Distinguish between owned and borrowed references to a SDL_Surface

In many SDL APIs that return a SDL_Surface *, the surface is considered
to be owned by the caller, and must be freed by the caller.

However, SDL_SetVideoMode and presumably SDL_GetVideoSurface return
a pointer to SDL's internal video surface, which will be freed by SDL
if necessary, and must not be freed by library users.
Incorrectly freeing this surface can lead to a use-after-free crash,
manifesting as a test failure in t/core_video.t.

Resolves: libsdl-org/sdl12-compat#305
Signed-off-by: Simon McVittie <smcv@debian.org>

Author: smcv

src/Core/Video.xs

@@ -10,6 +10,8 @@
 
 #include <SDL.h>
 
+typedef SDL_Surface SDL_Surface_borrowed;
+
 void _uinta_free(Uint16* av, int len_from_av_len)
 {
 	if( av != NULL)
@@ -56,7 +58,7 @@ See: L<http:/*www.libsdl.org/cgi/docwiki.cgi/SDL_API#head-813f033ec44914f267f321
 
 =cut
 
-SDL_Surface *
+SDL_Surface_borrowed *
 video_get_video_surface()
 	PREINIT:
 		char* CLASS = "SDL::Surface";
@@ -125,7 +127,7 @@ video_video_mode_ok ( width, height, bpp, flags )
 		RETVAL
 
 
-SDL_Surface *
+SDL_Surface_borrowed *
 video_set_video_mode ( width, height, bpp, flags )
 	int width
 	int height

src/helper.h

@@ -58,12 +58,13 @@ void objDESTROY(SV *bag, void (* callback)(void *object))
         Uint32 *threadid = (Uint32*)(pointers[2]);
 
         if(PERL_GET_CONTEXT == pointers[1]
-        && *threadid == SDL_ThreadID())
+        && (threadid == NULL || *threadid == SDL_ThreadID()))
         {
             pointers[0] = NULL;
-            if(object)
+            if(object && threadid != NULL)
                 callback(object);
-            safefree(threadid);
+            if (threadid != NULL)
+                safefree(threadid);
             safefree(pointers);
         }
     }

typemap

@@ -34,6 +34,7 @@ SDL_UserEvent * 	O_OBJECT
 SDL_QuitEvent * 	O_OBJECT
 SDL_keysym *		O_OBJECT
 SDL_Surface *		O_OBJECT
+SDL_Surface_borrowed *	O_BORROWED
 SDL_SysWMmsg *		T_PTR
 SDL_CD *		O_OBJECT
 SDL_CDtrack *		O_OBJECT
@@ -122,6 +123,17 @@ O_OBJECT
         XSRETURN_UNDEF;
     }
 
+O_BORROWED
+    if ($var) {
+        void** pointers  = malloc(3 * sizeof(void*));
+        pointers[0]      = (void*)$var;
+        pointers[1]      = (void*)PERL_GET_CONTEXT;
+        pointers[2]      = NULL;
+        sv_setref_pv( $arg, CLASS, (void*)pointers );
+    } else {
+        XSRETURN_UNDEF;
+    }
+
 INPUT
 
 O_OBJECT_NPGC
@@ -136,3 +148,14 @@ O_OBJECT
     } else {
         XSRETURN_UNDEF;
     }
+
+O_BORROWED
+    /* Same as O_OBJECT */
+    if( sv_isobject($arg) && (SvTYPE(SvRV($arg)) == SVt_PVMG) ) {
+        void** pointers = (void**)INT2PTR(void *, SvIV((SV *)SvRV( $arg )));
+        $var = ($type)(pointers[0]);
+    } else if ($arg == 0) {
+        XSRETURN(0);
+    } else {
+        XSRETURN_UNDEF;
+    }