flagutil: OptionalFlagValue has no parse function; use a con

Error message

flagutil: OptionalFlagValue has no parse function; use a constructor such as NewOptionalFlag

What it means

Returned when the zero-value OptionalFlagValue is used as a flag without one of the NewOptional* constructors, so it has no parse function registered. Construct the value with flagutil.NewOptionalBool/NewOptionalInt64/etc. instead of declaring a bare OptionalFlagValue.

Source

Thrown at go/flagutil/optional.go:124

// NewOptionalFlag returns a *OptionalFlagValue[T] with the given default value,
// pflag type name, parse function, and stringer.
// It returns an error if parse is nil, since a flag without a parse function cannot be set.
func NewOptionalFlag[T any](defaultVal T, typeName string, parse func(string) (T, error), stringer func(T) string) (*OptionalFlagValue[T], error) {
	if parse == nil {
		return nil, errors.New("flagutil: NewOptionalFlag requires a non-nil parse function")
	}
	return &OptionalFlagValue[T]{
		val:      defaultVal,
		typeName: typeName,
		parse:    parse,
		stringer: stringer,
	}, nil
}

func (f *OptionalFlagValue[T]) Set(arg string) error {
	if f.parse == nil {
		return errors.New("flagutil: OptionalFlagValue has no parse function; use a constructor such as NewOptionalFlag")
	}

	v, err := f.parse(arg)
	if err != nil {
		return err
	}

	f.val = v
	f.set = true

	return nil
}

func (f *OptionalFlagValue[T]) String() string {
	if f.stringer == nil {
		return fmt.Sprintf("%v", f.val)
	}

View on GitHub (pinned to 01a25a7d17)

Solutions

Retry the request
Increase topoReadPool size or throttle callers
Diagnose topo server latency causing long slot occupancy

Defensive patterns

Strategy: retry

Try / catch

v, err := cluster.GetVSchema(ctx, ks)
if err != nil && errors.Is(err, context.DeadlineExceeded) {
	v, err = cluster.GetVSchema(ctxWithRetry, ks)
}

Prevention

Serialize vschema reads or batch them via GetVSchemas
Increase topoReadPool size
Monitor topo read latency

When it happens

Trigger: Calling Cluster.GetVSchema while topoReadPool is exhausted and the request context expires during Acquire.

Common situations: Heavy concurrent vtadmin traffic; slow topo backend; short client deadlines.

Related errors

AI-assisted analysis of vitessio/vitess@01a25a7d17 (2026-09-01). Data as JSON: /api/errors/8a83e8c2a558d551. Report an issue: GitHub.