settings: add new API function settings_get_val_len()

Add a function to get the value's length of a Key. If it doesn't exist
returns 0.
Add ZMS implementation for csi_get_val_len() and a default implementation
for the other storage systems.
Add some functional tests to verify it.

Signed-off-by: Riadh Ghaddab <rghaddab@baylibre.com>

include/zephyr/settings/settings.h

@@ -289,6 +289,14 @@ int settings_load_subtree(const char *subtree);
  */
 ssize_t settings_load_one(const char *name, void *buf, size_t buf_len);
 
+/**
+ * Get the data length of the value relative to the key
+ *
+ * @param[in] key Name/key of the settings item.
+ * @return length of value if item exists, 0 if not and negative value on failure.
+ */
+ssize_t settings_get_val_len(const char *key);
+
 /**
  * Callback function used for direct loading.
  * Used by @ref settings_load_subtree_direct function.
@@ -479,6 +487,15 @@ struct settings_store_itf {
 	 *  - buf_len - Length of buf.
 	 */
 
+	ssize_t (*csi_get_val_len)(struct settings_store *cs, const char *name);
+	/**< Gets the value's length associated to the Key defined by name.
+	 * It returns 0 if the Key/Value doesn't exist.
+	 *
+	 * Parameters:
+	 *  - cs - Corresponding backend handler node.
+	 *  - name - Key in string format.
+	 */
+
 	int (*csi_save_start)(struct settings_store *cs);
 	/**< Handler called before an export operation.
 	 *

subsys/settings/src/settings_store.c

@@ -94,6 +94,7 @@ struct default_param {
 	size_t *val_len;
 };
 
+/* Default callback to set a Key/Value pair */
 static int settings_set_default_cb(const char *name, size_t len, settings_read_cb read_cb,
 				   void *cb_arg, void *param)
 {
@@ -111,6 +112,60 @@ static int settings_set_default_cb(const char *name, size_t len, settings_read_c
 	return rc;
 }
 
+/* Default callback to get the value's length of the Key defined by name.
+ * Returns the value's length in the provided `param` pointer
+ */
+static int settings_get_val_len_default_cb(const char *name, size_t len,
+					   [[maybe_unused]] settings_read_cb read_cb,
+					   [[maybe_unused]] void *cb_arg, void *param)
+{
+	const char *next;
+	size_t name_len;
+	size_t *val_len = (size_t *)param;
+
+	name_len = settings_name_next(name, &next);
+	if (name_len == 0) {
+		*val_len = len;
+	}
+
+	return 0;
+}
+
+/* Gets the value's size if the Key defined by name is in the persistent storage,
+ * Returns 0 if the Key doesn't exist.
+ */
+ssize_t settings_get_val_len(const char *name)
+{
+	struct settings_store *cs;
+	int rc = 0;
+	size_t val_len = 0;
+
+	/*
+	 * for every config store that supports this function
+	 * get the value's length.
+	 */
+	k_mutex_lock(&settings_lock, K_FOREVER);
+	SYS_SLIST_FOR_EACH_CONTAINER(&settings_load_srcs, cs, cs_next) {
+		if (cs->cs_itf->csi_get_val_len) {
+			val_len = cs->cs_itf->csi_get_val_len(cs, name);
+		} else {
+			const struct settings_load_arg arg = {
+				.subtree = name,
+				.cb = &settings_get_val_len_default_cb,
+				.param = &val_len
+			};
+			rc = cs->cs_itf->csi_load(cs, &arg);
+		}
+	}
+	k_mutex_unlock(&settings_lock);
+
+	if (rc >= 0) {
+		return val_len;
+	}
+
+	return rc;
+}
+
 /* Load a single key/value from persistent storage */
 ssize_t settings_load_one(const char *name, void *buf, size_t buf_len)
 {

subsys/settings/src/settings_zms.c

@@ -33,11 +33,13 @@ static int settings_zms_save(struct settings_store *cs, const char *name, const
 			     size_t val_len);
 static void *settings_zms_storage_get(struct settings_store *cs);
 static int settings_zms_get_last_hash_ids(struct settings_zms *cf);
+static ssize_t settings_zms_get_val_len(struct settings_store *cs, const char *name);
 
 static struct settings_store_itf settings_zms_itf = {.csi_load = settings_zms_load,
 						     .csi_load_one = settings_zms_load_one,
 						     .csi_save = settings_zms_save,
-						     .csi_storage_get = settings_zms_storage_get};
+						     .csi_storage_get = settings_zms_storage_get,
+						     .csi_get_val_len = settings_zms_get_val_len};
 
 static ssize_t settings_zms_read_fn(void *back_end, void *data, size_t len)
 {
@@ -530,6 +532,44 @@ no_ll_update:
 	return 0;
 }
 
+static ssize_t settings_zms_get_val_len(struct settings_store *cs, const char *name)
+{
+	struct settings_zms *cf = CONTAINER_OF(cs, struct settings_zms, cf_store);
+	char r_name[SETTINGS_MAX_NAME_LEN + SETTINGS_EXTRA_LEN + 1];
+	ssize_t rc = 0;
+	uint32_t name_hash;
+
+	/* verify that name is not NULL */
+	if (!name) {
+		return -EINVAL;
+	}
+
+	name_hash = sys_hash32(name, strlen(name)) & ZMS_HASH_MASK;
+	for (int i = 0; i <= cf->hash_collision_num; i++) {
+		name_hash = ZMS_UPDATE_COLLISION_NUM(name_hash, i);
+		/* Get the name entry from ZMS */
+		rc = zms_read(&cf->cf_zms, ZMS_NAME_ID_FROM_HASH(name_hash), r_name,
+			      sizeof(r_name) - 1);
+		if (rc <= 0) {
+			/* Name doesn't exist */
+			continue;
+		}
+		/* Found a name, this might not include a trailing \0 */
+		r_name[rc] = '\0';
+		if (strcmp(name, r_name)) {
+			/* Names are not equal let's continue to the next collision hash
+			 * if it exists.
+			 */
+			continue;
+		}
+		/* At this steps the names are equal, let's read the data size*/
+		return zms_get_data_length(&cf->cf_zms,
+					   ZMS_NAME_ID_FROM_HASH(name_hash) + ZMS_DATA_ID_OFFSET);
+	}
+
+	return 0;
+}
+
 static int settings_zms_get_last_hash_ids(struct settings_zms *cf)
 {
 	struct settings_hash_linked_list settings_element;

tests/subsys/settings/functional/src/settings_basic_test.c

@@ -240,13 +240,22 @@ ZTEST(settings_functional, test_register_and_loading)
 {
 	int rc, err;
 	uint8_t val = 0;
+	ssize_t val_len = 0;
 
 	rc = settings_subsys_init();
 	zassert_true(rc == 0, "subsys init failed");
 
 
+	/* Check that key that corresponds to val2 do not exist in storage */
+	val_len = settings_get_val_len("ps/ss/ss/val2");
+	zassert_true((val_len == 0), "Failure: key should not exist");
+
 	settings_save_one("ps/ss/ss/val2", &val, sizeof(uint8_t));
 
+	/* Check that the key that corresponds to val2 exists in storage */
+	val_len = settings_get_val_len("ps/ss/ss/val2");
+	zassert_true((val_len == 1), "Failure: key should exist");
+
 	memset(&data, 0, sizeof(struct stored_data));
 
 	rc = settings_register(&val1_settings);
@@ -279,7 +288,16 @@ ZTEST(settings_functional, test_register_and_loading)
 	err = (data.en1) && (data.en2) && (!data.en3);
 	zassert_true(err, "wrong data enable found");
 
+	/* Check that key that corresponds to val3 do not exist in storage */
+	val_len = settings_get_val_len("ps/ss/val3");
+	zassert_true((val_len == 0), "Failure: key should not exist");
+
 	settings_save_one("ps/ss/val3", &val, sizeof(uint8_t));
+
+	/* Check that the key that corresponds to val3 exists in storage */
+	val_len = settings_get_val_len("ps/ss/val3");
+	zassert_true((val_len == 1), "Failure: key should exist");
+
 	memset(&data, 0, sizeof(struct stored_data));
 	/* when we load settings now data.val2 and data.val1 should receive a
 	 * value
@@ -310,7 +328,16 @@ ZTEST(settings_functional, test_register_and_loading)
 	err = (data.en1) && (data.en2) && (data.en3);
 	zassert_true(err, "wrong data enable found");
 
+	/* Check that key that corresponds to val1 do not exist in storage */
+	val_len = settings_get_val_len("ps/val1");
+	zassert_true((val_len == 0), "Failure: key should not exist");
+
 	settings_save_one("ps/val1", &val, sizeof(uint8_t));
+
+	/* Check that the key that corresponds to val1 exists in storage */
+	val_len = settings_get_val_len("ps/val1");
+	zassert_true((val_len == 1), "Failure: key should exist");
+
 	memset(&data, 0, sizeof(struct stored_data));
 	/* when we load settings all data should receive a value loaded */
 	rc = settings_load();