libzfs: get rid of libzfs_handle::libzfs_mnttab

All users did a freopen() on it. Even some non-users did!
This is point-less ‒ just open the mtab when needed

If I understand Solaris' getextmntent(3C) correctly, the non-user
freopen()s are very likely an odd, twisted vestigial tail of that ‒
but it's got a completely different calling convention and caching
semantics than any platform we support

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11868

lib/libzfs/libzfs_dataset.c

@@ -808,13 +808,13 @@ libzfs_mnttab_init(libzfs_handle_t *hdl)
 static int
 libzfs_mnttab_update(libzfs_handle_t *hdl)
 {
+	FILE *mnttab;
 	struct mnttab entry;
 
-	/* Reopen MNTTAB to prevent reading stale data from open file */
-	if (freopen(MNTTAB, "re", hdl->libzfs_mnttab) == NULL)
+	if ((mnttab = fopen(MNTTAB, "re")) == NULL)
 		return (ENOENT);
 
-	while (getmntent(hdl->libzfs_mnttab, &entry) == 0) {
+	while (getmntent(mnttab, &entry) == 0) {
 		mnttab_node_t *mtn;
 		avl_index_t where;
 
@@ -840,6 +840,7 @@ libzfs_mnttab_update(libzfs_handle_t *hdl)
 		avl_add(&hdl->libzfs_mnttab_cache, mtn);
 	}
 
+	(void) fclose(mnttab);
 	return (0);
 }
 
@@ -871,6 +872,7 @@ int
 libzfs_mnttab_find(libzfs_handle_t *hdl, const char *fsname,
     struct mnttab *entry)
 {
+	FILE *mnttab;
 	mnttab_node_t find;
 	mnttab_node_t *mtn;
 	int ret = ENOENT;
@@ -881,16 +883,14 @@ libzfs_mnttab_find(libzfs_handle_t *hdl, const char *fsname,
 		if (avl_numnodes(&hdl->libzfs_mnttab_cache))
 			libzfs_mnttab_fini(hdl);
 
-		/* Reopen MNTTAB to prevent reading stale data from open file */
-		if (freopen(MNTTAB, "re", hdl->libzfs_mnttab) == NULL)
+		if ((mnttab = fopen(MNTTAB, "re")) == NULL)
 			return (ENOENT);
 
 		srch.mnt_special = (char *)fsname;
 		srch.mnt_fstype = MNTTYPE_ZFS;
-		if (getmntany(hdl->libzfs_mnttab, entry, &srch) == 0)
-			return (0);
-		else
-			return (ENOENT);
+		ret = getmntany(mnttab, entry, &srch) ? ENOENT : 0;
+		(void) fclose(mnttab);
+		return (ret);
 	}
 
 	pthread_mutex_lock(&hdl->libzfs_mnttab_cache_lock);

lib/libzfs/libzfs_mount.c

@@ -1528,6 +1528,7 @@ int
 zpool_disable_datasets(zpool_handle_t *zhp, boolean_t force)
 {
 	int used, alloc;
+	FILE *mnttab;
 	struct mnttab entry;
 	size_t namelen;
 	char **mountpoints = NULL;
@@ -1539,12 +1540,11 @@ zpool_disable_datasets(zpool_handle_t *zhp, boolean_t force)
 
 	namelen = strlen(zhp->zpool_name);
 
-	/* Reopen MNTTAB to prevent reading stale data from open file */
-	if (freopen(MNTTAB, "re", hdl->libzfs_mnttab) == NULL)
+	if ((mnttab = fopen(MNTTAB, "re")) == NULL)
 		return (ENOENT);
 
 	used = alloc = 0;
-	while (getmntent(hdl->libzfs_mnttab, &entry) == 0) {
+	while (getmntent(mnttab, &entry) == 0) {
 		/*
 		 * Ignore non-ZFS entries.
 		 */
@@ -1646,6 +1646,7 @@ zpool_disable_datasets(zpool_handle_t *zhp, boolean_t force)
 
 	ret = 0;
 out:
+	(void) fclose(mnttab);
 	for (i = 0; i < used; i++) {
 		if (datasets[i])
 			zfs_close(datasets[i]);

lib/libzfs/libzfs_util.c

@@ -1025,19 +1025,8 @@ libzfs_init(void)
 		return (NULL);
 	}
 
-#ifdef HAVE_SETMNTENT
-	if ((hdl->libzfs_mnttab = setmntent(MNTTAB, "re")) == NULL) {
-#else
-	if ((hdl->libzfs_mnttab = fopen(MNTTAB, "re")) == NULL) {
-#endif
-		(void) close(hdl->libzfs_fd);
-		free(hdl);
-		return (NULL);
-	}
-
 	if (libzfs_core_init() != 0) {
 		(void) close(hdl->libzfs_fd);
-		(void) fclose(hdl->libzfs_mnttab);
 		free(hdl);
 		return (NULL);
 	}
@@ -1056,7 +1045,6 @@ libzfs_init(void)
 		    &hdl->libzfs_max_nvlist))) {
 			errno = error;
 			(void) close(hdl->libzfs_fd);
-			(void) fclose(hdl->libzfs_mnttab);
 			free(hdl);
 			return (NULL);
 		}
@@ -1087,12 +1075,6 @@ void
 libzfs_fini(libzfs_handle_t *hdl)
 {
 	(void) close(hdl->libzfs_fd);
-	if (hdl->libzfs_mnttab)
-#ifdef HAVE_SETMNTENT
-		(void) endmntent(hdl->libzfs_mnttab);
-#else
-		(void) fclose(hdl->libzfs_mnttab);
-#endif
 	zpool_free_handles(hdl);
 	namespace_clear(hdl);
 	libzfs_mnttab_fini(hdl);
@@ -1139,10 +1121,6 @@ zfs_path_to_zhandle(libzfs_handle_t *hdl, const char *path, zfs_type_t argtype)
 		return (zfs_open(hdl, path, argtype));
 	}
 
-	/* Reopen MNTTAB to prevent reading stale data from open file */
-	if (freopen(MNTTAB, "re", hdl->libzfs_mnttab) == NULL)
-		return (NULL);
-
 	if (getextmntent(path, &entry, &statbuf) != 0)
 		return (NULL);