You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: delta.go
    path of file: ./repos/istio/pilot/pkg/xds
    
    the code of the file until where you have to start completion: // Copyright Istio Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package xds

import (
	"errors"
	"fmt"
	"strconv"
	"strings"
	"time"

	discovery "github.com/envoyproxy/go-control-plane/envoy/service/discovery/v3"
	"google.golang.org/grpc/codes"
	"google.golang.org/grpc/peer"
	"google.golang.org/grpc/status"

	"istio.io/istio/pilot/pkg/features"
	istiogrpc "istio.io/istio/pilot/pkg/grpc"
	"istio.io/istio/pilot/pkg/model"
	"istio.io/istio/pilot/pkg/networking/util"
	v3 "istio.io/istio/pilot/pkg/xds/v3"
	istiolog "istio.io/istio/pkg/log"
	"istio.io/istio/pkg/slices"
	"istio.io/istio/pkg/util/sets"
)

var deltaLog = istiolog.RegisterScope("delta", "delta xds debugging")

func (s *DiscoveryServer) StreamDeltas(stream DeltaDiscoveryStream) error {
	if knativeEnv != "" && firstRequest.Load() {
		// How scaling works in knative is the first request is the "loading" request. During
		// loading request, concurrency=1. Once that request is done, concurrency is enabled.
		// However, the XDS stream is long lived, so the first request would block all others. As a
		// result, we should exit the first request immediately; clients will retry.
		firstRequest.Store(false)
		return status.Error(codes.Unavailable, "server warmup not complete; try again")
	}
	// Check if server is ready to accept clients and process new requests.
	// Currently ready means caches have been synced and hence can build
	// clusters correctly. Without this check, InitContext() call below would
	// initialize with empty config, leading to reconnected Envoys loosing
	// configuration. This is an additional safety check inaddition to adding
	// cachesSynced logic to readiness probe to handle cases where kube-proxy
	// ip tables update latencies.
	// See https://github.com/istio/istio/issues/25495.
	if !s.IsServerReady() {
		return errors.New("server is not ready to serve discovery information")
	}

	ctx := stream.Context()
	peerAddr := "0.0.0.0"
	if peerInfo, ok := peer.FromContext(ctx); ok {
		peerAddr = peerInfo.Addr.String()
	}

	if err := s.WaitForRequestLimit(stream.Context()); err != nil {
		deltaLog.Warnf("ADS: %q exceeded rate limit: %v", peerAddr, err)
		return status.Errorf(codes.ResourceExhausted, "request rate limit exceeded: %v", err)
	}

	ids, err := s.authenticate(ctx)
	if err != nil {
		return status.Error(codes.Unauthenticated, err.Error())
	}
	if ids != nil {
		deltaLog.Debugf("Authenticated XDS: %v with identity %v", peerAddr, ids)
	} else {
		deltaLog.Debugf("Un
    
func (s *DiscoveryServer) shouldRespondDelta(con *Connection, request *discovery.DeltaDiscoveryRequest) bool {
	stype := v3.GetShortType(request.TypeUrl)

	// If there is an error in request that means previous response is erroneous.
	// We do not have to respond in that case. In this case request's version info
	// will be different from the version sent. But it is fragile to rely on that.
	if request.ErrorDetail != nil {
		errCode := codes.Code(request.ErrorDetail.Code)
		deltaLog.Warnf("ADS:%s: ACK ERROR %s %s:%s", stype, con.conID, errCode.String(), request.ErrorDetail.GetMessage())
		incrementXDSRejects(request.TypeUrl, con.proxy.ID, errCode.String())
		return false
	}

	previousInfo := con.proxy.GetWatchedResource(request.TypeUrl)

	// This can happen in two cases:
	// 1. Envoy initially send request to Istiod
	// 2. Envoy reconnect to Istiod i.e. Istiod does not have
	// information about this typeUrl, but Envoy sends response nonce - either
	// because Istiod is restarted or Envoy disconnects and reconnects.
	// We should always respond with the current resource names.
	if previousInfo == nil {
		con.proxy.Lock()
		defer con.proxy.Unlock()

		if len(request.InitialResourceVersions) > 0 {
			deltaLog.Debugf("ADS:%s: RECONNECT %s %s resources:%v", stype, con.conID, request.ResponseNonce, len(request.InitialResourceVersions))
		} else {
			deltaLog.Debugf("ADS:%s: INIT %s %s", stype, con.conID, request.ResponseNonce)
		}

		res, wildcard := deltaWatchedResources(nil, request)
		con.proxy.WatchedResources[request.TypeUrl] = &model.WatchedResource{
			TypeUrl:       request.TypeUrl,
			ResourceNames: res,
			Wildcard:      wildcard,
		}
		// For all EDS requests that we have already responded with in the same stream let us
		// force the response. It is important to respond to those requests for Envoy to finish
		// warming of those resources(Clusters).
		// This can happen with the following sequence
		// 1. Envoy disconnects and reconnects to Istiod.
		// 2. Envoy sends EDS request and we respond with it.
		// 3. Envoy sends CDS request and we respond with clusters.
		// 4. Envoy detects a change in cluster state and tries to warm those clusters and send EDS request for them.
		// 5. We should respond to the EDS request with Endpoints to let Envoy finish cluster warming.
		// Refer to https://github.com/envoyproxy/envoy/issues/13009 for more details.
		for _, dependent := range warmingDependencies(request.TypeUrl) {
			if dwr, exists := con.proxy.WatchedResources[dependent]; exists {
				dwr.AlwaysRespond = true
			}
		}
		return true
	}

	// If there is mismatch in the nonce, that is a case of expired/stale nonce.
	// A nonce becomes stale following a newer nonce being sent to Envoy.
	// TODO: due to concurrent unsubscribe, this probably doesn't make sense. Do we need any logic here?
	if request.ResponseNonce != "" && request.ResponseNonce != previousInfo.NonceSent {
		deltaLog.Debugf("ADS:%s: REQ %s Expired nonce received %s, sent %s", stype,
			con.conID, request.ResponseNonce, previousInfo.NonceSent)
		xdsExpiredNonce.With(typeTag.Value(v3.GetMetricType(request.TypeUrl))).Increment()
		return false
	}
	// If it comes here, that means nonce match. This an ACK. We should record
	// the ack details and respond if there is a change in resource names.
	var previousResources, currentResources []string
	var alwaysRespond bool
	con.proxy.UpdateWatchedResource(request.TypeUrl, func(wr *model.WatchedResource) *model.WatchedResource {
		previousResources = wr.ResourceNames
		currentResources, _ = deltaWatchedResources(previousResources, request)
		wr.NonceAcked = request.ResponseNonce
		wr.ResourceNames = currentResources
		alwaysRespond = wr.AlwaysRespond
		wr.AlwaysRespond = false
		return wr
	})

	subChanged := !slices.EqualUnordered(previousResources, currentResources)
	// Spontaneous DeltaDiscoveryRequests from the client.
	// This can be done to dynamically add or remove elements from the tracked resource_names set.
	// In this case response_nonce is empty.
	spontaneousReq := request.ResponseNonce == ""
	// It is invalid in the below two cases:
	// 1. no subscribed resources change from spontaneous delta request.
	// 2. subscribed resources changes from ACK.
	if spontaneousReq && !subChanged || !spontaneousReq && subChanged {
		deltaLog.Errorf("ADS:%s: Subscribed resources check mismatch: %v vs %v", stype, spontaneousReq, subChanged)
		if features.EnableUnsafeAssertions {
			panic(fmt.Sprintf("ADS:%s: Subscribed resources check mismatch: %v vs %v", stype, spontaneousReq, subChanged))
		}
	}

	// Envoy can send two DiscoveryRequests with same version and nonce
	// when it detects a new resource. We should respond if they change.
	if !subChanged {
		// We should always respond "alwaysRespond" marked requests to let Envoy finish warming
		// even though Nonce match and it looks like an ACK.
		if alwaysRespond {
			deltaLog.Infof("ADS:%s: FORCE RESPONSE %s for warming.", stype, con.conID)
			return true
		}

		deltaLog.Debugf("ADS:%s: ACK %s %s", stype, con.conID, request.ResponseNonce)
		return false
	}
	deltaLog.Debugf("ADS:%s: RESOURCE CHANGE previous resources: %v, new resources: %v %s %s", stype,
		previousResources, currentResources, con.conID, request.ResponseNonce)

	return true
}