vtgate panic kills connection when selecting database() when using OLAP workload

[aquarapid]

Scenario:

• Connect to vtgate
• set workload=olap;
• select database()

Result:

mysql> set workload=olap;
Query OK, 0 rows affected (0.00 sec)

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

In vtgate logs:

E0519 12:57:51.181414  160083 server.go:277] mysql_server caught panic:
runtime error: invalid memory address or nil pointer dereference
/usr/local/go/src/runtime/panic.go:212 (0x44d079)
        panicmem: panic(memoryError)
/usr/local/go/src/runtime/signal_unix.go:695 (0x44cec8)
        sigpanic: panicmem()
/home/jacques/go/src/vitess.io/vitess/go/vt/vtgate/evalengine/expressions.go:201 (0x7bd535)
        io/vitess/go/vt/vtgate/evalengine.(*BindVariable).Type: return e[b.Key].Type
/home/jacques/go/src/vitess.io/vitess/go/vt/vtgate/engine/projection.go:102 (0xb380a9)
        io/vitess/go/vt/vtgate/engine.(*Projection).addFields: Type: p.Exprs[i].Type(env),
/home/jacques/go/src/vitess.io/vitess/go/vt/vtgate/engine/projection.go:70 (0xb37ed8)
        io/vitess/go/vt/vtgate/engine.(*Projection).StreamExecute: p.addFields(result, bindVars)
/home/jacques/go/src/vitess.io/vitess/go/vt/vtgate/executor.go:1112 (0xb8ac26)
        io/vitess/go/vt/vtgate.(*Executor).StreamExecute: err = plan.Instructions.StreamExecute(vcursor, bindVars, true, func(qr *sqltypes.Result) error {
/home/jacques/go/src/vitess.io/vitess/go/vt/vtgate/vtgate.go:322 (0xba7591)
        io/vitess/go/vt/vtgate.(*VTGate).StreamExecute: err = vtg.executor.StreamExecute(
/home/jacques/go/src/vitess.io/vitess/go/vt/vtgate/plugin_mysql_server.go:208 (0xb93499)
        io/vitess/go/vt/vtgate.(*vtgateHandler).ComQuery: err := vh.vtg.StreamExecute(ctx, session, query, make(map[string]*querypb.BindVariable), callback)
/home/jacques/go/src/vitess.io/vitess/go/mysql/conn.go:1135 (0x8764b7)
        io/vitess/go/mysql.(*Conn).execQuery: err := handler.ComQuery(c, query, func(qr *sqltypes.Result) error {
/home/jacques/go/src/vitess.io/vitess/go/mysql/conn.go:818 (0x8974f2)
        io/vitess/go/mysql.(*Conn).handleNextCommand.func1: if err := c.execQuery(sql, handler, more); err != nil {
/home/jacques/go/src/vitess.io/vitess/go/mysql/conn.go:826 (0x8761ff)
        io/vitess/go/mysql.(*Conn).handleNextCommand: }()
/home/jacques/go/src/vitess.io/vitess/go/mysql/server.go:469 (0x892266)
        io/vitess/go/mysql.(*Listener).handle: err := c.handleNextCommand(l.handler)
/usr/local/go/src/runtime/asm_amd64.s:1373 (0x469100)
        goexit: BYTE    $0x90   // NOP

This is against a recent copy of master: 17dd6c6

[harshit-gangal]

Fixed by #6205