open-consul/agent/consul/rpc.go

package consul

import (
	"context"
	"crypto/tls"
	"encoding/binary"
	"errors"
	"fmt"
	"io"
	"net"
	"strings"
	"sync/atomic"
	"time"

	"github.com/armon/go-metrics"
	"github.com/armon/go-metrics/prometheus"
	connlimit "github.com/hashicorp/go-connlimit"
	"github.com/hashicorp/go-hclog"
	memdb "github.com/hashicorp/go-memdb"
	"github.com/hashicorp/go-raftchunking"
	"github.com/hashicorp/memberlist"
	msgpackrpc "github.com/hashicorp/net-rpc-msgpackrpc"
	"github.com/hashicorp/raft"
	"github.com/hashicorp/yamux"

	"github.com/hashicorp/consul/acl"
	"github.com/hashicorp/consul/agent/consul/state"
	"github.com/hashicorp/consul/agent/consul/wanfed"
	"github.com/hashicorp/consul/agent/metadata"
	"github.com/hashicorp/consul/agent/pool"
	"github.com/hashicorp/consul/agent/structs"
	"github.com/hashicorp/consul/lib"
	"github.com/hashicorp/consul/logging"
)

var RPCCounters = []prometheus.CounterDefinition{
	{
		Name: []string{"rpc", "accept_conn"},
		Help: "Increments when a server accepts an RPC connection.",
	},
	{
		Name: []string{"rpc", "raft_handoff"},
		Help: "Increments when a server accepts a Raft-related RPC connection.",
	},
	{
		Name: []string{"rpc", "request_error"},
		Help: "Increments when a server returns an error from an RPC request.",
	},
	{
		Name: []string{"rpc", "request"},
		Help: "Increments when a server receives a Consul-related RPC request.",
	},
	{
		Name: []string{"rpc", "cross-dc"},
		Help: "Increments when a server sends a (potentially blocking) cross datacenter RPC query.",
	},
	{
		Name: []string{"rpc", "query"},
		Help: "Increments when a server receives a new blocking RPC request, indicating the rate of new blocking query calls.",
	},
}

var RPCGauges = []prometheus.GaugeDefinition{
	{
		Name: []string{"rpc", "queries_blocking"},
		Help: "Shows the current number of in-flight blocking queries the server is handling.",
	},
}

var RPCSummaries = []prometheus.SummaryDefinition{
	{
		Name: []string{"rpc", "consistentRead"},
		Help: "Measures the time spent confirming that a consistent read can be performed.",
	},
}

const (
	// jitterFraction is a the limit to the amount of jitter we apply
	// to a user specified MaxQueryTime. We divide the specified time by
	// the fraction. So 16 == 6.25% limit of jitter. This same fraction
	// is applied to the RPCHoldTimeout
	jitterFraction = 16

	// Warn if the Raft command is larger than this.
	// If it's over 1MB something is probably being abusive.
	raftWarnSize = 1024 * 1024

	// enqueueLimit caps how long we will wait to enqueue
	// a new Raft command. Something is probably wrong if this
	// value is ever reached. However, it prevents us from blocking
	// the requesting goroutine forever.
	enqueueLimit = 30 * time.Second
)

var (
	ErrChunkingResubmit = errors.New("please resubmit call for rechunking")
)

func (s *Server) rpcLogger() hclog.Logger {
	return s.loggers.Named(logging.RPC)
}

// listen is used to listen for incoming RPC connections
func (s *Server) listen(listener net.Listener) {
	for {
		// Accept a connection
		conn, err := listener.Accept()
		if err != nil {
			if s.shutdown {
				return
			}
			s.rpcLogger().Error("failed to accept RPC conn", "error", err)
			continue
		}

		free, err := s.rpcConnLimiter.Accept(conn)
		if err != nil {
			s.rpcLogger().Error("rejecting RPC conn from because rpc_max_conns_per_client exceeded", "conn", logConn(conn))
			conn.Close()
			continue
		}
		// Wrap conn so it will be auto-freed from conn limiter when it closes.
		conn = connlimit.Wrap(conn, free)

		go s.handleConn(conn, false)
		metrics.IncrCounter([]string{"rpc", "accept_conn"}, 1)
	}
}

// logConn is a wrapper around memberlist's LogConn so that we format references
// to "from" addresses in a consistent way. This is just a shorter name.
func logConn(conn net.Conn) string {
	return memberlist.LogConn(conn)
}

// handleConn is used to determine if this is a Raft or
// Consul type RPC connection and invoke the correct handler
func (s *Server) handleConn(conn net.Conn, isTLS bool) {
	// Limit how long the client can hold the connection open before they send the
	// magic byte (and authenticate when mTLS is enabled). If `isTLS == true` then
	// this also enforces a timeout on how long it takes for the handshake to
	// complete since tls.Conn.Read implicitly calls Handshake().
	if s.config.RPCHandshakeTimeout > 0 {
		conn.SetReadDeadline(time.Now().Add(s.config.RPCHandshakeTimeout))
	}

	if !isTLS && s.tlsConfigurator.MutualTLSCapable() {
		// See if actually this is native TLS multiplexed onto the old
		// "type-byte" system.

		peekedConn, nativeTLS, err := pool.PeekForTLS(conn)
		if err != nil {
			if err != io.EOF {
				s.rpcLogger().Error(
					"failed to read first byte",
					"conn", logConn(conn),
					"error", err,
				)
			}
			conn.Close()
			return
		}

		if nativeTLS {
			s.handleNativeTLS(peekedConn)
			return
		}
		conn = peekedConn
	}

	// Read a single byte
	buf := make([]byte, 1)

	if _, err := conn.Read(buf); err != nil {
		if err != io.EOF {
			s.rpcLogger().Error("failed to read byte",
				"conn", logConn(conn),
				"error", err,
			)
		}
		conn.Close()
		return
	}
	typ := pool.RPCType(buf[0])

	// Reset the deadline as we aren't sure what is expected next - it depends on
	// the protocol.
	if s.config.RPCHandshakeTimeout > 0 {
		conn.SetReadDeadline(time.Time{})
	}

	// Enforce TLS if VerifyIncoming is set
	if s.tlsConfigurator.VerifyIncomingRPC() && !isTLS && typ != pool.RPCTLS && typ != pool.RPCTLSInsecure {
		s.rpcLogger().Warn("Non-TLS connection attempted with VerifyIncoming set", "conn", logConn(conn))
		conn.Close()
		return
	}

	// Switch on the byte
	switch typ {
	case pool.RPCConsul:
		s.handleConsulConn(conn)

	case pool.RPCRaft:
		metrics.IncrCounter([]string{"rpc", "raft_handoff"}, 1)
		s.raftLayer.Handoff(conn)

	case pool.RPCTLS:
		// Don't allow malicious client to create TLS-in-TLS for ever.
		if isTLS {
			s.rpcLogger().Error("TLS connection attempting to establish inner TLS connection", "conn", logConn(conn))
			conn.Close()
			return
		}
		conn = tls.Server(conn, s.tlsConfigurator.IncomingRPCConfig())
		s.handleConn(conn, true)

	case pool.RPCMultiplexV2:
		s.handleMultiplexV2(conn)

	case pool.RPCSnapshot:
		s.handleSnapshotConn(conn)

	case pool.RPCTLSInsecure:
		// Don't allow malicious client to create TLS-in-TLS for ever.
		if isTLS {
			s.rpcLogger().Error("TLS connection attempting to establish inner TLS connection", "conn", logConn(conn))
			conn.Close()
			return
		}
		conn = tls.Server(conn, s.tlsConfigurator.IncomingInsecureRPCConfig())
		s.handleInsecureConn(conn)

	case pool.RPCGRPC:
		s.grpcHandler.Handle(conn)

	default:
		if !s.handleEnterpriseRPCConn(typ, conn, isTLS) {
			s.rpcLogger().Error("unrecognized RPC byte",
				"byte", typ,
				"conn", logConn(conn),
			)
			conn.Close()
		}
	}
}

func (s *Server) handleNativeTLS(conn net.Conn) {
	s.rpcLogger().Trace(
		"detected actual TLS over RPC port",
		"conn", logConn(conn),
	)

	tlscfg := s.tlsConfigurator.IncomingALPNRPCConfig(pool.RPCNextProtos)
	tlsConn := tls.Server(conn, tlscfg)

	// Force the handshake to conclude.
	if err := tlsConn.Handshake(); err != nil {
		s.rpcLogger().Error(
			"TLS handshake failed",
			"conn", logConn(conn),
			"error", err,
		)
		conn.Close()
		return
	}

	// Reset the deadline as we aren't sure what is expected next - it depends on
	// the protocol.
	if s.config.RPCHandshakeTimeout > 0 {
		conn.SetReadDeadline(time.Time{})
	}

	var (
		cs        = tlsConn.ConnectionState()
		sni       = cs.ServerName
		nextProto = cs.NegotiatedProtocol

		transport = s.memberlistTransportWAN
	)

	s.rpcLogger().Trace(
		"accepted nativeTLS RPC",
		"sni", sni,
		"protocol", nextProto,
		"conn", logConn(conn),
	)

	switch nextProto {
	case pool.ALPN_RPCConsul:
		s.handleConsulConn(tlsConn)

	case pool.ALPN_RPCRaft:
		metrics.IncrCounter([]string{"rpc", "raft_handoff"}, 1)
		s.raftLayer.Handoff(tlsConn)

	case pool.ALPN_RPCMultiplexV2:
		s.handleMultiplexV2(tlsConn)

	case pool.ALPN_RPCSnapshot:
		s.handleSnapshotConn(tlsConn)

	case pool.ALPN_RPCGRPC:
		s.grpcHandler.Handle(conn)

	case pool.ALPN_WANGossipPacket:
		if err := s.handleALPN_WANGossipPacketStream(tlsConn); err != nil && err != io.EOF {
			s.rpcLogger().Error(
				"failed to ingest RPC",
				"sni", sni,
				"protocol", nextProto,
				"conn", logConn(conn),
				"error", err,
			)
		}

	case pool.ALPN_WANGossipStream:
		// No need to defer the conn.Close() here, the Ingest methods do that.
		if err := transport.IngestStream(tlsConn); err != nil {
			s.rpcLogger().Error(
				"failed to ingest RPC",
				"sni", sni,
				"protocol", nextProto,
				"conn", logConn(conn),
				"error", err,
			)
		}

	default:
		if !s.handleEnterpriseNativeTLSConn(nextProto, conn) {
			s.rpcLogger().Error(
				"discarding RPC for unknown negotiated protocol",
				"failed to ingest RPC",
				"protocol", nextProto,
				"conn", logConn(conn),
			)
			conn.Close()
		}
	}
}

// handleMultiplexV2 is used to multiplex a single incoming connection
// using the Yamux multiplexer
func (s *Server) handleMultiplexV2(conn net.Conn) {
	defer conn.Close()
	conf := yamux.DefaultConfig()
	// override the default because LogOutput conflicts with Logger
	conf.LogOutput = nil
	// TODO: should this be created once and cached?
	conf.Logger = s.logger.StandardLogger(&hclog.StandardLoggerOptions{InferLevels: true})
	server, _ := yamux.Server(conn, conf)
	for {
		sub, err := server.Accept()
		if err != nil {
			if err != io.EOF {
				s.rpcLogger().Error("multiplex conn accept failed",
					"conn", logConn(conn),
					"error", err,
				)
			}
			return
		}

		// In the beginning only RPC was supposed to be multiplexed
		// with yamux. In order to add the ability to multiplex network
		// area connections, this workaround was added.
		// This code peeks the first byte and checks if it is
		// RPCGossip, in which case this is handled by enterprise code.
		// Otherwise this connection is handled like before by the RPC
		// handler.
		// This wouldn't work if a normal RPC could start with
		// RPCGossip(6). In messagepack a 6 encodes a positive fixint:
		// https://github.com/msgpack/msgpack/blob/master/spec.md.
		// None of the RPCs we are doing starts with that, usually it is
		// a string for datacenter.
		peeked, first, err := pool.PeekFirstByte(sub)
		if err != nil {
			s.rpcLogger().Error("Problem peeking connection", "conn", logConn(sub), "err", err)
			sub.Close()
			return
		}
		sub = peeked
		switch first {
		case pool.RPCGossip:
			buf := make([]byte, 1)
			sub.Read(buf)
			go func() {
				if !s.handleEnterpriseRPCConn(pool.RPCGossip, sub, false) {
					s.rpcLogger().Error("unrecognized RPC byte",
						"byte", pool.RPCGossip,
						"conn", logConn(conn),
					)
					sub.Close()
				}
			}()
		default:
			go s.handleConsulConn(sub)
		}
	}
}

// handleConsulConn is used to service a single Consul RPC connection
func (s *Server) handleConsulConn(conn net.Conn) {
	defer conn.Close()
	rpcCodec := msgpackrpc.NewCodecFromHandle(true, true, conn, structs.MsgpackHandle)
	for {
		select {
		case <-s.shutdownCh:
			return
		default:
		}

		if err := s.rpcServer.ServeRequest(rpcCodec); err != nil {
			if err != io.EOF && !strings.Contains(err.Error(), "closed") {
				s.rpcLogger().Error("RPC error",
					"conn", logConn(conn),
					"error", err,
				)
				metrics.IncrCounter([]string{"rpc", "request_error"}, 1)
			}
			return
		}
		metrics.IncrCounter([]string{"rpc", "request"}, 1)
	}
}

// handleInsecureConsulConn is used to service a single Consul INSECURERPC connection
func (s *Server) handleInsecureConn(conn net.Conn) {
	defer conn.Close()
	rpcCodec := msgpackrpc.NewCodecFromHandle(true, true, conn, structs.MsgpackHandle)
	for {
		select {
		case <-s.shutdownCh:
			return
		default:
		}

		if err := s.insecureRPCServer.ServeRequest(rpcCodec); err != nil {
			if err != io.EOF && !strings.Contains(err.Error(), "closed") {
				s.rpcLogger().Error("INSECURERPC error",
					"conn", logConn(conn),
					"error", err,
				)
				metrics.IncrCounter([]string{"rpc", "request_error"}, 1)
			}
			return
		}
		metrics.IncrCounter([]string{"rpc", "request"}, 1)
	}
}

// handleSnapshotConn is used to dispatch snapshot saves and restores, which
// stream so don't use the normal RPC mechanism.
func (s *Server) handleSnapshotConn(conn net.Conn) {
	go func() {
		defer conn.Close()
		if err := s.handleSnapshotRequest(conn); err != nil {
			s.rpcLogger().Error("Snapshot RPC error",
				"conn", logConn(conn),
				"error", err,
			)
		}
	}()
}

func (s *Server) handleALPN_WANGossipPacketStream(conn net.Conn) error {
	defer conn.Close()

	transport := s.memberlistTransportWAN
	for {
		select {
		case <-s.shutdownCh:
			return nil
		default:
		}

		// Note: if we need to change this format to have additional header
		// information we can just negotiate a different ALPN protocol instead
		// of needing any sort of version field here.
		prefixLen, err := readUint32(conn, wanfed.GossipPacketMaxIdleTime)
		if err != nil {
			return err
		}

		// Avoid a memory exhaustion DOS vector here by capping how large this
		// packet can be to something reasonable.
		if prefixLen > wanfed.GossipPacketMaxByteSize {
			return fmt.Errorf("gossip packet size %d exceeds threshold of %d", prefixLen, wanfed.GossipPacketMaxByteSize)
		}

		lc := &limitedConn{
			Conn: conn,
			lr:   io.LimitReader(conn, int64(prefixLen)),
		}

		if err := transport.IngestPacket(lc, conn.RemoteAddr(), time.Now(), false); err != nil {
			return err
		}
	}
}

func readUint32(conn net.Conn, timeout time.Duration) (uint32, error) {
	// Since requests are framed we can easily just set a deadline on
	// reading that frame and then disable it for the rest of the body.
	if err := conn.SetReadDeadline(time.Now().Add(timeout)); err != nil {
		return 0, err
	}

	var v uint32
	if err := binary.Read(conn, binary.BigEndian, &v); err != nil {
		return 0, err
	}

	if err := conn.SetReadDeadline(time.Time{}); err != nil {
		return 0, err
	}

	return v, nil
}

type limitedConn struct {
	net.Conn
	lr io.Reader
}

func (c *limitedConn) Read(b []byte) (n int, err error) {
	return c.lr.Read(b)
}

// canRetry returns true if the given situation is safe for a retry.
func canRetry(args interface{}, err error) bool {
	// No leader errors are always safe to retry since no state could have
	// been changed.
	if structs.IsErrNoLeader(err) {
		return true
	}

	// If we are chunking and it doesn't seem to have completed, try again.
	if err != nil && strings.Contains(err.Error(), ErrChunkingResubmit.Error()) {
		return true
	}

	// Reads are safe to retry for stream errors, such as if a server was
	// being shut down.
	info, ok := args.(structs.RPCInfo)
	if ok && info.IsRead() && lib.IsErrEOF(err) {
		return true
	}

	return false
}

// ForwardRPC is used to forward an RPC request to a remote DC or to the local leader
// Returns a bool of if forwarding was performed, as well as any error
func (s *Server) ForwardRPC(method string, info structs.RPCInfo, args interface{}, reply interface{}) (bool, error) {
	var firstCheck time.Time

	// Handle DC forwarding
	dc := info.RequestDatacenter()
	if dc != s.config.Datacenter {
		// Local tokens only work within the current datacenter. Check to see
		// if we are attempting to forward one to a remote datacenter and strip
		// it, falling back on the anonymous token on the other end.
		if token := info.TokenSecret(); token != "" {
			done, ident, err := s.ResolveIdentityFromToken(token)
			if done {
				if err != nil && !acl.IsErrNotFound(err) {
					return false, err
				}
				if ident != nil && ident.IsLocal() {
					// Strip it from the request.
					info.SetTokenSecret("")
					defer info.SetTokenSecret(token)
				}
			}
		}

		err := s.forwardDC(method, dc, args, reply)
		return true, err
	}

	// Check if we can allow a stale read, ensure our local DB is initialized
	if info.IsRead() && info.AllowStaleRead() && !s.raft.LastContact().IsZero() {
		return false, nil
	}

CHECK_LEADER:
	// Fail fast if we are in the process of leaving
	select {
	case <-s.leaveCh:
		return true, structs.ErrNoLeader
	default:
	}

	// Find the leader
	isLeader, leader, rpcErr := s.getLeader()

	// Handle the case we are the leader
	if isLeader {
		return false, nil
	}

	// Handle the case of a known leader
	if leader != nil {
		rpcErr = s.connPool.RPC(s.config.Datacenter, leader.ShortName, leader.Addr,
			method, args, reply)
		if rpcErr != nil && canRetry(info, rpcErr) {
			goto RETRY
		}
		return true, rpcErr
	}

RETRY:
	// Gate the request until there is a leader
	if firstCheck.IsZero() {
		firstCheck = time.Now()
	}
	if time.Since(firstCheck) < s.config.RPCHoldTimeout {
		jitter := lib.RandomStagger(s.config.RPCHoldTimeout / jitterFraction)
		select {
		case <-time.After(jitter):
			goto CHECK_LEADER
		case <-s.leaveCh:
		case <-s.shutdownCh:
		}
	}

	// No leader found and hold time exceeded
	return true, rpcErr
}

// getLeader returns if the current node is the leader, and if not then it
// returns the leader which is potentially nil if the cluster has not yet
// elected a leader. In the case of not having a leader elected yet
// then a NoClusterLeader error gets returned. In the case of Raft having
// a leader but out internal tracking failing to find the leader we
// return a LeaderNotTracked error. Therefore if the err is nil AND
// the bool is false then the Server will be non-nil
func (s *Server) getLeader() (bool, *metadata.Server, error) {
	// Check if we are the leader
	if s.IsLeader() {
		return true, nil, nil
	}

	// Get the leader
	leader := s.raft.Leader()
	if leader == "" {
		return false, nil, structs.ErrNoLeader
	}

	// Lookup the server
	server := s.serverLookup.Server(leader)

	// if server is nil this indicates that while we have a Raft leader
	// something has caused that node to be considered unhealthy which
	// cascades into its removal from the serverLookup struct. In this case
	// we should not report no cluster leader but instead report a different
	// error so as not to confuse our users as to the what the root cause of
	// an issue might be.
	if server == nil {
		s.logger.Warn("Raft has a leader but other tracking of the node would indicate that the node is unhealthy or does not exist. The network may be misconfigured.", "leader", leader)
		return false, nil, structs.ErrLeaderNotTracked
	}

	return false, server, nil
}

// forwardDC is used to forward an RPC call to a remote DC, or fail if no servers
func (s *Server) forwardDC(method, dc string, args interface{}, reply interface{}) error {
	manager, server, ok := s.router.FindRoute(dc)
	if !ok {
		if s.router.HasDatacenter(dc) {
			s.rpcLogger().Warn("RPC request to DC is currently failing as no server can be reached", "datacenter", dc)
			return structs.ErrDCNotAvailable
		}
		s.rpcLogger().Warn("RPC request for DC is currently failing as no path was found",
			"datacenter", dc,
			"method", method,
		)
		return structs.ErrNoDCPath
	}

	metrics.IncrCounterWithLabels([]string{"rpc", "cross-dc"}, 1,
		[]metrics.Label{{Name: "datacenter", Value: dc}})
	if err := s.connPool.RPC(dc, server.ShortName, server.Addr, method, args, reply); err != nil {
		manager.NotifyFailedServer(server)
		s.rpcLogger().Error("RPC failed to server in DC",
			"server", server.Addr,
			"datacenter", dc,
			"method", method,
			"error", err,
		)
		return err
	}

	return nil
}

// keyringRPCs is used to forward an RPC request to a server in each dc. This
// will only error for RPC-related errors. Otherwise, application-level errors
// can be sent in the response objects.
func (s *Server) keyringRPCs(method string, args interface{}, dcs []string) (*structs.KeyringResponses, error) {

	errorCh := make(chan error, len(dcs))
	respCh := make(chan *structs.KeyringResponses, len(dcs))

	for _, dc := range dcs {
		go func(dc string) {
			rr := &structs.KeyringResponses{}
			if err := s.forwardDC(method, dc, args, &rr); err != nil {
				errorCh <- err
				return
			}
			respCh <- rr
		}(dc)
	}

	responses := &structs.KeyringResponses{}
	for i := 0; i < len(dcs); i++ {
		select {
		case err := <-errorCh:
			return nil, err
		case rr := <-respCh:
			responses.Add(rr)
		}
	}
	return responses, nil
}

type raftEncoder func(structs.MessageType, interface{}) ([]byte, error)

// raftApplyMsgpack encodes the msg using msgpack and calls raft.Apply. See
// raftApplyWithEncoder.
// Deprecated: use raftApplyMsgpack
func (s *Server) raftApply(t structs.MessageType, msg interface{}) (interface{}, error) {
	return s.raftApplyMsgpack(t, msg)
}

// raftApplyMsgpack encodes the msg using msgpack and calls raft.Apply. See
// raftApplyWithEncoder.
func (s *Server) raftApplyMsgpack(t structs.MessageType, msg interface{}) (interface{}, error) {
	return s.raftApplyWithEncoder(t, msg, structs.Encode)
}

// raftApplyProtobuf encodes the msg using protobuf and calls raft.Apply. See
// raftApplyWithEncoder.
func (s *Server) raftApplyProtobuf(t structs.MessageType, msg interface{}) (interface{}, error) {
	return s.raftApplyWithEncoder(t, msg, structs.EncodeProtoInterface)
}

// raftApplyWithEncoder encodes a message, and then calls raft.Apply with the
// encoded message. Returns the FSM response along with any errors. If the
// FSM.Apply response is an error it will be returned as the error return
// value with a nil response.
func (s *Server) raftApplyWithEncoder(
	t structs.MessageType,
	msg interface{},
	encoder raftEncoder,
) (response interface{}, err error) {
	if encoder == nil {
		return nil, fmt.Errorf("Failed to encode request: nil encoder")
	}
	buf, err := encoder(t, msg)
	if err != nil {
		return nil, fmt.Errorf("Failed to encode request: %v", err)
	}

	// Warn if the command is very large
	if n := len(buf); n > raftWarnSize {
		s.rpcLogger().Warn("Attempting to apply large raft entry", "size_in_bytes", n)
	}

	var chunked bool
	var future raft.ApplyFuture
	switch {
	case len(buf) <= raft.SuggestedMaxDataSize || t != structs.KVSRequestType:
		future = s.raft.Apply(buf, enqueueLimit)
	default:
		chunked = true
		future = raftchunking.ChunkingApply(buf, nil, enqueueLimit, s.raft.ApplyLog)
	}

	if err := future.Error(); err != nil {
		return nil, err
	}

	resp := future.Response()

	if chunked {
		// In this case we didn't apply all chunks successfully, possibly due
		// to a term change; resubmit
		if resp == nil {
			// This returns the error in the interface because the raft library
			// returns errors from the FSM via the future, not via err from the
			// apply function. Downstream client code expects to see any error
			// from the FSM (as opposed to the apply itself) and decide whether
			// it can retry in the future's response.
			return nil, ErrChunkingResubmit
		}
		// We expect that this conversion should always work
		chunkedSuccess, ok := resp.(raftchunking.ChunkingSuccess)
		if !ok {
			return nil, errors.New("unknown type of response back from chunking FSM")
		}
		resp = chunkedSuccess.Response
	}

	if err, ok := resp.(error); ok {
		return nil, err
	}
	return resp, nil
}

// queryFn is used to perform a query operation. If a re-query is needed, the
// passed-in watch set will be used to block for changes. The passed-in state
// store should be used (vs. calling fsm.State()) since the given state store
// will be correctly watched for changes if the state store is restored from
// a snapshot.
type queryFn func(memdb.WatchSet, *state.Store) error

// blockingQuery is used to process a potentially blocking query operation.
func (s *Server) blockingQuery(queryOpts structs.QueryOptionsCompat, queryMeta structs.QueryMetaCompat, fn queryFn) error {
	var cancel func()
	var ctx context.Context = &lib.StopChannelContext{StopCh: s.shutdownCh}

	var queriesBlocking uint64
	var queryTimeout time.Duration

	// Instrument all queries run
	metrics.IncrCounter([]string{"rpc", "query"}, 1)

	minQueryIndex := queryOpts.GetMinQueryIndex()
	// Fast path right to the non-blocking query.
	if minQueryIndex == 0 {
		goto RUN_QUERY
	}

	queryTimeout = queryOpts.GetMaxQueryTime()
	// Restrict the max query time, and ensure there is always one.
	if queryTimeout > s.config.MaxQueryTime {
		queryTimeout = s.config.MaxQueryTime
	} else if queryTimeout <= 0 {
		queryTimeout = s.config.DefaultQueryTime
	}

	// Apply a small amount of jitter to the request.
	queryTimeout += lib.RandomStagger(queryTimeout / jitterFraction)

	// wrap the base context with a deadline
	ctx, cancel = context.WithDeadline(ctx, time.Now().Add(queryTimeout))
	defer cancel()

	// instrument blockingQueries
	// atomic inc our server's count of in-flight blockingQueries and store the new value
	queriesBlocking = atomic.AddUint64(&s.queriesBlocking, 1)
	// atomic dec when we return from blockingQuery()
	defer atomic.AddUint64(&s.queriesBlocking, ^uint64(0))
	// set the gauge directly to the new value of s.blockingQueries
	metrics.SetGauge([]string{"rpc", "queries_blocking"}, float32(queriesBlocking))

RUN_QUERY:
	// Setup blocking loop
	// Update the query metadata.
	s.setQueryMeta(queryMeta)

	// Validate
	// If the read must be consistent we verify that we are still the leader.
	if queryOpts.GetRequireConsistent() {
		if err := s.consistentRead(); err != nil {
			return err
		}
	}

	// Run query

	// Operate on a consistent set of state. This makes sure that the
	// abandon channel goes with the state that the caller is using to
	// build watches.
	state := s.fsm.State()

	// We can skip all watch tracking if this isn't a blocking query.
	var ws memdb.WatchSet
	if minQueryIndex > 0 {
		ws = memdb.NewWatchSet()

		// This channel will be closed if a snapshot is restored and the
		// whole state store is abandoned.
		ws.Add(state.AbandonCh())
	}

	// Execute the queryFn
	err := fn(ws, state)
	// Note we check queryOpts.MinQueryIndex is greater than zero to determine if
	// blocking was requested by client, NOT meta.Index since the state function
	// might return zero if something is not initialized and care wasn't taken to
	// handle that special case (in practice this happened a lot so fixing it
	// systematically here beats trying to remember to add zero checks in every
	// state method). We also need to ensure that unless there is an error, we
	// return an index > 0 otherwise the client will never block and burn CPU and
	// requests.
	if err == nil && queryMeta.GetIndex() < 1 {
		queryMeta.SetIndex(1)
	}
	// block up to the timeout if we don't see anything fresh.
	if err == nil && minQueryIndex > 0 && queryMeta.GetIndex() <= minQueryIndex {
		if err := ws.WatchCtx(ctx); err == nil {
			// a non-nil error only occurs when the context is cancelled

			// If a restore may have woken us up then bail out from
			// the query immediately. This is slightly race-ey since
			// this might have been interrupted for other reasons,
			// but it's OK to kick it back to the caller in either
			// case.
			select {
			case <-state.AbandonCh():
			default:
				// loop back and look for an update again
				goto RUN_QUERY
			}
		}
	}
	return err
}

// setQueryMeta is used to populate the QueryMeta data for an RPC call
func (s *Server) setQueryMeta(m structs.QueryMetaCompat) {
	if s.IsLeader() {
		m.SetLastContact(0)
		m.SetKnownLeader(true)
	} else {
		m.SetLastContact(time.Since(s.raft.LastContact()))
		m.SetKnownLeader(s.raft.Leader() != "")
	}
}

// consistentRead is used to ensure we do not perform a stale
// read. This is done by verifying leadership before the read.
func (s *Server) consistentRead() error {
	defer metrics.MeasureSince([]string{"rpc", "consistentRead"}, time.Now())
	future := s.raft.VerifyLeader()
	if err := future.Error(); err != nil {
		return err //fail fast if leader verification fails
	}
	// poll consistent read readiness, wait for up to RPCHoldTimeout milliseconds
	if s.isReadyForConsistentReads() {
		return nil
	}
	jitter := lib.RandomStagger(s.config.RPCHoldTimeout / jitterFraction)
	deadline := time.Now().Add(s.config.RPCHoldTimeout)

	for time.Now().Before(deadline) {

		select {
		case <-time.After(jitter):
			// Drop through and check before we loop again.

		case <-s.shutdownCh:
			return fmt.Errorf("shutdown waiting for leader")
		}

		if s.isReadyForConsistentReads() {
			return nil
		}
	}

	return structs.ErrNotReadyForConsistentReads
}