zed.d/history_event-zfs-list-cacher.sh.in: parallelise, simplify

This:
  (a) improves the error log message,
  (b) locks per pool instead of globally,
  (c) locks the actual output file instead of /var/lock/zfs-list,
      which would otherwise linger there forever (well, still will,
      but you can remove it and it won't come back), and
  (d) preserves attributes of the output file
      instead of reverting them to 0:0 644

It is imperative that the previous commit
("zed-functions.sh: zed_lock(): don't truncate lock")
be included in any series that contains this one

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12042

cmd/zed/zed.d/history_event-zfs-list-cacher.sh.in

@@ -3,9 +3,8 @@
 # Track changes to enumerated pools for use in early-boot
 set -ef
 
-FSLIST_DIR="@sysconfdir@/zfs/zfs-list.cache"
-FSLIST_TMP="@runstatedir@/zfs-list.cache.new"
-FSLIST="${FSLIST_DIR}/${ZEVENT_POOL}"
+FSLIST="@sysconfdir@/zfs/zfs-list.cache/${ZEVENT_POOL}"
+FSLIST_TMP="@runstatedir@/zfs-list.cache@${ZEVENT_POOL}"
 
 # If the pool specific cache file is not writeable, abort
 [ -w "${FSLIST}" ] || exit 0
@@ -19,15 +18,15 @@ zed_check_cmd "${ZFS}" sort diff
 # If we are acting on a snapshot, we have nothing to do
 [ "${ZEVENT_HISTORY_DSNAME%@*}" = "${ZEVENT_HISTORY_DSNAME}" ] || exit 0
 
-# We obtain a lock on zfs-list to avoid any simultaneous writes.
+# We lock the output file to avoid simultaneous writes.
 # If we run into trouble, log and drop the lock
 abort_alter() {
-  zed_log_msg "Error updating zfs-list.cache!"
-  zed_unlock zfs-list
+  zed_log_msg "Error updating zfs-list.cache for ${ZEVENT_POOL}!"
+  zed_unlock "${FSLIST}"
 }
 
 finished() {
-  zed_unlock zfs-list
+  zed_unlock "${FSLIST}"
   trap - EXIT
   exit 0
 }
@@ -37,7 +36,7 @@ case "${ZEVENT_HISTORY_INTERNAL_NAME}" in
       ;;
 
     export)
-        zed_lock zfs-list
+        zed_lock "${FSLIST}"
         trap abort_alter EXIT
         echo > "${FSLIST}"
         finished
@@ -63,7 +62,7 @@ case "${ZEVENT_HISTORY_INTERNAL_NAME}" in
       ;;
 esac
 
-zed_lock zfs-list
+zed_lock "${FSLIST}"
 trap abort_alter EXIT
 
 PROPS="name,mountpoint,canmount,atime,relatime,devices,exec\
@@ -79,7 +78,7 @@ PROPS="name,mountpoint,canmount,atime,relatime,devices,exec\
 sort "${FSLIST_TMP}" -o "${FSLIST_TMP}"
 
 # Don't modify the file if it hasn't changed
-diff -q "${FSLIST_TMP}" "${FSLIST}" || mv "${FSLIST_TMP}" "${FSLIST}"
+diff -q "${FSLIST_TMP}" "${FSLIST}" || cat "${FSLIST_TMP}" > "${FSLIST}"
 rm -f "${FSLIST_TMP}"
 
 finished