go-delve/delve · error

count == 0 not allowed with a channel filter

Error message

count == 0 not allowed with a channel filter

What it means

When a channel filter is present, ListGoroutines requires a non-zero Count because paging of channel waiters is computed against Count (len(gs) == arg.Count decides whether more pages exist). Count==0 would make the channel query meaningless, so it is rejected.

Source

Thrown at service/rpc2/server.go:708

// For each group a maximum of MaxGroupMembers example goroutines are
// returned, as well as the total number of goroutines in the group.
func (s *RPCServer) ListGoroutines(arg ListGoroutinesIn, out *ListGoroutinesOut) error {
	//TODO(aarzilli): if arg contains a running goroutines filter (not negated)
	// and start == 0 and count == 0 then we can optimize this by just looking
	// at threads directly.

	var gs []*proc.G
	var nextg int
	var err error
	var gsLoaded bool

	for _, filter := range arg.Filters {
		if filter.Kind == api.GoroutineWaitingOnChannel {
			if filter.Negated {
				return errors.New("channel filter can not be negated")
			}
			if arg.Count == 0 {
				return errors.New("count == 0 not allowed with a channel filter")
			}
			if arg.EvalScope == nil {
				return errors.New("channel filter without eval scope")
			}
			gs, err = s.debugger.ChanGoroutines(arg.EvalScope.GoroutineID, arg.EvalScope.Frame, arg.EvalScope.DeferredCall, filter.Arg, arg.Start, arg.Count)
			if len(gs) == arg.Count {
				nextg = arg.Start + len(gs)
			} else {
				nextg = -1
			}
			gsLoaded = true
			break
		}
	}

	if !gsLoaded {
		gs, nextg, err = s.debugger.Goroutines(arg.Start, arg.Count)
	}

View on GitHub (pinned to a23773e6c3)

Solutions

1. Set Count to a positive value (e.g. page size or a large limit).
2. Default Count in the client when a channel filter is requested.
3. Drop the channel filter if unbounded listing is required (not supported).

Example fix

// before
in := api.ListGoroutinesIn{Filters: channelFilters} // Count defaults to 0
// after
in := api.ListGoroutinesIn{Count: 100, Filters: channelFilters}

if hasChannelFilter(filters) && count <= 0 {
    count = 100 // sensible default
}

err := client.Call("RPCServer.ListGoroutines", in, &out)
if err != nil && strings.Contains(err.Error(), "count == 0") {
    in.Count = 100
    err = client.Call("RPCServer.ListGoroutines", in, &out)
}

When it happens

Trigger: ListGoroutines RPC with Filters containing Kind=GoroutineWaitingOnChannel and arg.Count == 0.

Common situations: Clients that omit Count (zero value by default) when listing goroutines, then add a channel-wait filter.

Related errors

• channel filter can not be negated
• channel filter without eval scope
• could not find goroutine array
• mismatched length in addrs and addrpid
• can not call function with nil ReturnInfoLoadConfig

AI-assisted analysis of go-delve/delve@a23773e6c3 (2026-08-31). Data as JSON: /api/errors/21078791564aed9a. Report an issue: GitHub.