purego: fix returning a C function pointer as a Go func

func.go

@@ -409,8 +409,9 @@ func RegisterFunc(fptr any, cfn uintptr) {
 			v = reflect.NewAt(outType, unsafe.Pointer(&a1)).Elem()
 		case reflect.Func:
 			// wrap this C function in a nicely typed Go function
-			v = reflect.New(outType)
-			RegisterFunc(v.Interface(), syscall.a1)
+			if syscall.a1 != 0 {
+				RegisterFunc(v.Addr().Interface(), syscall.a1)
+			}
 		case reflect.String:
 			v.SetString(strings.GoString(syscall.a1))
 		case reflect.Float32:

func_test.go

@@ -239,6 +239,24 @@ func TestABI(t *testing.T) {
 			t.Fatalf("%s: got %q, want %q", cName, res, want)
 		}
 	}
+	{
+		const cName = "return_func_ptr"
+		var fn func() func(a, b int32) int32
+		purego.RegisterLibFunc(&fn, lib, cName)
+		add := fn()
+		const expect = 5
+		if res := add(2, 3); res != expect {
+			t.Fatalf("%s: got %d, want %d", cName, res, expect)
+		}
+	}
+	{
+		const cName = "return_null_func_ptr"
+		var fn func() func(a, b int32) int32
+		purego.RegisterLibFunc(&fn, lib, cName)
+		if fn() != nil {
+			t.Fatalf("%s: got a non-nil func, want nil", cName)
+		}
+	}
 }
 
 func TestABI_ArgumentPassing(t *testing.T) {

testdata/abitest/abi_test.c

@@ -197,3 +197,17 @@ double arm_float64_unaligned_on_stack(uintptr_t a1, uintptr_t a2, uintptr_t a3,
     return (double)a1 * 1 + (double)a2 * 2 + (double)a3 * 3 + (double)a4 * 4 +
            (double)a5 * 5 + a6;
 }
+
+typedef int32_t (*AddFunc)(int32_t, int32_t);
+
+int32_t returned_add(int32_t a, int32_t b) {
+    return a + b;
+}
+
+AddFunc return_func_ptr(void) {
+    return returned_add;
+}
+
+AddFunc return_null_func_ptr(void) {
+    return NULL;
+}