package xds import ( "crypto/sha256" "encoding/hex" "fmt" "sort" "sync/atomic" "time" envoy_cluster_v3 "github.com/envoyproxy/go-control-plane/envoy/config/cluster/v3" envoy_config_core_v3 "github.com/envoyproxy/go-control-plane/envoy/config/core/v3" envoy_endpoint_v3 "github.com/envoyproxy/go-control-plane/envoy/config/endpoint/v3" envoy_listener_v3 "github.com/envoyproxy/go-control-plane/envoy/config/listener/v3" envoy_route_v3 "github.com/envoyproxy/go-control-plane/envoy/config/route/v3" envoy_discovery_v3 "github.com/envoyproxy/go-control-plane/envoy/service/discovery/v3" "github.com/golang/protobuf/proto" "github.com/golang/protobuf/ptypes" "github.com/hashicorp/go-hclog" "google.golang.org/grpc/codes" "google.golang.org/grpc/status" "github.com/hashicorp/consul/agent/proxycfg" "github.com/hashicorp/consul/agent/structs" "github.com/hashicorp/consul/logging" ) // ADSDeltaStream is a shorter way of referring to this thing... type ADSDeltaStream = envoy_discovery_v3.AggregatedDiscoveryService_DeltaAggregatedResourcesServer // DeltaAggregatedResources implements envoy_discovery_v3.AggregatedDiscoveryServiceServer func (s *Server) DeltaAggregatedResources(stream ADSDeltaStream) error { defer s.activeStreams.Increment("v3")() // a channel for receiving incoming requests reqCh := make(chan *envoy_discovery_v3.DeltaDiscoveryRequest) reqStop := int32(0) go func() { for { req, err := stream.Recv() if atomic.LoadInt32(&reqStop) != 0 { return } if err != nil { s.Logger.Error("Error receiving new DeltaDiscoveryRequest; closing request channel", "error", err) close(reqCh) return } reqCh <- req } }() err := s.processDelta(stream, reqCh) if err != nil { s.Logger.Error("Error handling ADS delta stream", "xdsVersion", "v3", "error", err) } // prevents writing to a closed channel if send failed on blocked recv atomic.StoreInt32(&reqStop, 1) return err } const ( stateDeltaInit int = iota stateDeltaPendingInitialConfig stateDeltaRunning ) func (s *Server) processDelta(stream ADSDeltaStream, reqCh <-chan *envoy_discovery_v3.DeltaDiscoveryRequest) error { // Loop state var ( cfgSnap *proxycfg.ConfigSnapshot node *envoy_config_core_v3.Node stateCh <-chan *proxycfg.ConfigSnapshot watchCancel func() proxyID structs.ServiceID nonce uint64 // xDS requires a unique nonce to correlate response/request pairs ready bool // set to true after the first snapshot arrives ) var ( // resourceMap is the SoTW we are incrementally attempting to sync to envoy. // // type => name => proto resourceMap = emptyIndexedResources() // currentVersions is the the xDS versioning represented by Resources. // // type => name => version (as consul knows right now) currentVersions = make(map[string]map[string]string) ) generator := newResourceGenerator( s.Logger.Named(logging.XDS).With("xdsVersion", "v3"), s.CheckFetcher, s.CfgFetcher, true, ) // need to run a small state machine to get through initial authentication. var state = stateDeltaInit // Configure handlers for each type of request we currently care about. handlers := map[string]*xDSDeltaType{ ListenerType: newDeltaType(generator, stream, ListenerType, func(kind structs.ServiceKind) bool { return cfgSnap.Kind == structs.ServiceKindIngressGateway }), RouteType: newDeltaType(generator, stream, RouteType, func(kind structs.ServiceKind) bool { return cfgSnap.Kind == structs.ServiceKindIngressGateway }), ClusterType: newDeltaType(generator, stream, ClusterType, func(kind structs.ServiceKind) bool { // Mesh, Ingress, and Terminating gateways are allowed to inform CDS of // no clusters. return cfgSnap.Kind == structs.ServiceKindMeshGateway || cfgSnap.Kind == structs.ServiceKindTerminatingGateway || cfgSnap.Kind == structs.ServiceKindIngressGateway }), EndpointType: newDeltaType(generator, stream, EndpointType, nil), } // Endpoints are stored within a Cluster (and Routes // are stored within a Listener) so whenever the // enclosing resource is updated the inner resource // list is cleared implicitly. // // When this happens we should update our local // representation of envoy state to force an update. // // see: https://github.com/envoyproxy/envoy/issues/13009 handlers[ListenerType].childType = handlers[RouteType] handlers[ClusterType].childType = handlers[EndpointType] var authTimer <-chan time.Time extendAuthTimer := func() { authTimer = time.After(s.AuthCheckFrequency) } checkStreamACLs := func(cfgSnap *proxycfg.ConfigSnapshot) error { return s.authorize(stream.Context(), cfgSnap) } for { select { case <-authTimer: // It's been too long since a Discovery{Request,Response} so recheck ACLs. if err := checkStreamACLs(cfgSnap); err != nil { return err } extendAuthTimer() case req, ok := <-reqCh: if !ok { // reqCh is closed when stream.Recv errors which is how we detect client // going away. AFAICT the stream.Context() is only canceled once the // RPC method returns which it can't until we return from this one so // there's no point in blocking on that. return nil } generator.logTraceRequest("Incremental xDS v3", req) if req.TypeUrl == "" { return status.Errorf(codes.InvalidArgument, "type URL is required for ADS") } if node == nil && req.Node != nil { node = req.Node var err error generator.ProxyFeatures, err = determineSupportedProxyFeatures(req.Node) if err != nil { return status.Errorf(codes.InvalidArgument, err.Error()) } } if handler, ok := handlers[req.TypeUrl]; ok { if handler.Recv(req, generator.ProxyFeatures) { generator.Logger.Trace("subscribing to type", "typeUrl", req.TypeUrl) } } case cfgSnap = <-stateCh: newRes, err := generator.allResourcesFromSnapshot(cfgSnap) if err != nil { return status.Errorf(codes.Unavailable, "failed to generate all xDS resources from the snapshot: %v", err) } // index and hash the xDS structures newResourceMap := indexResources(generator.Logger, newRes) if s.ResourceMapMutateFn != nil { s.ResourceMapMutateFn(newResourceMap) } if err := populateChildIndexMap(newResourceMap); err != nil { return status.Errorf(codes.Unavailable, "failed to index xDS resource versions: %v", err) } newVersions, err := computeResourceVersions(newResourceMap) if err != nil { return status.Errorf(codes.Unavailable, "failed to compute xDS resource versions: %v", err) } resourceMap = newResourceMap currentVersions = newVersions ready = true } // Trigger state machine switch state { case stateDeltaInit: if node == nil { // This can't happen (tm) since stateCh is nil until after the first req // is received but lets not panic about it. continue } // Start authentication process, we need the proxyID proxyID = structs.NewServiceID(node.Id, parseEnterpriseMeta(node)) // Start watching config for that proxy stateCh, watchCancel = s.CfgMgr.Watch(proxyID) // Note that in this case we _intend_ the defer to only be triggered when // this whole process method ends (i.e. when streaming RPC aborts) not at // the end of the current loop iteration. We have to do it in the loop // here since we can't start watching until we get to this state in the // state machine. defer watchCancel() generator.Logger = generator.Logger.With("service_id", proxyID.String()) // enhance future logs generator.Logger.Trace("watching proxy, pending initial proxycfg snapshot for xDS") // Now wait for the config so we can check ACL state = stateDeltaPendingInitialConfig case stateDeltaPendingInitialConfig: if cfgSnap == nil { // Nothing we can do until we get the initial config continue } // Got config, try to authenticate next. state = stateDeltaRunning // Upgrade the logger switch cfgSnap.Kind { case structs.ServiceKindConnectProxy: case structs.ServiceKindTerminatingGateway: generator.Logger = generator.Logger.Named(logging.TerminatingGateway) case structs.ServiceKindMeshGateway: generator.Logger = generator.Logger.Named(logging.MeshGateway) case structs.ServiceKindIngressGateway: generator.Logger = generator.Logger.Named(logging.IngressGateway) } generator.Logger.Trace("Got initial config snapshot") // Lets actually process the config we just got or we'll mis responding fallthrough case stateDeltaRunning: // Check ACLs on every Discovery{Request,Response}. if err := checkStreamACLs(cfgSnap); err != nil { return err } // For the first time through the state machine, this is when the // timer is first started. extendAuthTimer() if !ready { generator.Logger.Trace("Skipping delta computation because we haven't gotten a snapshot yet") continue } var pendingTypes []string for typeUrl, handler := range handlers { if !handler.registered { continue } if len(handler.pendingUpdates) > 0 { pendingTypes = append(pendingTypes, typeUrl) } } if len(pendingTypes) > 0 { sort.Strings(pendingTypes) generator.Logger.Trace("Skipping delta computation because there are responses in flight", "pendingTypeUrls", pendingTypes) continue } generator.Logger.Trace("Invoking all xDS resource handlers and sending changed data if there are any") sentType := make(map[string]struct{}) // use this to only do one kind of mutation per type per execution for _, op := range xDSUpdateOrder { if _, sent := sentType[op.TypeUrl]; sent { continue } err, sent := handlers[op.TypeUrl].SendIfNew( cfgSnap.Kind, currentVersions[op.TypeUrl], resourceMap, &nonce, op.Upsert, op.Remove, ) if err != nil { return status.Errorf(codes.Unavailable, "failed to send %sreply for type %q: %v", op.errorLogNameReplyPrefix(), op.TypeUrl, err) } if sent { sentType[op.TypeUrl] = struct{}{} } } } } } var xDSUpdateOrder = []xDSUpdateOperation{ // 1. CDS updates (if any) must always be pushed first. {TypeUrl: ClusterType, Upsert: true}, // 2. EDS updates (if any) must arrive after CDS updates for the respective clusters. {TypeUrl: EndpointType, Upsert: true}, // 3. LDS updates must arrive after corresponding CDS/EDS updates. {TypeUrl: ListenerType, Upsert: true, Remove: true}, // 4. RDS updates related to the newly added listeners must arrive after CDS/EDS/LDS updates. {TypeUrl: RouteType, Upsert: true, Remove: true}, // 5. (NOT IMPLEMENTED YET IN CONSUL) VHDS updates (if any) related to the newly added RouteConfigurations must arrive after RDS updates. // {}, // 6. Stale CDS clusters and related EDS endpoints (ones no longer being referenced) can then be removed. {TypeUrl: ClusterType, Remove: true}, {TypeUrl: EndpointType, Remove: true}, // xDS updates can be pushed independently if no new // clusters/routes/listeners are added or if it’s acceptable to // temporarily drop traffic during updates. Note that in case of // LDS updates, the listeners will be warmed before they receive // traffic, i.e. the dependent routes are fetched through RDS if // configured. Clusters are warmed when adding/removing/updating // clusters. On the other hand, routes are not warmed, i.e., the // management plane must ensure that clusters referenced by a route // are in place, before pushing the updates for a route. } type xDSUpdateOperation struct { TypeUrl string Upsert bool Remove bool } func (op *xDSUpdateOperation) errorLogNameReplyPrefix() string { switch { case op.Upsert && op.Remove: return "upsert/remove " case op.Upsert: return "upsert " case op.Remove: return "remove " default: return "" } } type xDSDeltaType struct { generator *ResourceGenerator stream ADSDeltaStream typeURL string allowEmptyFn func(kind structs.ServiceKind) bool // childType is a type that in Envoy is actually stored within this type. // Upserts of THIS type should potentially trigger dependent named // resources within the child to be re-configured. childType *xDSDeltaType // registered indicates if this type has been requested at least once by // the proxy registered bool // wildcard indicates that this type was requested with no preference for // specific resource names. subscribe/unsubscribe are ignored. wildcard bool // sentToEnvoyOnce is true after we've sent one response to envoy. sentToEnvoyOnce bool // subscriptions is the set of currently subscribed envoy resources. // If wildcard == true, this will be empty. subscriptions map[string]struct{} // resourceVersions is the current view of CONFIRMED/ACKed updates to // envoy's view of the loaded resources. // // name => version resourceVersions map[string]string // pendingUpdates is a set of un-ACKed updates to the 'resourceVersions' // map. Once we get an ACK from envoy we'll update the resourceVersions map // and strike the entry from this map. // // nonce -> name -> {version} pendingUpdates map[string]map[string]PendingUpdate } func (t *xDSDeltaType) subscribed(name string) bool { if t.wildcard { return true } _, subscribed := t.subscriptions[name] return subscribed } type PendingUpdate struct { Remove bool Version string ChildResources []string // optional } func newDeltaType( generator *ResourceGenerator, stream ADSDeltaStream, typeUrl string, allowEmptyFn func(kind structs.ServiceKind) bool, ) *xDSDeltaType { return &xDSDeltaType{ generator: generator, stream: stream, typeURL: typeUrl, allowEmptyFn: allowEmptyFn, subscriptions: make(map[string]struct{}), resourceVersions: make(map[string]string), pendingUpdates: make(map[string]map[string]PendingUpdate), } } // Recv handles new discovery requests from envoy. // // Returns true the first time a type receives a request. func (t *xDSDeltaType) Recv(req *envoy_discovery_v3.DeltaDiscoveryRequest, sf supportedProxyFeatures) bool { if t == nil { return false // not something we care about } logger := t.generator.Logger.With("typeUrl", t.typeURL) registeredThisTime := false if !t.registered { // We are in the wildcard mode if the first request of a particular // type has empty subscription list t.wildcard = len(req.ResourceNamesSubscribe) == 0 t.registered = true registeredThisTime = true if sf.ForceLDSandCDSToAlwaysUseWildcardsOnReconnect { switch t.typeURL { case ListenerType, ClusterType: if !t.wildcard { t.wildcard = true logger.Trace("fixing Envoy bug fixed in 1.19.0 by inferring wildcard mode for type") } } } } /* DeltaDiscoveryRequest can be sent in the following situations: Initial message in a xDS bidirectional gRPC stream. As an ACK or NACK response to a previous DeltaDiscoveryResponse. In this case the response_nonce is set to the nonce value in the Response. ACK or NACK is determined by the absence or presence of error_detail. 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 must be omitted. */ /* DeltaDiscoveryRequest plays two independent roles. Any DeltaDiscoveryRequest can be either or both of: */ if req.ResponseNonce != "" { /* [2] (N)ACKing an earlier resource update from the server (using response_nonce, with presence of error_detail making it a NACK). */ if req.ErrorDetail == nil { logger.Trace("got ok response from envoy proxy", "nonce", req.ResponseNonce) t.ack(req.ResponseNonce) } else { logger.Error("got error response from envoy proxy", "nonce", req.ResponseNonce, "error", status.ErrorProto(req.ErrorDetail)) t.nack(req.ResponseNonce) } } if registeredThisTime && len(req.InitialResourceVersions) > 0 { /* Additionally, the first message (for a given type_url) of a reconnected gRPC stream has a third role: [3] informing the server of the resources (and their versions) that the client already possesses, using the initial_resource_versions field. */ logger.Trace("setting initial resource versions for stream", "resources", req.InitialResourceVersions) t.resourceVersions = req.InitialResourceVersions if !t.wildcard { for k := range req.InitialResourceVersions { t.subscriptions[k] = struct{}{} } } } if !t.wildcard { /* [1] informing the server of what resources the client has gained/lost interest in (using resource_names_subscribe and resource_names_unsubscribe), or */ for _, name := range req.ResourceNamesSubscribe { // A resource_names_subscribe field may contain resource names that // the server believes the client is already subscribed to, and // furthermore has the most recent versions of. However, the server // must still provide those resources in the response; due to // implementation details hidden from the server, the client may // have “forgotten” those resources despite apparently remaining // subscribed. // // NOTE: the server must respond with all resources listed in // resource_names_subscribe, even if it believes the client has the // most recent version of them. The reason: the client may have // dropped them, but then regained interest before it had a chance // to send the unsubscribe message. // // We handle that here by ALWAYS wiping the version so the diff // decides to send the value. _, alreadySubscribed := t.subscriptions[name] t.subscriptions[name] = struct{}{} // Reset the tracked version so we force a reply. if _, alreadyTracked := t.resourceVersions[name]; alreadyTracked { t.resourceVersions[name] = "" } if alreadySubscribed { logger.Trace("re-subscribing resource for stream", "resource", name) } else { logger.Trace("subscribing resource for stream", "resource", name) } } for _, name := range req.ResourceNamesUnsubscribe { if _, ok := t.subscriptions[name]; !ok { continue } delete(t.subscriptions, name) logger.Trace("unsubscribing resource for stream", "resource", name) // NOTE: we'll let the normal differential comparison handle cleaning up resourceVersions } } return registeredThisTime } func (t *xDSDeltaType) ack(nonce string) { pending, ok := t.pendingUpdates[nonce] if !ok { return } for name, obj := range pending { if obj.Remove { delete(t.resourceVersions, name) continue } t.resourceVersions[name] = obj.Version if t.childType != nil { // This branch only matters on UPDATE, since we already have // mechanisms to clean up orphaned resources. for _, childName := range obj.ChildResources { if _, exist := t.childType.resourceVersions[childName]; !exist { continue } if !t.subscribed(childName) { continue } t.generator.Logger.Trace( "triggering implicit update of resource", "typeUrl", t.typeURL, "resource", name, "childTypeUrl", t.childType.typeURL, "childResource", childName, ) // Basically manifest this as a re-subscribe/re-sync t.childType.resourceVersions[childName] = "" } } } t.sentToEnvoyOnce = true delete(t.pendingUpdates, nonce) } func (t *xDSDeltaType) nack(nonce string) { delete(t.pendingUpdates, nonce) } func (t *xDSDeltaType) SendIfNew( kind structs.ServiceKind, currentVersions map[string]string, // type => name => version (as consul knows right now) resourceMap *IndexedResources, nonce *uint64, upsert, remove bool, ) (error, bool) { if t == nil || !t.registered { return nil, false } logger := t.generator.Logger.With("typeUrl", t.typeURL) allowEmpty := t.allowEmptyFn != nil && t.allowEmptyFn(kind) // Zero length resource responses should be ignored and are the result of no // data yet. Notice that this caused a bug originally where we had zero // healthy endpoints for an upstream that would cause Envoy to hang waiting // for the EDS response. This is fixed though by ensuring we send an explicit // empty LoadAssignment resource for the cluster rather than allowing junky // empty resources. if len(currentVersions) == 0 && !allowEmpty { // Nothing to send yet return nil, false } resp, updates, err := t.createDeltaResponse(currentVersions, resourceMap, upsert, remove) if err != nil { return err, false } if resp == nil { return nil, false } *nonce++ resp.Nonce = fmt.Sprintf("%08x", *nonce) t.generator.logTraceResponse("Incremental xDS v3", resp) logger.Trace("sending response", "nonce", resp.Nonce) if err := t.stream.Send(resp); err != nil { return err, false } logger.Trace("sent response", "nonce", resp.Nonce) if t.childType != nil { // Capture the relevant child resource names on this pending update so // we can properly clean up the linked children when this change is // ACKed. for name, obj := range updates { if children, ok := resourceMap.ChildIndex[t.typeURL][name]; ok { obj.ChildResources = children updates[name] = obj } } } t.pendingUpdates[resp.Nonce] = updates return nil, true } func (t *xDSDeltaType) createDeltaResponse( currentVersions map[string]string, // name => version (as consul knows right now) resourceMap *IndexedResources, upsert, remove bool, ) (*envoy_discovery_v3.DeltaDiscoveryResponse, map[string]PendingUpdate, error) { // compute difference var ( hasRelevantUpdates = false updates = make(map[string]PendingUpdate) ) if t.wildcard { // First find things that need updating or deleting for name, envoyVers := range t.resourceVersions { currVers, ok := currentVersions[name] if !ok { if remove { hasRelevantUpdates = true } updates[name] = PendingUpdate{Remove: true} } else if currVers != envoyVers { if upsert { hasRelevantUpdates = true } updates[name] = PendingUpdate{Version: currVers} } } // Now find new things for name, currVers := range currentVersions { if _, known := t.resourceVersions[name]; known { continue } if upsert { hasRelevantUpdates = true } updates[name] = PendingUpdate{Version: currVers} } } else { // First find things that need updating or deleting // Walk the list of things currently stored in envoy for name, envoyVers := range t.resourceVersions { if t.subscribed(name) { if currVers, ok := currentVersions[name]; ok { if currVers != envoyVers { if upsert { hasRelevantUpdates = true } updates[name] = PendingUpdate{Version: currVers} } } } } // Now find new things not in envoy yet for name := range t.subscriptions { if _, known := t.resourceVersions[name]; known { continue } if currVers, ok := currentVersions[name]; ok { updates[name] = PendingUpdate{Version: currVers} if upsert { hasRelevantUpdates = true } } } } if !hasRelevantUpdates && t.sentToEnvoyOnce { return nil, nil, nil } // now turn this into a disco response resp := &envoy_discovery_v3.DeltaDiscoveryResponse{ // TODO(rb): consider putting something in SystemVersionInfo? TypeUrl: t.typeURL, } realUpdates := make(map[string]PendingUpdate) for name, obj := range updates { if obj.Remove { if remove { resp.RemovedResources = append(resp.RemovedResources, name) realUpdates[name] = PendingUpdate{Remove: true} } } else if upsert { resources, ok := resourceMap.Index[t.typeURL] if !ok { return nil, nil, fmt.Errorf("unknown type url: %s", t.typeURL) } res, ok := resources[name] if !ok { return nil, nil, fmt.Errorf("unknown name for type url %q: %s", t.typeURL, name) } any, err := ptypes.MarshalAny(res) if err != nil { return nil, nil, err } resp.Resources = append(resp.Resources, &envoy_discovery_v3.Resource{ Name: name, Resource: any, Version: obj.Version, }) realUpdates[name] = obj } } return resp, realUpdates, nil } func computeResourceVersions(resourceMap *IndexedResources) (map[string]map[string]string, error) { out := make(map[string]map[string]string) for typeUrl, resources := range resourceMap.Index { m, err := hashResourceMap(resources) if err != nil { return nil, fmt.Errorf("failed to hash resources for %q: %v", typeUrl, err) } out[typeUrl] = m } return out, nil } type IndexedResources struct { // Index is a map of typeURL => resourceName => resource Index map[string]map[string]proto.Message // ChildIndex is a map of typeURL => parentResourceName => list of // childResourceNames. This only applies if the child and parent do not // share a name. ChildIndex map[string]map[string][]string } func emptyIndexedResources() *IndexedResources { return &IndexedResources{ Index: map[string]map[string]proto.Message{ ListenerType: make(map[string]proto.Message), RouteType: make(map[string]proto.Message), ClusterType: make(map[string]proto.Message), EndpointType: make(map[string]proto.Message), }, ChildIndex: map[string]map[string][]string{ ListenerType: make(map[string][]string), ClusterType: make(map[string][]string), }, } } func populateChildIndexMap(resourceMap *IndexedResources) error { // LDS and RDS have a more complicated relationship. for name, res := range resourceMap.Index[ListenerType] { listener := res.(*envoy_listener_v3.Listener) rdsRouteNames, err := extractRdsResourceNames(listener) if err != nil { return err } resourceMap.ChildIndex[ListenerType][name] = rdsRouteNames } // CDS and EDS share exact names. for name := range resourceMap.Index[ClusterType] { resourceMap.ChildIndex[ClusterType][name] = []string{name} } return nil } func indexResources(logger hclog.Logger, resources map[string][]proto.Message) *IndexedResources { data := emptyIndexedResources() for typeURL, typeRes := range resources { for _, res := range typeRes { name := getResourceName(res) if name == "" { logger.Warn("skipping unexpected xDS type found in delta snapshot", "typeURL", typeURL) } else { data.Index[typeURL][name] = res } } } return data } func getResourceName(res proto.Message) string { // NOTE: this only covers types that we currently care about for LDS/RDS/CDS/EDS switch x := res.(type) { case *envoy_listener_v3.Listener: // LDS return x.Name case *envoy_route_v3.RouteConfiguration: // RDS return x.Name case *envoy_cluster_v3.Cluster: // CDS return x.Name case *envoy_endpoint_v3.ClusterLoadAssignment: // EDS return x.ClusterName default: return "" } } func hashResourceMap(resources map[string]proto.Message) (map[string]string, error) { m := make(map[string]string) for name, res := range resources { h, err := hashResource(res) if err != nil { return nil, fmt.Errorf("failed to hash resource %q: %v", name, err) } m[name] = h } return m, nil } // hashResource will take a resource and create a SHA256 hash sum out of the marshaled bytes func hashResource(res proto.Message) (string, error) { h := sha256.New() buffer := proto.NewBuffer(nil) buffer.SetDeterministic(true) err := buffer.Marshal(res) if err != nil { return "", err } h.Write(buffer.Bytes()) buffer.Reset() return hex.EncodeToString(h.Sum(nil)), nil }