nvpair: Constify string functions

After addressing coverity complaints involving `nvpair_name()`, the
compiler started complaining about dropping const. This lead to a rabbit
hole where not only `nvpair_name()` needed to be constified, but also
`nvpair_value_string()`, `fnvpair_value_string()` and a few other static
functions, plus variable pointers throughout the code. The result became
a fairly big change, so it has been split out into its own patch.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14612

lib/libzutil/os/linux/zutil_import_os.c

@@ -168,9 +168,9 @@ zpool_open_func(void *arg)
 	 * Add additional entries for paths described by this label.
 	 */
 	if (rn->rn_labelpaths) {
-		char *path = NULL;
-		char *devid = NULL;
-		char *env = NULL;
+		const char *path = NULL;
+		const char *devid = NULL;
+		const char *env = NULL;
 		rdsk_node_t *slice;
 		avl_index_t where;
 		int timeout;
@@ -769,7 +769,7 @@ no_dev:
  *    vdev_enc_sysfs_path: '/sys/class/enclosure/11:0:1:0/SLOT 4'
  */
 static void
-update_vdev_config_dev_sysfs_path(nvlist_t *nv, char *path)
+update_vdev_config_dev_sysfs_path(nvlist_t *nv, const char *path)
 {
 	char *upath, *spath;
 
@@ -795,7 +795,7 @@ sysfs_path_pool_vdev_iter_f(void *hdl_data, nvlist_t *nv, void *data)
 {
 	(void) hdl_data, (void) data;
 
-	char *path = NULL;
+	const char *path = NULL;
 	if (nvlist_lookup_string(nv, ZPOOL_CONFIG_PATH, &path) != 0)
 		return (1);
 
@@ -841,7 +841,7 @@ void
 update_vdev_config_dev_strs(nvlist_t *nv)
 {
 	vdev_dev_strs_t vds;
-	char *env, *type, *path;
+	const char *env, *type, *path;
 	uint64_t wholedisk = 0;
 
 	/*

lib/libzutil/zutil_import.c

@@ -233,7 +233,7 @@ fix_paths(libpc_handle_t *hdl, nvlist_t *nv, name_entry_t *names)
 	uint_t c, children;
 	uint64_t guid;
 	name_entry_t *ne, *best;
-	char *path;
+	const char *path;
 
 	if (nvlist_lookup_nvlist_array(nv, ZPOOL_CONFIG_CHILDREN,
 	    &child, &children) == 0) {
@@ -497,7 +497,7 @@ get_configs(libpc_handle_t *hdl, pool_list_t *pl, boolean_t active_ok,
 	uint_t i, nspares, nl2cache;
 	boolean_t config_seen;
 	uint64_t best_txg;
-	char *name, *hostname = NULL;
+	const char *name, *hostname = NULL;
 	uint64_t guid;
 	uint_t children = 0;
 	nvlist_t **child = NULL;
@@ -591,8 +591,8 @@ get_configs(libpc_handle_t *hdl, pool_list_t *pl, boolean_t active_ok,
 				 *	hostname (if available)
 				 */
 				uint64_t state, version;
-				char *comment = NULL;
-				char *compatibility = NULL;
+				const char *comment = NULL;
+				const char *compatibility = NULL;
 
 				version = fnvlist_lookup_uint64(tmp,
 				    ZPOOL_CONFIG_VERSION);
@@ -1161,12 +1161,12 @@ slice_cache_compare(const void *arg1, const void *arg2)
 
 static int
 label_paths_impl(libpc_handle_t *hdl, nvlist_t *nvroot, uint64_t pool_guid,
-    uint64_t vdev_guid, char **path, char **devid)
+    uint64_t vdev_guid, const char **path, const char **devid)
 {
 	nvlist_t **child;
 	uint_t c, children;
 	uint64_t guid;
-	char *val;
+	const char *val;
 	int error;
 
 	if (nvlist_lookup_nvlist_array(nvroot, ZPOOL_CONFIG_CHILDREN,
@@ -1204,7 +1204,8 @@ label_paths_impl(libpc_handle_t *hdl, nvlist_t *nvroot, uint64_t pool_guid,
  * The returned pointers are only valid as long as label remains valid.
  */
 int
-label_paths(libpc_handle_t *hdl, nvlist_t *label, char **path, char **devid)
+label_paths(libpc_handle_t *hdl, nvlist_t *label, const char **path,
+    const char **devid)
 {
 	nvlist_t *nvroot;
 	uint64_t pool_guid;
@@ -1473,7 +1474,7 @@ zpool_find_import_impl(libpc_handle_t *hdl, importargs_t *iarg,
 			}
 
 			if (iarg->poolname != NULL && !aux) {
-				char *pname;
+				const char *pname;
 
 				matched = nvlist_lookup_string(config,
 				    ZPOOL_CONFIG_POOL_NAME, &pname) == 0 &&
@@ -1547,7 +1548,7 @@ static int
 discover_cached_paths(libpc_handle_t *hdl, nvlist_t *nv,
     avl_tree_t *cache, pthread_mutex_t *lock)
 {
-	char *path = NULL;
+	const char *path = NULL;
 	ssize_t dl;
 	uint_t children;
 	nvlist_t **child;
@@ -1567,15 +1568,17 @@ discover_cached_paths(libpc_handle_t *hdl, nvlist_t *nv,
 		int ret;
 		char c = '\0';
 		if ((dl = zfs_dirnamelen(path)) == -1) {
-			path = (char *)".";
+			path = ".";
 		} else {
 			c = path[dl];
-			path[dl] = '\0';
+			((char *)path)[dl] = '\0';
+
 		}
 		ret = zpool_find_import_scan_dir(hdl, lock, cache,
 		    path, 0);
 		if (c != '\0')
-			path[dl] = c;
+			((char *)path)[dl] = c;
+
 		return (ret);
 	}
 	return (0);
@@ -1595,7 +1598,7 @@ zpool_find_import_cached(libpc_handle_t *hdl, importargs_t *iarg)
 	nvlist_t *raw, *src, *dst;
 	nvlist_t *pools;
 	nvpair_t *elem;
-	char *name;
+	const char *name;
 	uint64_t this_guid;
 	boolean_t active;
 
@@ -1825,10 +1828,10 @@ zpool_search_import(libpc_handle_t *hdl, importargs_t *import)
 }
 
 static boolean_t
-pool_match(nvlist_t *cfg, char *tgt)
+pool_match(nvlist_t *cfg, const char *tgt)
 {
 	uint64_t v, guid = strtoull(tgt, NULL, 0);
-	char *s;
+	const char *s;
 
 	if (guid != 0) {
 		if (nvlist_lookup_uint64(cfg, ZPOOL_CONFIG_POOL_GUID, &v) == 0)
@@ -1913,7 +1916,7 @@ for_each_vdev_cb(void *zhp, nvlist_t *nv, pool_vdev_iter_f func,
 	uint_t c, children;
 	int ret = 0;
 	int i;
-	char *type;
+	const char *type;
 
 	const char *list[] = {
 	    ZPOOL_CONFIG_SPARES,

lib/libzutil/zutil_import.h

@@ -33,8 +33,8 @@
 #define	IMPORT_ORDER_SCAN_OFFSET	10
 #define	IMPORT_ORDER_DEFAULT		100
 
-int label_paths(libpc_handle_t *hdl, nvlist_t *label, char **path,
-    char **devid);
+int label_paths(libpc_handle_t *hdl, nvlist_t *label, const char **path,
+    const char **devid);
 int zpool_find_import_blkid(libpc_handle_t *hdl, pthread_mutex_t *lock,
     avl_tree_t **slice_cache);