MySQL driver: on connect try setting wsrep_sync_wait, swallow error 1193

In Galera clusters wsrep_sync_wait=7 lets statements catch up all
pending sync between nodes first. This way new child rows await fresh parent
ones from other nodes not to run into foreign key errors. MySQL single nodes
will reject this with error 1193 "Unknown system variable" which is OK.

pkg/config/database.go

@@ -1,7 +1,9 @@
 package config
 
 import (
+	"context"
 	"database/sql"
+	"database/sql/driver"
 	"fmt"
 	"github.com/go-sql-driver/mysql"
 	icingadbDriver "github.com/icinga/icingadb/pkg/driver"
@@ -22,6 +24,8 @@ import (
 
 var registerDriverOnce sync.Once
 
+var errUnknownSysVar = &mysql.MySQLError{Number: 1193}
+
 // Database defines database client configuration.
 type Database struct {
 	Type       string           `yaml:"type" default:"mysql"`
@@ -82,7 +86,40 @@ func (d *Database) Open(logger *logging.Logger) (*icingadb.DB, error) {
 			return nil, errors.Wrap(err, "can't open mysql database")
 		}
 
-		connector := &icingadbDriver.RetryConnector{Connector: c, SqlDriver: &mysql.MySQLDriver{}, Logger: logger}
+		wsrepSyncWait := int64(d.Options.WsrepSyncWait)
+
+		// Setting "wsrep_sync_wait" for each session ensures that causality checks are performed before execution
+		// and that each statement is executed on a fully synchronized node. Doing so prevents foreign key violation
+		// when inserting into dependent tables on different MariaDB/MySQL nodes. When using MySQL single nodes, the
+		// "SET SESSION" command will fail with "Unknown system variable (1193)" and will therefore be silently dropped.
+		// https://mariadb.com/kb/en/galera-cluster-system-variables/#wsrep_sync_wait
+		var setGaleraOpts = func(ctx context.Context, conn driver.Conn) error {
+			const galeraOpts = "SET SESSION wsrep_sync_wait=?"
+
+			stmt, err := conn.(driver.ConnPrepareContext).PrepareContext(ctx, galeraOpts)
+			if err != nil {
+				err = errors.Wrap(err, "can't prepare "+galeraOpts)
+			} else {
+				_, err = stmt.(driver.StmtExecContext).ExecContext(ctx, []driver.NamedValue{{Value: wsrepSyncWait}})
+				if err != nil {
+					err = errors.Wrap(err, "can't execute "+galeraOpts)
+				}
+			}
+
+			if errors.Is(err, errUnknownSysVar) {
+				err = nil
+			}
+
+			if stmt != nil {
+				if errClose := stmt.Close(); errClose != nil && err == nil {
+					err = errors.Wrap(errClose, "can't close statement "+galeraOpts)
+				}
+			}
+
+			return err
+		}
+
+		connector := &icingadbDriver.RetryConnector{Connector: c, SqlDriver: &mysql.MySQLDriver{}, Logger: logger, InitConn: setGaleraOpts}
 		db = sqlx.NewDb(sql.OpenDB(connector), icingadbDriver.MySQL)
 	case "pgsql":
 		uri := &url.URL{