关于权限"android.permission.GLOBAL_SEARCH"

此前有一篇文章说明如何将自己的app作为一个外部数据源提供给android系统搜索：

http://blog.csdn.net/zhanglianyu00/article/details/61196900

里面提到安卓提供了一个权限android.permission.GLOBAL_SEARCH，供数据源提供方使用，来作为数据的保护机制。android官方文档对于此权限的说明较少，只有如下文字：

This permission can be used on content providers to allow the global search system to access their data. Typically it used when the provider has some permissions protecting it (which global search would not be expected to hold), and added as a read-only
permission to the path in the provider where global search queries are performed. This permission can not be held by regular applications; it is used by applications to protect themselves from everyone else besides global search.

https://developer.android.com/reference/android/Manifest.permission.html#GLOBAL_SEARCH

这里稍微深入，在android内部研究一下这个权限。

通过search android源代码，可以找到这个权限是在这个工程中定义 frameworks/base/core/res。这个工程在android内部托管各种框架层使用的资源。查看frameworks/base/core/res/AndroidManifest.xml，找到此权限的定义：

    <!-- @SystemApi This permission can be used on content providers to allow the global

         search system to access their data.  Typically it used when the

         provider has some permissions protecting it (which global search

         would not be expected to hold), and added as a read-only permission

         to the path in the provider where global search queries are

         performed.  This permission can not be held by regular applications;

         it is used by applications to protect themselves from everyone else

         besides global search. -->

    <permission android:name="android.permission.GLOBAL_SEARCH"

        android:protectionLevel="signature|privileged" />

看到保护级别是signature，再复习一下这两个flag的含义（https://developer.android.com/reference/android/R.attr.html#protectionLevel ）：

signature：

A permission that the system is to grant only if the requesting application is signed with the same certificate as the application that declared the permission. If the certificates match, the system automatically grants the permission without notifying
the user or asking for the user's explicit approval.

privileged：

Additional flag from base permission type: this permission can also be granted to any applications installed as privileged apps on the system image. Please avoid using this option, as the signature protection level should be sufficient for most needs and
works regardless of exactly where applications are installed. This permission flag is used for certain special situations where multiple vendors have applications built in to a system image which need to share specific features explicitly because they are
being built together.

可见需要与frameworks/base/core/res签名相同或者是由系统授权的应用（譬如vendor）才能使用这个权限。

frameworks/base/core/res是一个系统工程，查看其mk文件 frameworks/base/core/res/Android.mk，看到签名信息：

LOCAL_CERTIFICATE := platform

android一共有4种系统签名：

testkey：用于普通apk

platform：用于系统核心apk

shared：用于Launcher、Contacts等重要apk

media：用于系统的多媒体和下载类apk

（参见build/target/product/security/）

由此引申出一个问题，由签名为platform的工程定义的signature级别的permission，只对platform开放，还是对上述四种都开放？

在一个shared签名的系统应用中测试添加使用该permission，编译，成功。所以可见对于所有系统签名都可用。

对于纯第三方App，使用Android Studio开发时是无法添加这个use permission的，会提示错误：

Permission is only granted to system apps.