Requesting and allowing WRITE_EXTERNAL_STORAGE per

问题:

this is regarding the new model of runtime permissions introduced in Android Marshmallow when requesting Manifest.permission.WRITE_EXTERNAL_STORAGE permission.

In short, what I am experiencing is that if I request (and the user allows) Manifest.permission.WRITE_EXTERNAL_STORAGE permission, the app won't be able to read and write from the external storage directory until I destroy and restart the app.

This is what I am doing/experiencing:

My app starts from a state where:

ContextCompat.checkSelfPermission(this, Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED

This is, I don't have permissions to to access external storage.

Then, I request permission to Manifest.permission.WRITE_EXTERNAL_STORAGE just as Google explains

private void requestWriteExternalStoragePermission() {
    // Should we show an explanation?
    if (ActivityCompat.shouldShowRequestPermissionRationale(this, Manifest.permission.WRITE_EXTERNAL_STORAGE)) {
        new AlertDialog.Builder(this)
                .setTitle("Inform and request")
                .setMessage("You need to enable permissions, bla bla bla")
                .setPositiveButton(R.string.ok, new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialog, int which) {
                        ActivityCompat.requestPermissions(MendeleyActivity.this, new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, RC_PERMISSION_WRITE_EXTERNAL_STORAGE);
                    }
                })
                .show();
    } else {
        ActivityCompat.requestPermissions(MendeleyActivity.this, new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, RC_PERMISSION_WRITE_EXTERNAL_STORAGE);
    }
}

Once the user allows the permission, onRequestPermissionsResult gets invoked.

@Override
public void onRequestPermissionsResult(int requestCode,  String permissions[], int[] grantResults) {
    switch (requestCode) {
        case RC_PERMISSION_WRITE_EXTERNAL_STORAGE: {
            // If request is cancelled, the result arrays are empty.
            if (grantResults.length > 0 && PackageManager.PERMISSION_GRANTED
                // allowed 
            } else {
                // denied
            }
            break;
        }
    }
}

The allowed block is executed, confirming the user has granted permissions.

Immediately after this, if I don't destroy and open the app again, I still have no access permission to external storage. More specifically:

hasWriteExternalStoragePermission();                  // returns true
Environment.getExternalStorageDirectory().canRead();  // RETURNS FALSE!!
Environment.getExternalStorageDirectory().canWrite(); // RETURNS FALSE!!

So, it seems the Android runtime thinks I have permissions, but the file system doesn't... Indeed, trying to access Environment.getExternalStorageDirectory() throws the exception:

android.system.ErrnoException: open failed: EACCES (Permission denied)
at libcore.io.Posix.open(Native Method)
at libcore.io.BlockGuardOs.open(BlockGuardOs.java:186)
at libcore.io.IoBridge.open(IoBridge.java:438)
at java.io.FileOutputStream.<init>(FileOutputStream.java:87) 
at java.io.FileOutputStream.<init>(FileOutputStream.java:72) 

If I now destroy the app and open it again, the behaviour becomes as it should, being able to read and write in the external storage folder.

Is anyone experiencing this?

I am using one official emulator with:

• Latest Android 6.0 (API 23) API 23, Rev 1.
• Emulator running Intel x86 Atom System Image, API 23, Rev 1.

I build the app with:

android {
    compileSdkVersion 23
    buildToolsVersion "22.0.1"

    defaultConfig {
        minSdkVersion 16
        targetSdkVersion 23
    }
...
}

If someone confirms this and I am not the only one I guess we'll need to open a bug, but I hope I am doing something wrong, as I think such a core feature is unlikely to be buggy in the SDK.

回答1:

http://developer.android.com/reference/android/Manifest.permission.html#WRITE_EXTERNAL_STORAGE:

Starting in API level 19, this permission is not required to read/write files in your application-specific directories returned by getExternalFilesDir(String) and getExternalCacheDir().

"Runtime permission requesting" starts at API level 23, obviously above 19, so the permission is not required anymore, unless you're accessing the data outside of the folder pointed by getExternalFilesDir(). So I believe this is a bug of the emulator.

On lower targets below level 19, which don't support requesting permission at runtime, just calim the permission in manifest and it will work.

回答2:

I had the same problem. Turns out this seems to be a bigger issue. Changing the permission to write to the external storage changes the GID for this process (on the linux side). In order to change the ID the process has to be restarted. Next time you open the app, the new groupID is set and the permission is granted.

Long story short, I'm afraid this is not a bug in the emulator but in fact a bigger issue with Linux and Android.

I "solved" this by asking for permission the first time the app is executed and restarting it when the permission is given like this:

PackageManager packageManager = getPackageManager();
                    Intent intent = packageManager.getLaunchIntentForPackage(getPackageName());
                    ComponentName componentName = intent.getComponent();
                    Intent mainIntent = IntentCompat.makeRestartActivityTask(componentName);
                    startActivity(mainIntent);
                    System.exit(0);

You may try to create a service running in the background (having another process id) and giving it the permission. That way you would only need to restart the service and not the complete app. On the down side this might make more work for you.

Hope this helps.

--- EDIT ---

The user M66B (https://stackoverflow.com/a/32473449/1565635) found a list of the related gids. Further information can be found here: https://android.googlesource.com/platform/frameworks/base/+/master/data/etc/platform.xml