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Catfriend1 edited this page Jul 7, 2020 · 13 revisions

UPDATE 2020/07/08

Android 11 is being expected to be the first version since 4.4 that supports writing to external SD card again using Syncthing. Stay tuned.

Why SD card use together with Syncthing requires special attention

According to Google's document:

https://source.android.com/devices/storage/traditional.html

It is true if you did not format the external SD card as adoptable storage but left it as portable. Doing this prevents any native linux app from writing to this card freely. The directories on the SD card that an app can write to are restricted to the following places, for example:

  • /storage/[ABCD-EFGH]/Android/data/com.github.catfriend1.syncthingandroid.[debug]/files
  • /storage/[ABCD-EFGH]/Android/media/com.github.catfriend1.syncthingandroid.[debug]

In Syncthing-Fork, when you are prompted for the directory, you are automatically taken to a writable directory by the built-in directory chooser to help finding out these writable directories with ease. Just create your folder you wish to be used by Syncthing below the suggested directory that opens up on the UI.

To sum up:

  • You CAN write to your internal storage and external SD card using Syncthing-Fork.
  • Adoptable Storage counts as "internal" storage.
  • You can NOT decide freely where Syncthing-Fork has write access due to the above mentioned Android restriction if the SD card was formatted as PORTABLE.

To ease your technical understanding of the issue and prevent the opening of tickets caused by misunderstandings:

  • Syncthing-Fork consists of two components:
  1. "libsyncthingnative.so" - A cross-compiled linux binary which we often call "SyncthingNative". It contains all synchronization functionality and carries out everything required to read, transfer and store files.
  2. The wrapper which represents the UI. It mainly controls the start and stop of the SyncthingNative binary to save your battery and monitor health of the syncing process.

Android forces SD card access to be read only upon SyncthingNative outside the above mentioned directory paths. People often asked on the issue tracker and forum why the Syncthing-Fork app refrains from asking for SD card directory tree permission using the WRITE_EXTERNAL_STORAGE permission. It might disappoint you, but this was already tried. If the Java app (2.) asks for permission and the user grants it, Android still forces the SD card paths to be accessed read-only by SyncthingNative (1.). This is why we don't ask the user for a directory tree permission like for example file manager 3rd party apps would have done it. File manager apps are technically different from Syncthing because they run as Java code and not as Native code.

Implementing Syncthing as Java code instead of using the Native would involve a LOT of work, and so far until the time of writing this, no one stepped up to do it yet. If you want to help with this and are familiar with Go, Java and Android, read through the discussion in issues #29 and #1008. They outline the steps to solve this issue pretty good. Short summary: SynchtingNative would have to be transformed into a gomobile library with goreverse bindings to be called from the wrapper's java code.

Special ways to bypass the Android SD card write access restriction for IT nerds or experts

  • You may ROOT your phone at your own risk. Syncthing-Fork has an option in its settings to enable the execution of SyncthingNative with root privileges. BUT: Executing SyncthingNative with such a high privilege level may also open up security issues (in theory).
  • You may use the ExtSDCard Write Enabler Magisk Module.
  • Android 6, 7: If you are on a rooted phone, you could use this Xposed module to give Syncthing permission to write to the external SD card storage.
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