Use truncate mode to replace the contents of existing files

[cketti]

Type of change

• Feature
• Bugfix
• Technical
• Other :

Content

Instances where ContentResolver.openOutputStream(Uri) is used to potentially open existing files for writing have been fixed.

Motivation and context

ContentResolver.openOutputStream(Uri) does not truncate existing files. If the amount of data written is smaller than the file size, you end up with new data at the beginning of the file followed by old data at the end of the file.

Some places where ContentResolver.openOutputStream(Uri) is used have not been changed because they only deal with newly created files and are thus not a problem.

Tests

1. Create a large text file with arbitrary content on your Android device (1 MB should do)
2. Go to Settings → Security & Privacy
3. Select Export E2E room keys
4. In the system file picker, select the file created in step 1, confirm that you want to overwrite the file
5. Complete the export flow (enter passwords, etc)
6. Notice that the file size hasn't changed. Only the beginning of the file has been overwritten with the exported data.

The other export functionality can be tested in a similar way.

Tested devices

• Physical
• Emulator
• OS version(s): Android 12

Checklist

• Changes has been tested on an Android device or Android emulator with API 21
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/vector-im/element-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request includes a new file under ./changelog.d. See https://github.com/vector-im/element-android/blob/develop/CONTRIBUTING.md#changelog
• Pull request includes screenshots or videos if containing UI changes
• Pull request includes a sign off
• You've made a self review of your PR
• If you have modified the screen flow, or added new screens to the application, you have updated the test UiAllScreensSanityTest.allScreensTest()

Signed-off-by: cketti <ck@cketti.de>

[ouchadam]

looping in product, we might want to warn the user if we're overwriting files

[bmarty]

Thanks for your contribution!

I understand the pb and I think you fixed it properly.

The file replacement warning is handled by the system, so this is OK to me.

Thanks!

[bmarty]

Nit: maybe creating a quick extension like this could help to avoid code duplication:

    fun Context.safeOpenOutputStream(uri: Uri): OutputStream? {
        return contentResolver.openOutputStream(uri, "wt")
    }

[cketti]

👍
Done

[bmarty]

Thanks for the fast update!