Add libretro support

[RobLoach]

This introduces libretro.h virtual file system support for PhysFS, allowing to use PhysFS directly from a libretro core through RETRO_ENVIRONMENT_GET_VFS_INTERFACE.

I've put together a physfs_test_libretro core to test out the functionality. It was originally written for the chailove core.

A few considerations...

1. In both Playdate, and libretro now, PHYSFS_init() requires you to pass in some user data pertaining to the environment as an argv0 argument. As you mentioned in the comments, it seems like a cheat. Would it make sense to introduce a function like PHYSFS_setPlatformData(void*) or something?
2. I didn't include the CMakeLists.txt update, as this does depend on some of libretro-common, and libretro-common doesn't use CMake at all. Would be nice to include CMake support, but I'm unsure how to best accomplish it. For reference, it depends on the following from libretro-common...

   include/libretro.h
   rthreads/rthreads.c
   

3. Haven't tested this with UTF-8 paths yet
4. Unsure of the pref directory strategy so far

[icculus]

It was originally written for the chailove core.

Before we go any further: is this code able to be contributed under a zlib license? ChaiLove uses the MIT license.

[RobLoach]

Good call. As the original author of ChaiLove, I'm okay to re-license this code to zlib. I got the 👍 from phcoder too.

[RobLoach]

For the base directory, it's currently using getcwd(), but this may not be the best solution. Maybe use RETRO_ENVIRONMENT_GET_LIBRETRO_PATH instead? There's also RETRO_ENVIRONMENT_GET_GAME_INFO_EXT, but that may not be set when PHYSFS_init() is called.

[icculus]

This is meant to be where to find game data files...which for a libretro core is either going to be the engine's data or the game/rom-specific data...I'd lean towards the latter, if they aren't the same thing.

[RobLoach]

Good brainstorming, thanks. Made an update to...

1. Try to get the game directory via GET_GAME_INFO_EXT, which is a directory in which the content file exists
2. Otherwise, use GET_SYSTEM_DIRECTORY, as that may make sense for data files
3. If both fail, use /

[icculus]

In both Playdate, and libretro now

(AND android...this is a big mess, and I'll probably change this in PhysicsFS 4 to not take a const char *, but this API has been otherwise-stable for like...23 years now, so I'm hesitant to change it. :) Hack it in for now.)

[sezero]

May I suggest something like the following to make it buildable in C90 mode and avoid errors if ELOOP isn't available:

diff --git a/src/physfs_platform_libretro.c b/src/physfs_platform_libretro.c
index 58694ba..dcc0c87 100644
--- a/src/physfs_platform_libretro.c
+++ b/src/physfs_platform_libretro.c
@@ -306,9 +306,10 @@ void *__PHYSFS_platformOpenAppend(const char *filename)
 PHYSFS_sint64 __PHYSFS_platformRead(void *opaque, void *buffer,
                                     PHYSFS_uint64 len)
 {
+    PHYSFS_sint64 retval;
     BAIL_IF(physfs_platform_libretro_vfs == NULL || physfs_platform_libretro_vfs->read == NULL, PHYSFS_ERR_NOT_INITIALIZED, -1);
     BAIL_IF(opaque == NULL || buffer == NULL, PHYSFS_ERR_INVALID_ARGUMENT, -1);
-    PHYSFS_sint64 retval = (PHYSFS_sint64) physfs_platform_libretro_vfs->read((struct retro_vfs_file_handle *) opaque, buffer, (uint64_t) len);
+    retval = (PHYSFS_sint64) physfs_platform_libretro_vfs->read((struct retro_vfs_file_handle *) opaque, buffer, (uint64_t) len);
     BAIL_IF(retval == -1, PHYSFS_ERR_IO, -1);
     return retval;
 } /* __PHYSFS_platformRead */
@@ -317,9 +318,10 @@ PHYSFS_sint64 __PHYSFS_platformRead(void *opaque, void *buffer,
 PHYSFS_sint64 __PHYSFS_platformWrite(void *opaque, const void *buffer,
                                      PHYSFS_uint64 len)
 {
+    PHYSFS_sint64 retval;
     BAIL_IF(physfs_platform_libretro_vfs == NULL || physfs_platform_libretro_vfs->write == NULL, PHYSFS_ERR_NOT_INITIALIZED, -1);
     BAIL_IF(opaque == NULL || buffer == NULL, PHYSFS_ERR_INVALID_ARGUMENT, -1);
-    PHYSFS_sint64 retval = (PHYSFS_sint64) physfs_platform_libretro_vfs->write((struct retro_vfs_file_handle *) opaque, buffer, (uint64_t)len);
+    retval = (PHYSFS_sint64) physfs_platform_libretro_vfs->write((struct retro_vfs_file_handle *) opaque, buffer, (uint64_t)len);
     BAIL_IF(retval == -1, PHYSFS_ERR_IO, -1);
     return retval;
 } /* __PHYSFS_platformWrite */
@@ -386,7 +388,9 @@ static PHYSFS_ErrorCode errcodeFromErrnoError(const int err)
         case WSAEDQUOT: return PHYSFS_ERR_NO_SPACE;
 #endif
         case EIO: return PHYSFS_ERR_IO;
+#if defined(ELOOP)
         case ELOOP: return PHYSFS_ERR_SYMLINK_LOOP;
+#endif
         case EMLINK: return PHYSFS_ERR_NO_SPACE;
         case ENAMETOOLONG: return PHYSFS_ERR_BAD_FILENAME;
         case ENOENT: return PHYSFS_ERR_NOT_FOUND;

[RobLoach]

Did another test of this with https://github.com/robloach/physfs_test_libretro ... Looking pretty good. If there's anything else you can think of adding, feel free to let me know.