// execute runs a prepared query in the local DC without any failover. We don't
// apply any sorting options or ACL checks at this level - it should be done up above.
func (p *PreparedQuery) execute(query *structs.PreparedQuery,
	reply *structs.PreparedQueryExecuteResponse) error {
	state := p.srv.fsm.State()
	_, nodes, err := state.CheckServiceNodes(query.Service.Service)
	if err != nil {
		return err
	}

	// Filter out any unhealthy nodes.
	nodes = nodes.Filter(query.Service.OnlyPassing)

	// Apply the tag filters, if any.
	if len(query.Service.Tags) > 0 {
		nodes = tagFilter(query.Service.Tags, nodes)
	}

	// Capture the nodes and pass the DNS information through to the reply.
	reply.Service = query.Service.Service
	reply.Nodes = nodes
	reply.DNS = query.DNS

	// Stamp the result for this datacenter.
	reply.Datacenter = p.srv.config.Datacenter

	return nil
}

// preparedQueryExecute executes a prepared query.
func (s *HTTPServer) preparedQueryExecute(id string, resp http.ResponseWriter, req *http.Request) (interface{}, error) {
	args := structs.PreparedQueryExecuteRequest{
		QueryIDOrName: id,
	}
	s.parseSource(req, &args.Source)
	if done := s.parse(resp, req, &args.Datacenter, &args.QueryOptions); done {
		return nil, nil
	}
	if err := parseLimit(req, &args.Limit); err != nil {
		return nil, fmt.Errorf("Bad limit: %s", err)
	}

	var reply structs.PreparedQueryExecuteResponse
	endpoint := s.agent.getEndpoint(preparedQueryEndpoint)
	if err := s.agent.RPC(endpoint+".Execute", &args, &reply); err != nil {
		// We have to check the string since the RPC sheds
		// the specific error type.
		if err.Error() == consul.ErrQueryNotFound.Error() {
			resp.WriteHeader(404)
			resp.Write([]byte(err.Error()))
			return nil, nil
		}
		return nil, err
	}

	// Use empty list instead of nil.
	if reply.Nodes == nil {
		reply.Nodes = make(structs.CheckServiceNodes, 0)
	}
	return reply, nil
}

// ExecuteRemote is used when a local node doesn't have any instances of a
// service available and needs to probe remote DCs. This sends the full query
// over since the remote side won't have it in its state store, and this doesn't
// do the failover logic since that's already being run on the originating DC.
// We don't want things to fan out further than one level.
func (p *PreparedQuery) ExecuteRemote(args *structs.PreparedQueryExecuteRemoteRequest,
	reply *structs.PreparedQueryExecuteResponse) error {
	if done, err := p.srv.forward("PreparedQuery.ExecuteRemote", args, args, reply); done {
		return err
	}
	defer metrics.MeasureSince([]string{"consul", "prepared-query", "execute_remote"}, time.Now())

	// We have to do this ourselves since we are not doing a blocking RPC.
	p.srv.setQueryMeta(&reply.QueryMeta)
	if args.RequireConsistent {
		if err := p.srv.consistentRead(); err != nil {
			return err
		}
	}

	// Run the query locally to see what we can find.
	if err := p.execute(&args.Query, reply); err != nil {
		return err
	}

	// We don't bother trying to do an RTT sort here since we are by
	// definition in another DC. We just shuffle to make sure that we
	// balance the load across the results.
	reply.Nodes.Shuffle()

	// Apply the limit if given.
	if args.Limit > 0 && len(reply.Nodes) > args.Limit {
		reply.Nodes = reply.Nodes[:args.Limit]
	}

	return nil
}

// Execute runs a prepared query and returns the results. This will perform the
// failover logic if no local results are available. This is typically called as
// part of a DNS lookup, or when executing prepared queries from the HTTP API.
func (p *PreparedQuery) Execute(args *structs.PreparedQueryExecuteRequest,
	reply *structs.PreparedQueryExecuteResponse) error {
	if done, err := p.srv.forward("PreparedQuery.Execute", args, args, reply); done {
		return err
	}
	defer metrics.MeasureSince([]string{"consul", "prepared-query", "execute"}, time.Now())

	// We have to do this ourselves since we are not doing a blocking RPC.
	p.srv.setQueryMeta(&reply.QueryMeta)
	if args.RequireConsistent {
		if err := p.srv.consistentRead(); err != nil {
			return err
		}
	}

	// Try to locate the query.
	state := p.srv.fsm.State()
	_, query, err := state.PreparedQueryLookup(args.QueryIDOrName)
	if err != nil {
		return err
	}
	if query == nil {
		return ErrQueryNotFound
	}

	// Execute the query for the local DC.
	if err := p.execute(query, reply); err != nil {
		return err
	}

	// Shuffle the results in case coordinates are not available if they
	// requested an RTT sort.
	reply.Nodes.Shuffle()
	if err := p.srv.sortNodesByDistanceFrom(args.Source, reply.Nodes); err != nil {
		return err
	}

	// Apply the limit if given.
	if args.Limit > 0 && len(reply.Nodes) > args.Limit {
		reply.Nodes = reply.Nodes[:args.Limit]
	}

	// In the happy path where we found some healthy nodes we go with that
	// and bail out. Otherwise, we fail over and try remote DCs, as allowed
	// by the query setup.
	if len(reply.Nodes) == 0 {
		wrapper := &queryServerWrapper{p.srv}
		if err := queryFailover(wrapper, query, args.Limit, args.QueryOptions, reply); err != nil {
			return err
		}
	}

	return nil
}

// preparedQueryExecute executes a prepared query.
func (s *HTTPServer) preparedQueryExecute(id string, resp http.ResponseWriter, req *http.Request) (interface{}, error) {
	args := structs.PreparedQueryExecuteRequest{
		QueryIDOrName: id,
		Agent: structs.QuerySource{
			Node:       s.agent.config.NodeName,
			Datacenter: s.agent.config.Datacenter,
		},
	}
	s.parseSource(req, &args.Source)
	if done := s.parse(resp, req, &args.Datacenter, &args.QueryOptions); done {
		return nil, nil
	}
	if err := parseLimit(req, &args.Limit); err != nil {
		return nil, fmt.Errorf("Bad limit: %s", err)
	}

	var reply structs.PreparedQueryExecuteResponse
	endpoint := s.agent.getEndpoint(preparedQueryEndpoint)
	if err := s.agent.RPC(endpoint+".Execute", &args, &reply); err != nil {
		// We have to check the string since the RPC sheds
		// the specific error type.
		if err.Error() == consul.ErrQueryNotFound.Error() {
			resp.WriteHeader(404)
			resp.Write([]byte(err.Error()))
			return nil, nil
		}
		return nil, err
	}

	// Note that we translate using the DC that the results came from, since
	// a query can fail over to a different DC than where the execute request
	// was sent to. That's why we use the reply's DC and not the one from
	// the args.
	translateAddresses(s.agent.config, reply.Datacenter, reply.Nodes)

	// Use empty list instead of nil.
	if reply.Nodes == nil {
		reply.Nodes = make(structs.CheckServiceNodes, 0)
	}
	return reply, nil
}

// execute runs a prepared query in the local DC without any failover. We don't
// apply any sorting options at this level - it should be done up above.
func (p *PreparedQuery) execute(query *structs.PreparedQuery,
	reply *structs.PreparedQueryExecuteResponse) error {
	state := p.srv.fsm.State()
	_, nodes, err := state.CheckServiceNodes(query.Service.Service)
	if err != nil {
		return err
	}

	// This is kind of a paranoia ACL check, in case something changed with
	// the token from the time the query was registered. Note that we use
	// the token stored with the query, NOT the passed-in one, which is
	// critical to how queries work (the query becomes a proxy for a lookup
	// using the ACL it was created with).
	acl, err := p.srv.resolveToken(query.Token)
	if err != nil {
		return err
	}
	if acl != nil && !acl.ServiceRead(query.Service.Service) {
		p.srv.logger.Printf("[WARN] consul.prepared_query: Execute of prepared query for service '%s' denied due to ACLs", query.Service.Service)
		return permissionDeniedErr
	}

	// Filter out any unhealthy nodes.
	nodes = nodes.Filter(query.Service.OnlyPassing)

	// Apply the tag filters, if any.
	if len(query.Service.Tags) > 0 {
		nodes = tagFilter(query.Service.Tags, nodes)
	}

	// Capture the nodes and pass the DNS information through to the reply.
	reply.Service = query.Service.Service
	reply.Nodes = nodes
	reply.DNS = query.DNS

	// Stamp the result for this datacenter.
	reply.Datacenter = p.srv.config.Datacenter

	return nil
}

// queryFailover runs an algorithm to determine which DCs to try and then calls
// them to try to locate alternative services.
func queryFailover(q queryServer, query *structs.PreparedQuery,
	limit int, options structs.QueryOptions,
	reply *structs.PreparedQueryExecuteResponse) error {

	// Pull the list of other DCs. This is sorted by RTT in case the user
	// has selected that.
	nearest, err := q.GetOtherDatacentersByDistance()
	if err != nil {
		return err
	}

	// This will help us filter unknown DCs supplied by the user.
	known := make(map[string]struct{})
	for _, dc := range nearest {
		known[dc] = struct{}{}
	}

	// Build a candidate list of DCs to try, starting with the nearest N
	// from RTTs.
	var dcs []string
	index := make(map[string]struct{})
	if query.Service.Failover.NearestN > 0 {
		for i, dc := range nearest {
			if !(i < query.Service.Failover.NearestN) {
				break
			}

			dcs = append(dcs, dc)
			index[dc] = struct{}{}
		}
	}

	// Then add any DCs explicitly listed that weren't selected above.
	for _, dc := range query.Service.Failover.Datacenters {
		// This will prevent a log of other log spammage if we do not
		// attempt to talk to datacenters we don't know about.
		if _, ok := known[dc]; !ok {
			q.GetLogger().Printf("[DEBUG] consul.prepared_query: Skipping unknown datacenter '%s' in prepared query", dc)
			continue
		}

		// This will make sure we don't re-try something that fails
		// from the NearestN list.
		if _, ok := index[dc]; !ok {
			dcs = append(dcs, dc)
		}
	}

	// Now try the selected DCs in priority order.
	failovers := 0
	for _, dc := range dcs {
		// This keeps track of how many iterations we actually run.
		failovers++

		// Be super paranoid and set the nodes slice to nil since it's
		// the same slice we used before. We know there's nothing in
		// there, but the underlying msgpack library has a policy of
		// updating the slice when it's non-nil, and that feels dirty.
		// Let's just set it to nil so there's no way to communicate
		// through this slice across successive RPC calls.
		reply.Nodes = nil

		// Note that we pass along the limit since it can be applied
		// remotely to save bandwidth. We also pass along the consistency
		// mode information we were given, so that applies to the remote
		// query as well.
		remote := &structs.PreparedQueryExecuteRemoteRequest{
			Datacenter:   dc,
			Query:        *query,
			Limit:        limit,
			QueryOptions: options,
		}
		if err := q.ForwardDC("PreparedQuery.ExecuteRemote", dc, remote, reply); err != nil {
			q.GetLogger().Printf("[WARN] consul.prepared_query: Failed querying for service '%s' in datacenter '%s': %s", query.Service.Service, dc, err)
			continue
		}

		// We can stop if we found some nodes.
		if len(reply.Nodes) > 0 {
			break
		}
	}

	// Set this at the end because the response from the remote doesn't have
	// this information.
	reply.Failovers = failovers

	return nil
}

// Execute runs a prepared query and returns the results. This will perform the
// failover logic if no local results are available. This is typically called as
// part of a DNS lookup, or when executing prepared queries from the HTTP API.
func (p *PreparedQuery) Execute(args *structs.PreparedQueryExecuteRequest,
	reply *structs.PreparedQueryExecuteResponse) error {
	if done, err := p.srv.forward("PreparedQuery.Execute", args, args, reply); done {
		return err
	}
	defer metrics.MeasureSince([]string{"consul", "prepared-query", "execute"}, time.Now())

	// We have to do this ourselves since we are not doing a blocking RPC.
	p.srv.setQueryMeta(&reply.QueryMeta)
	if args.RequireConsistent {
		if err := p.srv.consistentRead(); err != nil {
			return err
		}
	}

	// Try to locate the query.
	state := p.srv.fsm.State()
	_, query, err := state.PreparedQueryResolve(args.QueryIDOrName)
	if err != nil {
		return err
	}
	if query == nil {
		return ErrQueryNotFound
	}

	// Execute the query for the local DC.
	if err := p.execute(query, reply); err != nil {
		return err
	}

	// If they supplied a token with the query, use that, otherwise use the
	// token passed in with the request.
	token := args.QueryOptions.Token
	if query.Token != "" {
		token = query.Token
	}
	if err := p.srv.filterACL(token, &reply.Nodes); err != nil {
		return err
	}

	// TODO (slackpad) We could add a special case here that will avoid the
	// fail over if we filtered everything due to ACLs. This seems like it
	// might not be worth the code complexity and behavior differences,
	// though, since this is essentially a misconfiguration.

	// Shuffle the results in case coordinates are not available if they
	// requested an RTT sort.
	reply.Nodes.Shuffle()

	// Build the query source. This can be provided by the client, or by
	// the prepared query. Client-specified takes priority.
	qs := args.Source
	if qs.Datacenter == "" {
		qs.Datacenter = args.Agent.Datacenter
	}
	if query.Service.Near != "" && qs.Node == "" {
		qs.Node = query.Service.Near
	}

	// Respect the magic "_agent" flag.
	if qs.Node == "_agent" {
		qs.Node = args.Agent.Node
	}

	// Perform the distance sort
	err = p.srv.sortNodesByDistanceFrom(qs, reply.Nodes)
	if err != nil {
		return err
	}

	// If we applied a distance sort, make sure that the node queried for is in
	// position 0, provided the results are from the same datacenter.
	if qs.Node != "" && reply.Datacenter == qs.Datacenter {
		for i, node := range reply.Nodes {
			if node.Node.Node == qs.Node {
				reply.Nodes[0], reply.Nodes[i] = reply.Nodes[i], reply.Nodes[0]
				break
			}

			// Put a cap on the depth of the search. The local agent should
			// never be further in than this if distance sorting was applied.
			if i == 9 {
				break
			}
		}
	}

	// Apply the limit if given.
	if args.Limit > 0 && len(reply.Nodes) > args.Limit {
		reply.Nodes = reply.Nodes[:args.Limit]
	}

	// In the happy path where we found some healthy nodes we go with that
	// and bail out. Otherwise, we fail over and try remote DCs, as allowed
	// by the query setup.
	if len(reply.Nodes) == 0 {
		wrapper := &queryServerWrapper{p.srv}
//.........这里部分代码省略.........

// Execute runs a prepared query and returns the results. This will perform the
// failover logic if no local results are available. This is typically called as
// part of a DNS lookup, or when executing prepared queries from the HTTP API.
func (p *PreparedQuery) Execute(args *structs.PreparedQueryExecuteRequest,
	reply *structs.PreparedQueryExecuteResponse) error {
	if done, err := p.srv.forward("PreparedQuery.Execute", args, args, reply); done {
		return err
	}
	defer metrics.MeasureSince([]string{"consul", "prepared-query", "execute"}, time.Now())

	// We have to do this ourselves since we are not doing a blocking RPC.
	p.srv.setQueryMeta(&reply.QueryMeta)
	if args.RequireConsistent {
		if err := p.srv.consistentRead(); err != nil {
			return err
		}
	}

	// Try to locate the query.
	state := p.srv.fsm.State()
	_, query, err := state.PreparedQueryResolve(args.QueryIDOrName)
	if err != nil {
		return err
	}
	if query == nil {
		return ErrQueryNotFound
	}

	// Execute the query for the local DC.
	if err := p.execute(query, reply); err != nil {
		return err
	}

	// If they supplied a token with the query, use that, otherwise use the
	// token passed in with the request.
	token := args.QueryOptions.Token
	if query.Token != "" {
		token = query.Token
	}
	if err := p.srv.filterACL(token, &reply.Nodes); err != nil {
		return err
	}

	// TODO (slackpad) We could add a special case here that will avoid the
	// fail over if we filtered everything due to ACLs. This seems like it
	// might not be worth the code complexity and behavior differences,
	// though, since this is essentially a misconfiguration.

	// Shuffle the results in case coordinates are not available if they
	// requested an RTT sort.
	reply.Nodes.Shuffle()
	if err := p.srv.sortNodesByDistanceFrom(args.Source, reply.Nodes); err != nil {
		return err
	}

	// Apply the limit if given.
	if args.Limit > 0 && len(reply.Nodes) > args.Limit {
		reply.Nodes = reply.Nodes[:args.Limit]
	}

	// In the happy path where we found some healthy nodes we go with that
	// and bail out. Otherwise, we fail over and try remote DCs, as allowed
	// by the query setup.
	if len(reply.Nodes) == 0 {
		wrapper := &queryServerWrapper{p.srv}
		if err := queryFailover(wrapper, query, args.Limit, args.QueryOptions, reply); err != nil {
			return err
		}
	}

	return nil
}