Unloading unused extensions

[piotrrzysko]

Unused extensions should be automatically unloaded. SQLite does this when the connections that loaded the extensions are closed (see https://www.sqlite.org/loadext.html, section Persistent Loadable Extensions).

See this thread for more information: #1447 (comment) - specifically this comment, as some extensions cannot be released on connection close.

[PThorpe92]

This was touched on a bit in the referenced comment, but to add some context: vfs module extensions specifically cannot be tied to a connection because the user needs to be able to .open some.db some_vfs. if the extension was loaded and the library owned by the connection that existed when that command was called it would be dropped. Also as mentioned the module's destructor needs to be able to call close on any files, so the library needs to remain loaded until afterwards. I think it makes sense that libraries for vfs modules should remain loaded for the life of the program, so they show up in .vfslist.

Other libraries can be refcounted and owned by the connection and a reference can be held by any open modules to ensure they live at least as long as their modules.