Loadable extension

[vrischmann]

Add the ability to build a loadable extension (with .load myextension.so for example).

Buliding a loadable extension is problematic with Zig because the way sqlite3ext.h works is that it redefines the sqlite API with defines, like this:

#define sqlite3_open_v2 sqlite3_api->open_v2

This is not supported by translate-c, I looked into ways to make it work but it's not straightforward:

• if there's already a definition for a function the aliasing macros won't do anything: translate-c keeps generating the code for the function defined in sqlite3.h
• even if there's no definition already, the code generated will fail to compile because it references the variable sqlite3_api which is not defined
• and even if the sqlite3_api is defined before, the generated code fails to compile for another reason:

  pub const struct_api_routines = extern struct {
  open_v2: ?*const fn ([*c]const u8) callconv(.C) c_int,
  };
  pub export var api: [*c]struct_api_routines = null;
  pub const sqlite3_open_v2 = api.*.open_v2;

  I ~/t/zig-translate-c $ zig build-exe main.zig
  foobar.zig:457:29: error: cannot load runtime value in comptime block
  pub const sqlite3_open_v2 = api.*.open_v2;
                          ^~~

To make it work requires quite a bit of work:

• all function definitions in sqlite3.h must be removed
• all aliasing macros from sqlite3ext.h must be removed
• wrappers must be created in zig to forwards calls to sqlite3_api

The two first steps are done with a tool which can be run like this:

$ zig build preprocess-files

The last step is easy enough but will take some time to do.

[jiacai2050]

To make it work requires quite a bit of work:

How you tried solution like this:

• https://github.com/riyaz-ali/sqlite/blob/master/bridge/bridge.h

The Go library has similar issue with Zig, the author define a bridge header to redefine most functions of sqlite_api, then in Go side, it only need to include bridge.h, not sqlite3ext.h.

I think Zig can also use this method to avoid modify sqlite3.h and sqlite3ext.h