net: lwm2m: add cache filtering

Introduce `lwm2m_set_cache_filter()` so applications can drop cached
samples before they reach the LwM2M SEND path.

Fixes #91590

Signed-off-by: Julien Vermillard <[email protected]>

Author: Julien Vermillard

doc/connectivity/networking/api/lwm2m.rst

@@ -532,9 +532,9 @@ Read and Write operations
 Full content of data cache is written into a payload when any READ, SEND or NOTIFY operation
 internally reads the content of a given resource. This has a side effect that any read callbacks
 registered for a that resource are ignored when cache is enabled.
-Data is written into a cache when any of the ``lwm2m_set_*`` functions are called. To filter
-the data entering the cache, application may register a validation callback using
-:c:func:`lwm2m_register_validate_callback`.
+Data is written into a cache when any of the ``lwm2m_set_*`` functions are called. Applications can
+register a cache filter callback with :c:func:`lwm2m_set_cache_filter` to drop otherwise valid samples
+based on application-specific rules.
 
 Limitations
 ===========

include/zephyr/net/lwm2m.h

@@ -348,6 +348,20 @@ struct lwm2m_time_series_elem {
 	};
 };
 
+/**
+ * @typedef lwm2m_cache_filter_cb_t
+ * @brief Callback type for filtering cached time series samples.
+ *
+ * Returning false skips storing the provided sample in the LwM2M cache.
+ *
+ * @param path    Object path of the cached resource.
+ * @param element Sample produced by the engine for the cache.
+ *
+ * @return true to keep the sample, false to discard it.
+ */
+typedef bool (*lwm2m_cache_filter_cb_t)(const struct lwm2m_obj_path *path,
+					const struct lwm2m_time_series_elem *element);
+
 /**
  * @brief Asynchronous callback to get a resource buffer and length.
  *
@@ -1590,6 +1604,21 @@ struct lwm2m_ctx *lwm2m_rd_client_ctx(void);
 int lwm2m_enable_cache(const struct lwm2m_obj_path *path, struct lwm2m_time_series_elem *data_cache,
 		       size_t cache_len);
 
+/**
+ * @brief Register a filtering callback for cached resource samples.
+ *
+ * The callback is invoked whenever the LwM2M engine attempts to append a new
+ * sample to the resource cache. Returning false prevents the sample from being
+ * stored. Passing a NULL callback removes any previously registered filter.
+ *
+ * @param path      LwM2M path to the cached resource.
+ * @param filter_cb Callback used to decide whether samples should be cached.
+ *
+ * @return 0 for success or a negative errno code in case of error.
+ */
+int lwm2m_set_cache_filter(const struct lwm2m_obj_path *path,
+			   lwm2m_cache_filter_cb_t filter_cb);
+
 /**
  * @brief Security modes as defined in LwM2M Security object.
  */

subsys/net/lib/lwm2m/lwm2m_registry.c

@@ -1466,6 +1466,7 @@ lwm2m_cache_entry_allocate(const struct lwm2m_obj_path *path)
 	for (i = 0; i < ARRAY_SIZE(lwm2m_cache_entries); i++) {
 		if (lwm2m_cache_entries[i].path.level == 0) {
 			lwm2m_cache_entries[i].path = *path;
+			lwm2m_cache_entries[i].filter_cb = NULL;
 			sys_slist_append(&lwm2m_timed_cache_list, &lwm2m_cache_entries[i].node);
 			return &lwm2m_cache_entries[i];
 		}
@@ -1542,6 +1543,14 @@ static void lwm2m_engine_cache_write(const struct lwm2m_engine_obj_field *obj_fi
 		break;
 	}
 
+	if (cache_entry->filter_cb &&
+	    !cache_entry->filter_cb(&cache_entry->path, &elements)) {
+		LOG_DBG("Cache filter dropped sample for %u/%u/%u",
+			cache_entry->path.obj_id, cache_entry->path.obj_inst_id,
+			cache_entry->path.res_id);
+		return;
+	}
+
 	if (!lwm2m_cache_write(cache_entry, &elements)) {
 		LOG_WRN("Data cache full");
 	}
@@ -1628,6 +1637,29 @@ int lwm2m_enable_cache(const struct lwm2m_obj_path *path, struct lwm2m_time_seri
 #endif /* CONFIG_LWM2M_RESOURCE_DATA_CACHE_SUPPORT */
 }
 
+int lwm2m_set_cache_filter(const struct lwm2m_obj_path *path,
+			   lwm2m_cache_filter_cb_t filter_cb)
+{
+#if defined(CONFIG_LWM2M_RESOURCE_DATA_CACHE_SUPPORT)
+	struct lwm2m_time_series_resource *cache_entry;
+
+	if (path == NULL) {
+		return -EINVAL;
+	}
+
+	cache_entry = lwm2m_cache_entry_get_by_object(path);
+	if (cache_entry == NULL) {
+		return -ENOENT;
+	}
+
+	cache_entry->filter_cb = filter_cb;
+
+	return 0;
+#else
+	return -ENOTSUP;
+#endif /* CONFIG_LWM2M_RESOURCE_DATA_CACHE_SUPPORT */
+}
+
 #if defined(CONFIG_LWM2M_RESOURCE_DATA_CACHE_SUPPORT)
 static int lwm2m_engine_data_cache_init(void)
 {

subsys/net/lib/lwm2m/lwm2m_registry.h

@@ -209,6 +209,8 @@ struct lwm2m_time_series_resource {
 	sys_snode_t node;
 	/* Resource Path url */
 	struct lwm2m_obj_path path;
+	/* Optional filter for cached samples */
+	lwm2m_cache_filter_cb_t filter_cb;
 	/* Ring buffer */
 	struct ring_buf rb;
 };

tests/net/lib/lwm2m/lwm2m_registry/src/lwm2m_registry.c

@@ -628,6 +628,7 @@ ZTEST(lwm2m_registry, test_resource_cache)
 	/* Resource cache is turned off */
 	zassert_is_null(lwm2m_cache_entry_get_by_object(&path));
 	zassert_equal(lwm2m_enable_cache(&path, &e, 1), -ENOTSUP);
+	zassert_equal(lwm2m_set_cache_filter(&path, NULL), -ENOTSUP);
 	zassert_false(lwm2m_cache_write(NULL, NULL));
 	zassert_false(lwm2m_cache_read(NULL, NULL));
 	zassert_equal(lwm2m_cache_size(NULL), 0);