PoC: C25-preview defer approach

Derived from https://gustedt.wordpress.com/2025/01/06/simple-defer-ready-to-use/

Author: ppannuto

libtock-sync/crypto/hmac.c

@@ -1,9 +1,6 @@
 #include "hmac.h"
 
-// would be in a libtock-sync/support.h or internal.h or similar
-#define SCOPED_ALLOW(method, buffer, length) \
-  inline void unallow_##buffer(returncode_t* sentinel##buffer) { if (sentinel##buffer == RETURNCODE_SUCCESS) method(NULL, 0); } \
-  returncode_t ret_##buffer __attribute__((cleanup(unallow_##buffer))) = method(buffer, length);  \
+#include <libtock/defer.h>
 
 returncode_t libtocksync_hmac_simple(libtock_hmac_algorithm_t hmac_type,
                                      uint8_t* key_buffer, uint32_t key_length,
@@ -14,19 +11,17 @@ returncode_t libtocksync_hmac_simple(libtock_hmac_algorithm_t hmac_type,
   ret = libtock_hmac_command_set_algorithm((uint32_t) hmac_type);
   if (ret != RETURNCODE_SUCCESS) return ret;
 
-  // Macro expands to this:
-  //
-  // inline void unallow_key_buffer(returncode_t* sentinel_key_buffer) { if (sentinel_key_buffer == RETURNCODE_SUCCESS) libtock_hmac_set_readonly_allow_key_buffer(NULL, 0); }
-  // returncode_t ret_key_buffer __attribute__((cleanup(unallow_key_buffer))) = libtock_hmac_set_readonly_allow_key_buffer(key_buffer, key_length);
-
-  SCOPED_ALLOW(libtock_hmac_set_readonly_allow_key_buffer, key_buffer, key_length);
-  if (ret_key_buffer != RETURNCODE_SUCCESS) return ret_key_buffer;
+  ret = libtock_hmac_set_readonly_allow_key_buffer(key_buffer, key_length);
+  if (ret != RETURNCODE_SUCCESS) return ret;
+  defer { libtock_hmac_set_readonly_allow_key_buffer(NULL, 0); };
 
-  SCOPED_ALLOW(libtock_hmac_set_readonly_allow_data_buffer, input_buffer, input_length);
-  if (ret_input_buffer != RETURNCODE_SUCCESS) return ret_input_buffer;
+  ret = libtock_hmac_set_readonly_allow_data_buffer(input_buffer, input_length);
+  if (ret != RETURNCODE_SUCCESS) return ret;
+  defer { libtock_hmac_set_readonly_allow_data_buffer(NULL, 0); };
 
-  SCOPED_ALLOW(libtock_hmac_set_readwrite_allow_destination_buffer, hmac_buffer, hmac_length);
-  if (ret_hmac_buffer != RETURNCODE_SUCCESS) return ret_hmac_buffer;
+  ret = libtock_hmac_set_readwrite_allow_destination_buffer(hmac_buffer, hmac_length);
+  if (ret != RETURNCODE_SUCCESS) return ret;
+  defer { libtock_hmac_set_readwrite_allow_destination_buffer(NULL, 0); };
 
   ret = libtock_hmac_command_run();
   if (ret != RETURNCODE_SUCCESS) return ret;

libtock/defer.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define __DEFER__(F, V) \
+  auto void F(int*); \
+  [[gnu::cleanup(F)]] int V; \
+  auto void F(int*)
+
+#define defer __DEFER(__COUNTER__)
+#define __DEFER(N) __DEFER_(N)
+#define __DEFER_(N) __DEFER__(__DEFER_FUNCTION_ ## N, __DEFER_VARIABLE_ ## N)
+
+#ifdef __cplusplus
+}
+#endif