MDEV-22159: Don't redirect as root to a tmp file not owned by root

Also add a check for tmp file being empty and bail out with a clear
error message in such a case, as mysqld_safe prevents normal stderr
from being displayed anywhere and would fail silently on this.

scripts/mysqld_safe.sh

@@ -263,7 +263,9 @@ wsrep_recover_position() {
   fi
 
   if [ -f $wr_logfile ]; then
-    [ "$euid" = "0" ] && chown $user $wr_logfile
+    # NOTE! Do not change ownership of the temporary file, as on newer kernel
+    # versions fs.protected_regular is set to '2' and redirecting output with >
+    # as root to a file not owned by root will fail with "Permission denied"
     chmod 600 $wr_logfile
   else
     log_error "WSREP: mktemp failed"
@@ -278,6 +280,11 @@ wsrep_recover_position() {
 
   eval "$mysqld_cmd --wsrep_recover $wr_options 2> $wr_logfile"
 
+  if [ ! -s "$wr_logfile" ]; then
+    log_error "Log file $wr_logfile was empty, cannot proceed. Is system running fs.protected_regular?"
+    exit 1
+  fi
+
   local rp="$(grep 'WSREP: Recovered position:' $wr_logfile)"
   if [ -z "$rp" ]; then
     local skipped="$(grep WSREP $wr_logfile | grep 'skipping position recovery')"