[AD] file interface fopen

[Matthew Leverton <meffer@xxxxxxxxxx>]

I've been working with A5's file interfaces, namely creating an A4
PACKFILE / DATAFILE loader, and the current setup of creating file
interfaces bugs me. I recently proposed al_fopen_f on a.cc, but after
playing around some more, that's not really what I want.

The problem in a nutshell is that the file interface is responsible
for creating its own structure and therefore must have access to the
Allegro internals, which of course is bad. So my proposal is:

* have Allegro internally be responsible for creating the ALLEGRO_FILE
struct, and populating the vtable
* pass that structure to the fi_fopen hook
* if fi_fopen returns false, destroy the struct, and return NULL

In addition to that, there's a new private field on the ALLEGRO_FILE
struct, called void* userdata, accessible via:

* al_get_file_userdata(file)
* al_set_file_userdata(file, ptr)

Also, there's a new way to initialize a file (with the vtable set to
the current default one):

* ALLEGRO_FILE al_create_file();

Now that function doesn't always make sense (and in fact would crash
subsequent calls with the stdio/physfs systems since they assume an
extended struct), but the file interface struct could contain a bool
to enable/disable this behavior.

So, for example, the memfile addon looks like in al_open_memfile:

al_store_state(&state, ALLEGRO_STATE_NEW_FILE_INTERFACE);
al_set_new_file_interface(&memfile_vtable);
memfile = al_create_file();
al_set_file_userdata(memfile, userdata);
al_restore_state(&state);

and

static size_t memfile_fread(ALLEGRO_FILE *fp, void *ptr, size_t size)
{
   ALLEGRO_FILE_MEMFILE *mf = al_get_file_userdata(fp);
...

It no longer needs any access to the internals.

It might be useful to integrate the userdata into the al_create_file,
such that the hook can validate it and return true/false. Also,
perhaps the set_file_userdata() could also optionally call a hook that
notifies the interface that the userdata changed.

I've attached a patch against 5.1. It's designed to not disrupt
anything in 5.0... in fact, this implementation could wait until 5.2.

It simply adds "fi_fopen2" which is given preference over "fi_fopen"
in the al_fopen function. I didn't feel like updating the physfs and
stdio drivers from the current hacky struct inheritance to the
userdata pointer.

The patch may have some bugs ... I didn't test it.

(Also, I've never looked at ALLEGRO_FS_INTERFACE, so I don't know if
there's any sort of similar issue with that.)

--
Matthew Leverton

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