MODE_MULTI_PROCESS for SharedPreferences isn't working

Question

I have a SyncAdapter running on its own process separately from the main app process.

I'm using a static wrapper class around my SharedPreferences that creates a static object on process load (Application's onCreate) like so:

myPrefs = context.getSharedPreferences(MY_FILE_NAME, Context.MODE_MULTI_PROCESS | Context.MODE_PRIVATE);

The wrapper has get and set methods, like so:

public static String getSomeString() {
    return myPrefs.getString(SOME_KEY, null);
}

public static void setSomeString(String str) {
    myPrefs.edit().putString(SOME_KEY, str).commit();
}

Both SyncAdapter and app uses this wrapper class to edit and get from the prefs, this works sometimes but a lot of times I see the SyncAdapter getting old/missing prefs on accesses to the prefs, while the main app sees the recent changes properly.

According to the docs I think the MODE_MULTI_PROCESS flag should work as I expect it to, allowing both processes to see latest changes, but it doesn't work.

Update:

Per x90's suggestion, I've tried refraining from using a static SharedPreferences object and instead calling getSharedPreferences on each get/set method. This caused a new issue, where the prefs file gets deleted (!!!) on multi-process simultaneous access. i.e. I see in the logcat:

(process 1): getName => "Name"
(process 2): getName => null
(process 1): getName => null

and from that point all the prefs saved on the SharedPreferences object were deleted.

This is probably a result of another warning I see in the log:

W/FileUtils(21552): Failed to chmod(/data/data/com.my_company/shared_prefs/prefs_filename.xml): libcore.io.ErrnoException: chmod failed: ENOENT (No such file or directory)

P.S this is not a deterministic issue, I saw the above logs after a crash happened, but couldn't recreate yet on the same device, and until now it didn't seem to happen on other devices.

ANOTHER UPDATE:

I've filed a bug report on this, after writing a small testing method to confirm this is indeed an Android issue, star it at https://code.google.com/p/android/issues/detail?id=66625

Answer 1

I gave a very quick look at Google's code and apparently Context.MODE_MULTI_PROCESS is not an actual way to ensure process-safety of SharedPreferences.

SharedPreferences itself is not process-safe. (That's probably why SharedPreferences documentation says "currently this class does not support use across multiple processes. This will be added later.")

MODE_MULTI_PROCESS just works in conjunction with every Context.getSharedPreferences(String name, int mode) call: when you retrieve an instance of SharedPreferences specifying the MODE_MULTI_PROCESS flag android will reload the preferences file to be up to date with any (eventual) concurrent modification that occurred to it. If you then keep that instance as a class (static or not) member, the preference file won't be reloaded again.

Using Context.getSharedPreferences(...) every time you want to write or read into preferences is not process-safe either, but I guess it's probably the closest that you can get to it at the moment.

If you don't actually need to read the same preference from the different processes, then a workaround could be to use different preferences files for the different processes.