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//调用installd执行/system/etc/updatecmds下命令脚本
if (DEBUG_UPGRADE) Log.v(TAG, "Running installd update commands");
          mInstaller.moveFiles();

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// Prune any system packages that no longer exist.
           final List<String> possiblyDeletedUpdatedSystemApps = new ArrayList<String>();
           if (!mOnlyCore) {
               Iterator<PackageSetting> psit = mSettings.mPackages.values().iterator();
               while (psit.hasNext()) {
                   PackageSetting ps = psit.next();

                   /*
 * If this is not a system app, it can't be a
 * disable system app.
 */
                   if ((ps.pkgFlags & ApplicationInfo.FLAG_SYSTEM) == 0) {
                       continue;//忽略普通应用
                   }

                   /*
 * If the package is scanned, it's not erased.
 */
                   final PackageParser.Package scannedPkg = mPackages.get(ps.name);
                   if (scannedPkg != null) {
                       /*
 * If the system app is both scanned and in the
 * disabled packages list, then it must have been
 * added via OTA. Remove it from the currently
 * scanned package so the previously user-installed
 * application can be scanned.
 */
                        // packages.xml中<updated-package>修饰的package会被记录到mSettings中的disable列表中去
                        // 这说明扫描的系统app是带有升级包的
                       if (mSettings.isDisabledSystemPackageLPr(ps.name)) {
                           logCriticalInfo(Log.WARN, "Expecting better updated system app for "
                                   + ps.name + "; removing system app. Last known codePath="
                                   + ps.codePathString + ", installStatus=" + ps.installStatus
                                   + ", versionCode=" + ps.versionCode + "; scanned versionCode="
                                   + scannedPkg.mVersionCode);
                           removePackageLI(ps, true);//将其从mPackages中移除
                           mExpectingBetter.put(ps.name, ps.codePath);// 将其添加到mExpectingBetter，后续处理
                       }

                       continue;
                   }

                  // 运行到这里说明packages.xml中记录的app，这次没有扫描到。
                   if (!mSettings.isDisabledSystemPackageLPr(ps.name)) {
                     // 如果这个app在packages.xml也不属于<updated-package>
                     // 意味着这个应用是残留在packages.xml中的，可能还会剩下沙箱数据，因此也要删掉
                       psit.remove();
                       logCriticalInfo(Log.WARN, "System package " + ps.name
                               + " no longer exists; wiping its data");
                       removeDataDirsLI(null, ps.name);
                   } else {
                     // 如果这个app在packages.xml属于<updated-package>
                     // 将其加入possiblyDeletedUpdatedSystemApps
                       final PackageSetting disabledPs = mSettings.getDisabledSystemPkgLPr(ps.name);
                       if (disabledPs.codePath == null || !disabledPs.codePath.exists()) {
                           possiblyDeletedUpdatedSystemApps.add(ps.name);
                       }
                   }
               }
           }

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//扫描并删除未成功安装的apk包（针对第三方app）
 //look for any incomplete package installations
ArrayList<PackageSetting> deletePkgsList = mSettings.getListOfIncompleteInstallPackagesLPr();
//clean up list
for(int i = 0; i < deletePkgsList.size(); i++) {
    //clean up here
    cleanupInstallFailedPackage(deletePkgsList.get(i));
}

// 删除临时文件
//delete tmp files
deleteTempPackageFiles();

// 把从mSettings中没有关联任何应用的SharedUserSetting对象删掉
mSettings.pruneSharedUsersLPw();

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EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_PMS_DATA_SCAN_START,
        SystemClock.uptimeMillis());
scanDirLI(mAppInstallDir, 0, scanFlags | SCAN_REQUIRE_KNOWN, 0);

scanDirLI(mDrmAppPrivateInstallDir, PackageParser.PARSE_FORWARD_LOCK,
        scanFlags | SCAN_REQUIRE_KNOWN, 0);

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/data/app

/data/app-priv

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/**
 * Remove disable package settings for any updated system
 * apps that were removed via an OTA. If they're not a
 * previously-updated app, remove them completely.
 * Otherwise, just revoke their system-level permissions.
 */
for (String deletedAppName : possiblyDeletedUpdatedSystemApps) {
    // 在扫描了用户app目录之后，再次尝试查找是否有这些app
    PackageParser.Package deletedPkg = mPackages.get(deletedAppName);
    mSettings.removeDisabledSystemPackageLPw(deletedAppName);

    String msg;
    // 依旧没有，那么就删除他们的数据目录
    if (deletedPkg == null) {
        msg = "Updated system package " + deletedAppName
                + " no longer exists; wiping its data";
        removeDataDirsLI(null, deletedAppName);
    } else {
      // 找到了，说明是在用户app目录中找到的，那么移除系统权限
        msg = "Updated system app + " + deletedAppName
                + " no longer present; removing system privileges for "
                + deletedAppName;

        deletedPkg.applicationInfo.flags &= ~ApplicationInfo.FLAG_SYSTEM;

        PackageSetting deletedPs = mSettings.mPackages.get(deletedAppName);
        deletedPs.pkgFlags &= ~ApplicationInfo.FLAG_SYSTEM;
    }
    logCriticalInfo(Log.WARN, msg);
}

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    /**
 * Make sure all system apps that we expected to appear on
 * the userdata partition actually showed up. If they never
 * appeared, crawl back and revive the system version.
 */
    for (int i = 0; i < mExpectingBetter.size(); i++) {
        final String packageName = mExpectingBetter.keyAt(i);
        if (!mPackages.containsKey(packageName)) {
            final File scanFile = mExpectingBetter.valueAt(i);

            logCriticalInfo(Log.WARN, "Expected better " + packageName
                    + " but never showed up; reverting to system");

            //确保是在
            ///system/app-priv
            ///system/app
            ///vendor/app
            ///oem/app
            // 这四个目录中。
            final int reparseFlags;
            if (FileUtils.contains(privilegedAppDir, scanFile)) {
                reparseFlags = PackageParser.PARSE_IS_SYSTEM
                        | PackageParser.PARSE_IS_SYSTEM_DIR
                        | PackageParser.PARSE_IS_PRIVILEGED;
            } else if (FileUtils.contains(systemAppDir, scanFile)) {
                reparseFlags = PackageParser.PARSE_IS_SYSTEM
                        | PackageParser.PARSE_IS_SYSTEM_DIR;
            } else if (FileUtils.contains(vendorAppDir, scanFile)) {
                reparseFlags = PackageParser.PARSE_IS_SYSTEM
                        | PackageParser.PARSE_IS_SYSTEM_DIR;
            } else if (FileUtils.contains(oemAppDir, scanFile)) {
                reparseFlags = PackageParser.PARSE_IS_SYSTEM
                        | PackageParser.PARSE_IS_SYSTEM_DIR;
            } else {
                Slog.e(TAG, "Ignoring unexpected fallback path " + scanFile);
                continue;
            }

            // 会将其加入mSettings的mPackages中
            mSettings.enableSystemPackageLPw(packageName);

            /// 重新扫描这些文件
            try {
                scanPackageLI(scanFile, reparseFlags, scanFlags, 0, null);
            } catch (PackageManagerException e) {
                Slog.e(TAG, "Failed to parse original system package: "
                        + e.getMessage());
            }
        }
    }
}
  // 清除mExpectingBetter
  mExpectingBetter.clear();

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mInstallerService = new PackageInstallerService(context, this);