MDEV-15442 xtrabackup-v2 SST donor stuck in DONOR/DESYNCED state when joiner is killed

Monitor mysqld process and if it is terminated during sst stream kill xtrabackup-v2 script and children

scripts/wsrep_sst_xtrabackup-v2.sh

@@ -801,6 +801,27 @@ check_for_version()
     fi
 }
 
+monitor_process()
+{
+    local sst_stream_pid=$1
+
+    while true ; do
+
+        if ! ps --pid "${WSREP_SST_OPT_PARENT}" &>/dev/null; then
+            wsrep_log_error "Parent mysqld process (PID:${WSREP_SST_OPT_PARENT}) terminated unexpectedly." 
+            kill -- -"${WSREP_SST_OPT_PARENT}"
+            exit 32
+        fi
+
+        if ! ps --pid "${sst_stream_pid}" &>/dev/null; then 
+            break
+        fi
+
+        sleep 0.1
+
+    done
+}
+
 
 if [[ ! -x `which $INNOBACKUPEX_BIN` ]];then 
     wsrep_log_error "innobackupex not in path: $PATH"
@@ -1102,7 +1123,7 @@ then
 
         MAGIC_FILE="${DATA}/${INFO_FILE}"
         wsrep_log_info "Waiting for SST streaming to complete!"
-        wait $jpid
+        monitor_process $jpid
 
         get_proc