Cleanup: Address Clang's static analyzer's unused code complaints

These were categorized as the following:

 * Dead assignment		23

 * Dead increment		4

 * Dead initialization		6

 * Dead nested assignment	18

Most of these are harmless, but since actual issues can hide among them,
we correct them.

That said, there were a few return values that were being ignored that
appeared to merit some correction:

 * `destroy_callback()` in `cmd/zfs/zfs_main.c` ignored the error from
   `destroy_batched()`. We handle it by returning -1 if there is an
   error.

 * `zfs_do_upgrade()` in `cmd/zfs/zfs_main.c` ignored the error from
   `zfs_for_each()`. We handle it by doing a binary OR of the error
   value from the subsequent `zfs_for_each()` call to the existing
   value. This is how errors are mostly handled inside `zfs_for_each()`.

 * `get_zap_prop()` in `module/zfs/zcp_get.c` ignored the
   `dsl_prop_get_ds()` when the property is not of type string. We
   return an error if it does. There is a small concern that the
   `zfs_get_temporary_prop()` call would handle things, but in the case
   that it does not, we would be pushing an uninitialized numval onto
   the lua stack. It is expected that `dsl_prop_get_ds()` will succeed
   anytime that `zfs_get_temporary_prop()` does, so that not giving it a
   chance to fix things is not a problem.

 * `draid_merge_impl()` in `tests/zfs-tests/cmd/draid.c` used
   `nvlist_add_nvlist()` twice in ways in which errors are expected to
   be impossible, so we switch to `fnvlist_add_nvlist()`.

A few notable ones did not merit use of the return value, so we
suppressed it with `(void)`:

 * `write_free_diffs()` in `lib/libzfs/libzfs_diff.c` ignored the error
   value from `describe_free()`. A look through the commit history
   revealed that this was intentional.

 * `arc_evict_hdr()` in `module/zfs/arc.c` did not need to use the
   returned handle from `arc_hdr_realloc()` because it is already
   referenced in lists.

 * `spa_vdev_detach()` in `module/zfs/spa.c` has a comment explicitly
   saying not to use the error from `vdev_label_init()` because whatever
   causes the error could be the reason why a detach is being done.

Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>

Author: ryao

cmd/raidz_test/raidz_test.c

@@ -146,8 +146,6 @@ static void process_options(int argc, char **argv)
 	memcpy(o, &rto_opts_defaults, sizeof (*o));
 
 	while ((opt = getopt(argc, argv, "TDBSvha:er:o:d:s:t:")) != -1) {
-		value = 0;
-
 		switch (opt) {
 		case 'a':
 			value = strtoull(optarg, NULL, 0);

cmd/zfs/zfs_main.c

@@ -1453,8 +1453,13 @@ destroy_callback(zfs_handle_t *zhp, void *data)
 	if (zfs_get_type(zhp) == ZFS_TYPE_SNAPSHOT) {
 		cb->cb_snap_count++;
 		fnvlist_add_boolean(cb->cb_batchedsnaps, name);
-		if (cb->cb_snap_count % 10 == 0 && cb->cb_defer_destroy)
+		if (cb->cb_snap_count % 10 == 0 && cb->cb_defer_destroy) {
 			error = destroy_batched(cb);
+			if (error != 0) {
+				zfs_close(zhp);
+				return (-1);
+			}
+		}
 	} else {
 		error = destroy_batched(cb);
 		if (error != 0 ||
@@ -2576,7 +2581,7 @@ zfs_do_upgrade(int argc, char **argv)
 		cb.cb_foundone = B_FALSE;
 		cb.cb_newer = B_TRUE;
 
-		ret = zfs_for_each(0, NULL, flags, ZFS_TYPE_FILESYSTEM,
+		ret |= zfs_for_each(0, NULL, flags, ZFS_TYPE_FILESYSTEM,
 		    NULL, NULL, 0, upgrade_list_callback, &cb);
 
 		if (!cb.cb_foundone && !found) {

lib/libefi/rdwr_efi.c

@@ -423,7 +423,6 @@ efi_alloc_and_read(int fd, struct dk_gpt **vtoc)
 		void *tmp;
 		length = (int) sizeof (struct dk_gpt) +
 		    (int) sizeof (struct dk_part) * (vptr->efi_nparts - 1);
-		nparts = vptr->efi_nparts;
 		if ((tmp = realloc(vptr, length)) == NULL) {
 			/* cppcheck-suppress doubleFree */
 			free(vptr);
@@ -565,10 +564,9 @@ int
 efi_rescan(int fd)
 {
 	int retry = 10;
-	int error;
 
 	/* Notify the kernel a devices partition table has been updated */
-	while ((error = ioctl(fd, BLKRRPART)) != 0) {
+	while ((ioctl(fd, BLKRRPART)) != 0) {
 		if ((--retry == 0) || (errno != EBUSY)) {
 			(void) fprintf(stderr, "the kernel failed to rescan "
 			    "the partition table: %d\n", errno);

lib/libzfs/libzfs_dataset.c

@@ -2006,7 +2006,7 @@ zfs_prop_inherit(zfs_handle_t *zhp, const char *propname, boolean_t received)
 	if ((ret = changelist_prefix(cl)) != 0)
 		goto error;
 
-	if ((ret = zfs_ioctl(zhp->zfs_hdl, ZFS_IOC_INHERIT_PROP, &zc)) != 0) {
+	if (zfs_ioctl(zhp->zfs_hdl, ZFS_IOC_INHERIT_PROP, &zc) != 0) {
 		changelist_free(cl);
 		return (zfs_standard_error(hdl, errno, errbuf));
 	} else {

lib/libzfs/libzfs_diff.c

@@ -377,7 +377,7 @@ write_free_diffs(FILE *fp, differ_info_t *di, dmu_diff_record_t *dr)
 			if (zc.zc_obj > dr->ddr_last) {
 				break;
 			}
-			err = describe_free(fp, di, zc.zc_obj, fobjname,
+			(void) describe_free(fp, di, zc.zc_obj, fobjname,
 			    MAXPATHLEN);
 		} else if (errno == ESRCH) {
 			break;

lib/libzfs/libzfs_pool.c

@@ -2214,7 +2214,6 @@ zpool_import_props(libzfs_handle_t *hdl, nvlist_t *config, const char *newname,
 			    ((policy.zlp_rewind & ZPOOL_TRY_REWIND) != 0), nv);
 		}
 		nvlist_free(nv);
-		return (0);
 	}
 
 	return (ret);

lib/libzfs/libzfs_sendrecv.c

@@ -2117,9 +2117,9 @@ send_prelim_records(zfs_handle_t *zhp, const char *from, int fd,
 			fnvlist_add_boolean(hdrnv, "raw");
 		}
 
-		if ((err = gather_nvlist(zhp->zfs_hdl, tofs,
+		if (gather_nvlist(zhp->zfs_hdl, tofs,
 		    from, tosnap, recursive, raw, doall, replicate, skipmissing,
-		    verbose, backup, holds, props, &fss, fsavlp)) != 0) {
+		    verbose, backup, holds, props, &fss, fsavlp) != 0) {
 			return (zfs_error(zhp->zfs_hdl, EZFS_BADBACKUP,
 			    errbuf));
 		}

lib/libzfs/libzfs_util.c

@@ -1249,7 +1249,7 @@ zcmd_read_dst_nvlist(libzfs_handle_t *hdl, zfs_cmd_t *zc, nvlist_t **nvlp)
 static void
 zprop_print_headers(zprop_get_cbdata_t *cbp, zfs_type_t type)
 {
-	zprop_list_t *pl = cbp->cb_proplist;
+	zprop_list_t *pl;
 	int i;
 	char *title;
 	size_t len;

lib/libzutil/os/linux/zutil_device_path_os.c

@@ -428,7 +428,6 @@ dm_get_underlying_path(const char *dm_name)
 	char *tmp = NULL;
 	char *path = NULL;
 	char *dev_str;
-	int size;
 	char *first_path = NULL;
 	char *enclosure_path;
 
@@ -450,7 +449,7 @@ dm_get_underlying_path(const char *dm_name)
 	else
 		dev_str = tmp;
 
-	if ((size = asprintf(&tmp, "/sys/block/%s/slaves/", dev_str)) == -1) {
+	if ((asprintf(&tmp, "/sys/block/%s/slaves/", dev_str)) == -1) {
 		tmp = NULL;
 		goto end;
 	}
@@ -479,8 +478,7 @@ dm_get_underlying_path(const char *dm_name)
 			if (!enclosure_path)
 				continue;
 
-			if ((size = asprintf(
-			    &path, "/dev/%s", ep->d_name)) == -1)
+			if ((asprintf(&path, "/dev/%s", ep->d_name)) == -1)
 				path = NULL;
 			free(enclosure_path);
 			break;
@@ -499,7 +497,7 @@ dm_get_underlying_path(const char *dm_name)
 		 * enclosure devices.  Throw up out hands and return the first
 		 * underlying path.
 		 */
-		if ((size = asprintf(&path, "/dev/%s", first_path)) == -1)
+		if ((asprintf(&path, "/dev/%s", first_path)) == -1)
 			path = NULL;
 	}
 

module/icp/algs/blake3/blake3.c

@@ -189,9 +189,7 @@ static void chunk_state_update(const blake3_ops_t *ops,
 		input_len -= BLAKE3_BLOCK_LEN;
 	}
 
-	size_t take = chunk_state_fill_buf(ctx, input, input_len);
-	input += take;
-	input_len -= take;
+	chunk_state_fill_buf(ctx, input, input_len);
 }
 
 static output_t chunk_state_output(const blake3_chunk_state_t *ctx)