Expose offline database merge API

[LukasPaczos]

Closes #12692.

[asheemmamoowala]

How/Where is this handled? I don't see any reference to this exception type anywhere else.

[LukasPaczos]

Good catch, we pushing the error message through MergeOfflineRegionsCallback#onError. The doc needs to be updated.

[LukasPaczos]

This one is ready for a review but still relies on #12876 and waits for a fix for I/O errors thrown by locally generated databases.

[kkaefer]

Debugged the crash and found that it was crashing with SQLITE_IOERR, and extended error code SQLITE_IOERR_GETTEMPPATH. In our database merging code, we're using temporary tables, which may generate temporary files. Since Android doesn't have a default temporary directory, we have to explicitly set one.

[kkaefer]

• When the test is started without other tests being run prior to it, the database doesn't exist yet and we get "Merge database has incorrect user_version". (Note that the main database doesn't exist yet, the database that we are merging from does exist)
• Once the test has run, the main database already contains the region. Subsequent runs will not exercise the merging code (and not trigger the temp file crash). We should restore the original database before every run

[LukasPaczos]

When the test is started without other tests being run prior to it, the database doesn't exist yet and we get "Merge database has incorrect user_version". (Note that the main database doesn't exist yet, the database that we are merging from does exist)

I haven't noticed these issues locally or during firebase runs and capturing from an internal convo that it might be dependent on other variables like run profiles.

Once the test has run, the main database already contains the region. Subsequent runs will not exercise the merging code (and not trigger the temp file crash). We should restore the original database before every run

Each firebase run is performed on a clean install, so we should exercise the merging code with every push. The instrumentation test is also deleting the region, the only things that persist are tiles. When it comes to the example activity, the region can be removed via the DeleteRegionActivity example activity that manages a list of locally stored regions.

[kkaefer]

Each firebase run is performed on a clean install

yeah, but it means that local testing will be much harder.

[LukasPaczos]

it means that local testing will be much harder

True, I can try removing the database when exiting the test activity.

[LukasPaczos]

Removing database would require recreating the singleton FileSource instance or exposing OfflineDatabase::removeExisting, otherwise, the DB will be recreated only when resources are requested and fail once. I feel like it's better to clear app's local storage and restart the process instead.

[tobrun]

🚀