MDEV-8743: wsrep_sst_common close FDs > 2

To prevent accidential overwriting of files due to mysqld file
descriptors being open we ensure where that only FDs 1, 2 and 3
are available to SST scripts. Assumed is mysqld has thse still
mapped to stdin (unused), stdout and stderr.

scripts/wsrep_sst_common.sh

@@ -16,6 +16,28 @@
 
 # This is a common command line parser to be sourced by other SST scripts
 
+# Close file descriptors numbered above 2 just in case mysqld or the
+# wsrep_provider left a file descriptor open. We don't want a broken SST
+# script or called program from overwriting a tablespace because
+# its file descriptor was left open.
+if [ -d /proc/self/fd ]; then
+  for fd in /proc/self/fd/*; do
+    fd=${fd##*/}
+    # While bash would support the below syntax other sh's don't
+    #[ $fd -gt 2 ] && exec {fd}>&-
+    [ $fd -gt 2 ] && eval "exec ${fd}>&-" 2> /dev/null
+  done
+else
+  upper_no_file=$(ulimit -n)
+  [ "$upper_no_file" = "unlimited" ] || [ -z "$upper_no_file" ] && upper_no_file=1024
+
+  while [ $upper_no_file -gt 2 ]
+  do
+     eval "exec ${upper_no_file}>&-"
+     upper_no_file=$(( "$upper_no_file" - 1 ))
+  done
+fi
+
 set -u
 
 WSREP_SST_OPT_BYPASS=0