Mapbox-gl-native: [Qt] "database is locked" when opening the same db in different threads

[brunoabinader]

Got this on CircleCI with parallel execution:

[ RUN      ] OfflineDatabase.ConcurrentUse
[WARNING] {memcheck-amd64-}[Database]: Removing existing incompatible offline database
terminate called after throwing an instance of 'mapbox::sqlite::Exception'
  what():  database is locked Unable to fetch row

[brunoabinader]

/cc @tmpsantos

[tmpsantos]

We are not sharing the DefaultFileSource on Qt. We should do that by default, unless the user defines a path for the cache that is different from the default path, otherwise it would break the QtLocation plugin API.

[tmpsantos]

Still seeing this! https://circleci.com/gh/mapbox/mapbox-gl-native/10564

[tmpsantos]

That test is wrong, not sure how it is working to be honest. The sqlite API is not thread safe unless we use SQLITE_OPEN_FULLMUTEX which we should not use.

[jfirebaugh]

The test is correct. We need sqlite in at least "multi-thread" mode, where multiple threads with independent connections are safe; that's what the test is testing.

If the current Qt setup is operating in "single-thread" mode, then that needs to be fixed.

We do not depend on "serialized" mode, but there's no harm if the sqlite implementation is in that mode.

https://sqlite.org/threadsafe.html

[jfirebaugh]

The log message "Removing existing incompatible offline database" (OfflineDatabase::removeExisting()) may be the key here. I don't think that should be happening with this test.

[tmpsantos]

@jfirebaugh thanks a lot for the new insight on this issue. I'm suspecting that QSqlDatabase can only do single-thread to keep compatibility across multiple platforms it supports. I will investigate the Qt source code further to reach a more solid conclusion.

[brunoabinader]

Qt Sqlite plugin for QSqlDatabase does not allow setting e.g. SQLITE_OPEN_FULLMUTEX when opening the connection: https://doc.qt.io/qt-5/qsqldatabase.html#setConnectOptions

[tmpsantos]

:rage4: https://circleci.com/gh/mapbox/mapbox-gl-native/11804

[tmpsantos]

I think I finally found the issue. The busy timeout we set in mapboxgl is overflowing in Qt (signed -> unsigned) and for some reason Qt silently ignores the error, leaving the default timeout that is very low.