MDEV-24845 Oddities around innodb_fatal_semaphore_wait_threshold and global.innodb_disallow_writes

This commit adds a mtr test for reproducing a test scenario where despite of
innodb_disallow_writes blocking, writes to file system can still happen.

The test launches a garbd node, which triggers one of the cluster node to switch to
SST donor state. In this state, all disk activity should be halted, and e.g.
innodb_disallow_writes has been set. The test records md5sum aggregate over mariadb
data directory when the node enters the donor state, and records another md5sum
when the node leaves the donor state. If there is no IO activity in data directory, these
hashes should be equal.

For this test, the Donor state processing, has beeen instrumented so that, SST donor thread can be
stopped when entering the donor state. The test uses this new dbug sync point,
to control when to record the md5sums.

New SST script was added: wsrep_sst_backup, and garbd uses backup method to lauch the donor
node to call this script, and to enter in donor state.

The backup script could be later extended as general purpose backup method for the cluster.

This commit fixes also one race condition happening in wsrep_sst_rsync, like this:
* wsrep_rsync_sst script requests for flush tables,
  and then waits in a loop until mariadbd has created file tables_flushed,
  as confirmation that FLUSH TABLES has completed
* mariadbd's SST donor thread, wakes for the flush table request and then performs FTWRL,
  and after this it creates the tables_flushed file
* note that SST script will now continue to startup rsync sending
* mariadbd's SST donor thread now calls for sst_disallow_writes(),
  so that innodb would setup disk IO blockage, however rsyncing may already be ongoing at this point

This race condition is fixed in this commit, by performing all disk IO blocking before
creating the tables_flushed file.

Reviewed-by: Jan Lindström <jan.lindstrom@mariadb.com>

2 files changed, 113 insertions, 0 deletions

diff --git a/scripts/CMakeLists.txt b/scripts/CMakeLists.txt
index 41b4e556835..bd8aac00012 100644
--- a/scripts/CMakeLists.txt
+++ b/scripts/CMakeLists.txt
@@ -275,6 +275,7 @@ ELSE()
       wsrep_sst_mysqldump
       wsrep_sst_rsync
       wsrep_sst_mariabackup
+      wsrep_sst_backup
     )
     # The following script is sourced from other SST scripts, so it should
     # not be made executable.
diff --git a/scripts/wsrep_sst_backup.sh b/scripts/wsrep_sst_backup.sh
new file mode 100644
index 00000000000..55e11ddffc0
--- /dev/null
+++ b/scripts/wsrep_sst_backup.sh
@@ -0,0 +1,112 @@
+#!/usr/bin/env bash
+
+set -ue
+
+# Copyright (C) 2017-2021 MariaDB
+# Copyright (C) 2010-2014 Codership Oy
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; version 2 of the License.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; see the file COPYING. If not, write to the
+# Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston
+# MA  02110-1335  USA.
+
+# This is a reference script for rsync-based state snapshot transfer
+
+RSYNC_REAL_PID=0   # rsync process id
+STUNNEL_REAL_PID=0 # stunnel process id
+
+OS="$(uname)"
+[ "$OS" = 'Darwin' ] && export -n LD_LIBRARY_PATH
+
+# Setting the path for lsof on CentOS
+export PATH="/usr/sbin:/sbin:$PATH"
+
+. $(dirname "$0")/wsrep_sst_common
+
+MAGIC_FILE="$WSREP_SST_OPT_DATA/backup_sst_complete"
+rm -rf "$MAGIC_FILE"
+
+WSREP_LOG_DIR=${WSREP_LOG_DIR:-""}
+# if WSREP_LOG_DIR env. variable is not set, try to get it from my.cnf
+if [ -z "$WSREP_LOG_DIR" ]; then
+    WSREP_LOG_DIR=$(parse_cnf mysqld innodb-log-group-home-dir '')
+fi
+
+if [ -n "$WSREP_LOG_DIR" ]; then
+    # handle both relative and absolute paths
+    WSREP_LOG_DIR=$(cd $WSREP_SST_OPT_DATA; mkdir -p "$WSREP_LOG_DIR"; cd $WSREP_LOG_DIR; pwd -P)
+else
+    # default to datadir
+    WSREP_LOG_DIR=$(cd $WSREP_SST_OPT_DATA; pwd -P)
+fi
+
+if [ "$WSREP_SST_OPT_ROLE" = 'donor' ]
+then
+
+    [ -f "$MAGIC_FILE"      ] && rm -f "$MAGIC_FILE"
+
+    RC=0
+
+    if [ $WSREP_SST_OPT_BYPASS -eq 0 ]; then
+
+        FLUSHED="$WSREP_SST_OPT_DATA/tables_flushed"
+        ERROR="$WSREP_SST_OPT_DATA/sst_error"
+
+        [ -f "$FLUSHED" ] && rm -f "$FLUSHED"
+        [ -f "$ERROR"   ] && rm -f "$ERROR"
+
+         echo "flush tables"
+
+        # Wait for :
+        # (a) Tables to be flushed, AND
+        # (b) Cluster state ID & wsrep_gtid_domain_id to be written to the file, OR
+        # (c) ERROR file, in case flush tables operation failed.
+
+        while [ ! -r "$FLUSHED" ] && \
+                ! grep -q -F ':' '--' "$FLUSHED" >/dev/null 2>&1
+        do
+            # Check whether ERROR file exists.
+            if [ -f "$ERROR" ]; then
+                # Flush tables operation failed.
+                rm -f "$ERROR"
+                exit 255
+            fi
+            sleep 0.2
+        done
+
+        STATE=$(cat "$FLUSHED")
+        rm -f "$FLUSHED"
+
+
+    else # BYPASS
+
+        wsrep_log_info "Bypassing state dump."
+    fi
+
+    echo 'continue' # now server can resume updating data
+
+    echo "$STATE" > "$MAGIC_FILE"
+
+    echo "done $STATE"
+
+elif [ "$WSREP_SST_OPT_ROLE" = 'joiner' ]
+then
+    wsrep_log_error "Unrecognized role: '$WSREP_SST_OPT_ROLE'"
+    exit 22 # EINVAL
+
+
+else
+    wsrep_log_error "Unrecognized role: '$WSREP_SST_OPT_ROLE'"
+    exit 22 # EINVAL
+fi
+
+exit 0