FLUSH TABLES causes vtgate panic

[deepthi]

Overview of the Issue

Trying to execute flush tables from mysql client causes a panic in vtgate.

Reproduction Steps

Run the local example starter script (101_initial_cluster.sh). Insert some data using the insert script as documented in the website Getting Started guide.

$ mysql
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.7.9-vitess-10.0.0-SNAPSHOT Version: 10.0.0-SNAPSHOT (Git revision fbc5f9f13 branch 'master') built on Wed Mar  3 12:14:17 PST 2021 by <...> using go1.15.7 linux/amd64

mysql> flush tables;
ERROR 2013 (HY000): Lost connection to MySQL server during query

Operating system and Environment details

Ubuntu 18.04

Log Fragments

E0304 10:38:30.617328   30134 server.go:303] mysql_server caught panic:
runtime error: invalid memory address or nil pointer dereference
/home/deepthi/.gvm/gos/go1.15.7/src/runtime/panic.go:212 (0x44fff2)
        panicmem: panic(memoryError)
/home/deepthi/.gvm/gos/go1.15.7/src/runtime/signal_unix.go:742 (0x44fe72)
        sigpanic: panicmem()
/home/deepthi/go/src/vitess.io/vitess/go/vt/srvtopo/resolver.go:194 (0xd4621b)
        io/vitess/go/vt/srvtopo.(*Resolver).ResolveDestinations: if err := destination.Resolve(allShards, func(shard string) error {
/home/deepthi/go/src/vitess.io/vitess/go/vt/vtgate/vcursor_impl.go:487 (0xe975a4)
        io/vitess/go/vt/vtgate.(*vcursorImpl).ResolveDestinations: return vc.resolver.ResolveDestinations(vc.ctx, keyspace, vc.tabletType, ids, destinations)
/home/deepthi/go/src/vitess.io/vitess/go/vt/vtgate/engine/send.go:77 (0xd781a2)
        io/vitess/go/vt/vtgate/engine.(*Send).Execute: rss, _, err := vcursor.ResolveDestinations(s.Keyspace.Name, nil, []key.Destination{s.TargetDestination})
/home/deepthi/go/src/vitess.io/vitess/go/vt/vtgate/plan_execute.go:173 (0xea617a)
        io/vitess/go/vt/vtgate.(*Executor).executePlan.func1: qr, err := plan.Instructions.Execute(vcursor, bindVars, true)
/home/deepthi/go/src/vitess.io/vitess/go/vt/vtgate/plan_execute.go:118 (0xe80b8d)
        io/vitess/go/vt/vtgate.(*Executor).newExecute: return e.executePlan(ctx, plan, vcursor, bindVars, execStart)(logStats, safeSession)
/home/deepthi/go/src/vitess.io/vitess/go/vt/vtgate/executor.go:187 (0xe6eaba)
        io/vitess/go/vt/vtgate.(*Executor).execute: stmtType, qr, err := e.newExecute(ctx, safeSession, sql, bindVars, logStats)
/home/deepthi/go/src/vitess.io/vitess/go/vt/vtgate/executor.go:155 (0xe6e6c4)
        io/vitess/go/vt/vtgate.(*Executor).Execute: stmtType, result, err := e.execute(ctx, safeSession, sql, bindVars, logStats)
/home/deepthi/go/src/vitess.io/vitess/go/vt/vtgate/vtgate.go:280 (0xe9eb77)
        io/vitess/go/vt/vtgate.(*VTGate).Execute: qr, err = vtg.executor.Execute(ctx, "Execute", NewSafeSession(session), sql, bindVariables)
/home/deepthi/go/src/vitess.io/vitess/go/vt/vtgate/plugin_mysql_server.go:218 (0xe82a25)
        io/vitess/go/vt/vtgate.(*vtgateHandler).ComQuery: session, result, err := vh.vtg.Execute(ctx, session, query, make(map[string]*querypb.BindVariable))
/home/deepthi/go/src/vitess.io/vitess/go/mysql/conn.go:1241 (0xbbf7ab)
        io/vitess/go/mysql.(*Conn).execQuery: err := handler.ComQuery(c, query, func(qr *sqltypes.Result) error {
/home/deepthi/go/src/vitess.io/vitess/go/mysql/conn.go:1226 (0xbbf2d5)
        io/vitess/go/mysql.(*Conn).handleComQuery: res := c.execQuery(sql, handler, more)
/home/deepthi/go/src/vitess.io/vitess/go/mysql/conn.go:874 (0xbbc367)
        io/vitess/go/mysql.(*Conn).handleNextCommand: return c.handleComQuery(handler, data)
/home/deepthi/go/src/vitess.io/vitess/go/mysql/server.go:495 (0xbdec51)
        io/vitess/go/mysql.(*Listener).handle: kontinue := c.handleNextCommand(l.handler)
/home/deepthi/go/src/vitess.io/vitess/go/mysql/server.go:286 (0xbe4e44)
        io/vitess/go/mysql.(*Listener).Accept.func1: l.handle(conn, connectionID, acceptTime)
/home/deepthi/.gvm/gos/go1.15.7/src/runtime/asm_amd64.s:1374 (0x472180)
        goexit: BYTE    $0x90   // NOP