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"
@@ -20,7 +22,10 @@ import (
 	"time"
 )
 
-var registerDriverOnce sync.Once
+var (
+	registerDriverOnce sync.Once
+	wsrepSyncWait      int64
+)
 
 // Database defines database client configuration.
 type Database struct {
@@ -77,12 +82,14 @@ func (d *Database) Open(logger *logging.Logger) (*icingadb.DB, error) {
 			}
 		}
 
+		wsrepSyncWait = int64(d.Options.WsrepSyncWait)
+
 		c, err := mysql.NewConnector(config)
 		if err != nil {
 			return nil, errors.Wrap(err, "can't open mysql database")
 		}
 
-		connector := &icingadbDriver.RetryConnector{Connector: c, Logger: logger}
+		connector := &icingadbDriver.RetryConnector{Connector: c, Logger: logger, InitConn: setGaleraOpts}
 		db = sqlx.NewDb(sql.OpenDB(connector), icingadbDriver.MySQL)
 	case "pgsql":
 		uri := &url.URL{
@@ -182,3 +189,39 @@ func (d *Database) isUnixAddr() bool {
 func unknownDbType(t string) error {
 	return errors.Errorf(`unknown database type %q, must be one of: "mysql", "pgsql"`, t)
 }
+
+var errUnknownSysVar = &mysql.MySQLError{Number: 1193}
+
+// setGaleraOpts tries SET SESSION wsrep_sync_wait.
+//
+// This ensures causality checks will take place before execution,
+// ensuring that every statement is executed on a fully synced node.
+// https://mariadb.com/kb/en/galera-cluster-system-variables/#wsrep_sync_wait
+//
+// It prevents running into foreign key errors while inserting into linked tables on different MySQL nodes.
+// Error 1193 "Unknown system variable" is ignored to support MySQL single nodes.
+func setGaleraOpts(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 err != nil && 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
+}