Android: tailscale app does not request storage permission

[ppaga]

What is the issue?

On Android Pixel 7a: when trying to taildrop a file from linux, I get
500 Internal Server Error: open redacted.3e790f11: permission denied
and no notification on the phone.

When checking permissions, the tailscale app only has the notification permission and no other requested.

Steps to reproduce

No response

Are there any recent changes that introduced the issue?

No response

OS

No response

OS version

No response

Tailscale version

No response

Other software

No response

Bug report

No response

[kari-ts]

This is due to our use of getExternalStoragePublicDirectory, which is deprecated in Android 10+ because of changes to Android's management of scoped storage. The fix is to use Storage Access Framework instead.

[kari-ts]

SAF is designed to give users control over which directories and files an app can access, so it requires users select where Taildropped files are saved.
We currently set the directory during initialization when creating LocalBackend, but there are cases where Tailscale may be running without the app having been opened by the user (eg if Always On is enabled and the device has been restarted but the app hasn't been opened).
Since we will be adding an additional directory picker, this should get shown in MainActivity to users who haven't selected the Taildrop destination before when we are asking for other permissions.
Context.getExternalFilesDirs gives a directory on external storage that's private to the app and doesn't require permission - when a user has not selected the Taildrop location, Taildropped files should get saved there, and a notification shown explaining this and giving them the option to open up the directory picker, and once the location has been selected, files should be transferred to that location.