nanocoap_cache: add helper function to check if entry is stale

sys/include/net/nanocoap/cache.h

@@ -23,6 +23,7 @@
 #define NET_NANOCOAP_CACHE_H
 
 #include <assert.h>
+#include <stdbool.h>
 #include <stdint.h>
 #include "clist.h"
 #include "net/nanocoap.h"
@@ -89,7 +90,7 @@ typedef struct {
      * @brief absolute system time in seconds until which this cache entry
      * is considered valid.
      */
-    ztimer_now_t max_age;
+    uint32_t max_age;
 } nanocoap_cache_entry_t;
 
 /**
@@ -230,6 +231,21 @@ void nanocoap_cache_key_generate(const coap_pkt_t *req, uint8_t *cache_key);
  */
 ssize_t nanocoap_cache_key_compare(uint8_t *cache_key1, uint8_t *cache_key2);
 
+/**
+ * @brief   Check if the Max-Age of a cache entry has passed
+ *
+ * @param[in] ce    A cache entry
+ * @param[in] now   The current time
+ *
+ * @return true, if Max-Age of cache entry has passed.
+ * @return false, if Max-Age of cache entry has not yet passed.
+ */
+static inline bool nanocoap_cache_entry_is_stale(const nanocoap_cache_entry_t *ce, uint32_t now)
+{
+    /* see https://en.wikipedia.org/w/index.php?title=Serial_number_arithmetic&oldid=1085516466#General_solution */
+    return ((int)(now - ce->max_age) > 0);
+}
+
 #ifdef __cplusplus
 }
 #endif

sys/net/application_layer/gcoap/gcoap.c

@@ -1253,8 +1253,7 @@ static bool _cache_lookup(gcoap_request_memo_t *memo,
         /* cache hit, methods are equal, and cache entry is not stale */
         if (*ce &&
             ((*ce)->request_method == coap_get_code(pdu)) &&
-            /* if now < (*ce)->max_age we have an overflow => result large as (*ce)->max_age */
-            ((unsigned)(now - (*ce)->max_age) > (*ce)->max_age)) {
+            !nanocoap_cache_entry_is_stale(*ce, now)) {
             return true;
         }
     }

tests/unittests/tests-nanocoap_cache/tests-nanocoap_cache.c

@@ -231,7 +231,7 @@ static void test_nanocoap_cache__max_age(void)
     /* the absolute time of max-age should be at approx. now + 30 sec
        (1 sec buffer) */
     now = ztimer_now(ZTIMER_SEC);
-    TEST_ASSERT(c->max_age < (now + 31));
+    TEST_ASSERT(nanocoap_cache_entry_is_stale(c, now + 31));
 
     /* delete previously added cache entry */
     nanocoap_cache_del(c);
@@ -251,7 +251,10 @@ static void test_nanocoap_cache__max_age(void)
     /* the absolute time of max-age should be at approx. now + 60 sec
        (1 sec buffer) */
     now = ztimer_now(ZTIMER_SEC);
-    TEST_ASSERT(c->max_age < (now + 61));
+    TEST_ASSERT(nanocoap_cache_entry_is_stale(c, now + 61));
+    /* check overflow cases */
+    c->max_age = UINT32_MAX - 40;
+    TEST_ASSERT(nanocoap_cache_entry_is_stale(c, 20));
 }
 
 Test *tests_nanocoap_cache_tests(void)