galera SST scripts: removing obsolete xtrabackup_pid support

Removed handling of the long-unsupported xtrabackup_pid file,
as it is not even created by modern versions of mariabackup.
Instead, added stopping of the asynchronous process that
mariabackup runs (if it is still active) to the exception
handler.

scripts/wsrep_sst_mariabackup.sh

@@ -687,16 +687,16 @@ cleanup_at_exit()
     fi
 
     if [ "$WSREP_SST_OPT_ROLE" = 'joiner' ]; then
-        wsrep_log_info "Removing the sst_in_progress file"
-        wsrep_cleanup_progress_file
-    else
         if [ -n "$BACKUP_PID" ]; then
-            if check_pid "$BACKUP_PID" 1; then
+            if check_pid $BACKUP_PID; then
                 wsrep_log_error \
                     "mariadb-backup process is still running. Killing..."
-                cleanup_pid $CHECK_PID "$BACKUP_PID"
+                cleanup_pid $CHECK_PID
             fi
         fi
+        wsrep_log_info "Removing the sst_in_progress file"
+        wsrep_cleanup_progress_file
+    else
         [ -f "$DATA/$IST_FILE" ] && rm -f "$DATA/$IST_FILE" || :
     fi
 
@@ -1220,9 +1220,6 @@ if [ "$WSREP_SST_OPT_ROLE" = 'donor' ]; then
             exit 22
         fi
 
-        # mariadb-backup implicitly writes PID to fixed location in $xtmpdir
-        BACKUP_PID="$xtmpdir/xtrabackup_pid"
-
     else # BYPASS FOR IST
 
         wsrep_log_info "Bypassing the SST for IST"
@@ -1402,7 +1399,7 @@ else # joiner
         fi
         mkdir -p "$DATA/.sst"
         (recv_joiner "$DATA/.sst" "$stagemsg-SST" 0 0 0) &
-        jpid=$!
+        BACKUP_PID=$!
         wsrep_log_info "Proceeding with SST"
 
         get_binlog
@@ -1447,6 +1444,7 @@ else # joiner
 
         # Deleting files from previous SST and legacy files from old versions:
         [ -f "$DATA/xtrabackup_binary" ]      && rm -f "$DATA/xtrabackup_binary"
+        [ -f "$DATA/xtrabackup_pid" ]         && rm -f "$DATA/xtrabackup_pid"
         [ -f "$DATA/xtrabackup_checkpoints" ] && rm -f "$DATA/xtrabackup_checkpoints"
         [ -f "$DATA/xtrabackup_info" ]        && rm -f "$DATA/xtrabackup_info"
       # [ -f "$DATA/xtrabackup_slave_info" ]  && rm -f "$DATA/xtrabackup_slave_info"
@@ -1457,7 +1455,8 @@ else # joiner
         MAGIC_FILE="$DATA/$INFO_FILE"
 
         wsrep_log_info "Waiting for SST streaming to complete!"
-        monitor_process $jpid
+        monitor_process $BACKUP_PID
+        BACKUP_PID=""
 
         if [ ! -s "$DATA/xtrabackup_checkpoints" ]; then
             wsrep_log_error "xtrabackup_checkpoints missing," \