Racket - NULL pointer handling

SWIG now converts a C/C++ NULL pointer into a null value by calling
scheme_make_null(), so that scheme's null? is true for a NULL C/C++
pointer value.

Consistency with Guile and needed for a pending commit for handling
NULL and std::unique_ptr.

CHANGES.current

@@ -7,6 +7,10 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.1.0 (in progress)
 ===========================
 
+2022-08-26: wsfulton
+            [Racket] SWIG now converts a C/C++ NULL pointer into a null value by calling
+            scheme_make_null(), so that scheme's null? is true for a NULL C/C++ pointer value.
+
 2022-08-18: wsfulton
             [Racket] Add support for std::unique_ptr in std_unique_ptr.i.
             Add support for std::auto_ptr in std_auto_ptr.i.

Examples/test-suite/guile/null_pointer_runme.scm

@@ -0,0 +1,3 @@
+(dynamic-call "scm_init_null_pointer_module" (dynamic-link "./libnull_pointer"))
+(load "testsuite.scm")
+(load "../schemerunme/null_pointer.scm")

Examples/test-suite/mzscheme/null_pointer_runme.scm

@@ -0,0 +1,3 @@
+(load-extension "null_pointer.so")
+
+(load "../schemerunme/null_pointer.scm")

Examples/test-suite/schemerunme/null_pointer.scm

@@ -0,0 +1,8 @@
+(define null '())
+(unless (funk null)
+  (error "funk(null) does not return true"))
+
+(unless (null? (getnull))
+  (error "NULL pointer should be null"))
+
+(exit 0)

Lib/mzscheme/mzrun.swg

@@ -144,6 +144,7 @@ mz_free_swig(void *p, void *data) {
 
 static Scheme_Object *
 SWIG_MzScheme_NewPointerObj(void *ptr, swig_type_info *type, int owner) {
+  if (ptr) {
     struct swig_mz_proxy *new_proxy;
     new_proxy = (struct swig_mz_proxy *) scheme_malloc(sizeof(struct swig_mz_proxy));
     new_proxy->mztype = swig_type;
@@ -154,6 +155,9 @@ SWIG_MzScheme_NewPointerObj(void *ptr, swig_type_info *type, int owner) {
       scheme_add_finalizer(new_proxy, mz_free_swig, NULL);
     }
     return (Scheme_Object *) new_proxy;
+  } else {
+    return scheme_make_null();
+  }
 }
 
 static int