram(
+                            "Android.MultiInstanceMigration.FailedToRenameMetadataFile", true);
+                }
             }
         }
 
         // 2. Move files from other state directories.
         for (int i = TabModelSelectorImpl.CUSTOM_TABS_SELECTOR_INDEX;
                 i < TabWindowManager.MAX_SIMULTANEOUS_SELECTORS; i++) {
             // Skip the directory we're migrating to.
             if (i == 0) continue;
 
             File otherStateDir = new File(
                     TabPersistentStore.getOrCreateBaseStateDirectory(), Integer.toString(i));
             if (otherStateDir == null || !otherStateDir.exists()) continue;
 
             // Rename tab state file.
-            metadataFile = new File(otherStateDir, LEGACY_SAVED_STATE_FILE);
-            if (metadataFile.exists()) {
-                if (!metadataFile.renameTo(new File(stateDir, getStateFileName(i)))) {
-                    Log.e(TAG, "Failed to rename file: " + metadataFile);
+            oldMetadataFile = new File(otherStateDir, LEGACY_SAVED_STATE_FILE);
+            if (oldMetadataFile.exists()) {
+                if (!oldMetadataFile.renameTo(new File(stateDir, getStateFileName(i)))) {
+                    Log.e(TAG, "Failed to rename file: " + oldMetadataFile);
                 }
             }
 
             // Rename tab files.
             File[] files = otherStateDir.listFiles();
             if (files != null) {
                 for (File file : files) {
                     if (TabState.parseInfoFromFilename(file.getName()) != null) {
                         // Custom tabs does not currently use tab files. Delete them rather than
                         // migrating.
@@ skipping 173 matching lines @@
                     } catch (Exception e) {
                         Log.e(TAG, "Unable to read state for " + metadataFile.getName() + ": " + e);
                     } finally {
                         StreamUtil.closeQuietly(stream);
                     }
                 }
             }
         }
     }
 }