// ReadOnlyCmd updates the read timestamp cache and waits for any
// overlapping writes currently processing through Raft ahead of us to
// clear via the read queue.
func (r *Range) ReadOnlyCmd(method string, args proto.Request, reply proto.Response) error {
	header := args.Header()
	r.Lock()
	r.tsCache.Add(header.Key, header.EndKey, header.Timestamp)
	var wg sync.WaitGroup
	r.readQ.AddRead(header.Key, header.EndKey, &wg)
	r.Unlock()
	wg.Wait()

	// It's possible that arbitrary delays (e.g. major GC, VM
	// de-prioritization, etc.) could cause the execution of this read
	// command to occur AFTER the range replica has lost leadership.
	//
	// There is a chance that we waited on writes, and although they
	// were committed to the log, they weren't successfully applied to
	// this replica's state machine. We re-verify leadership before
	// reading to make sure that all pending writes are persisted.
	//
	// There are some elaborate cases where we might have lost
	// leadership and then regained it during the delay, but this is ok
	// because any writes during that period necessarily had higher
	// timestamps. This is because the read-timestamp-cache prevents it
	// for the active leader and leadership changes force the
	// read-timestamp-cache to reset its high water mark.
	if !r.IsLeader() {
		// TODO(spencer): when we happen to know the leader, fill it in here via replica.
		return &proto.NotLeaderError{}
	}
	return r.executeCmd(method, args, reply)
}

// ExecuteCmd synchronously runs Store.ExecuteCmd. The store is looked
// up from the store map if specified by header.Replica; otherwise,
// the command is being executed locally, and the replica is
// determined via lookup of header.Key in the ranges slice.
func (kv *LocalKV) ExecuteCmd(method string, args proto.Request, replyChan interface{}) {
	// If the replica isn't specified in the header, look it up.
	var err error
	var store *storage.Store
	// If we aren't given a Replica, then a little bending over
	// backwards here. We need to find the Store, but all we have is the
	// Key. So find its Range locally, and pull out its Replica which we
	// use to find the Store. This lets us use the same codepath below
	// (store.ExecuteCmd) for both locally and remotely originated
	// commands.
	header := args.Header()
	if header.Replica.NodeID == 0 {
		if repl := kv.lookupReplica(header.Key); repl != nil {
			header.Replica = *repl
		} else {
			err = util.Errorf("unable to lookup range replica for key %q", string(header.Key))
		}
	}
	if err == nil {
		store, err = kv.GetStore(&header.Replica)
	}
	reply := reflect.New(reflect.TypeOf(replyChan).Elem().Elem()).Interface().(proto.Response)
	if err != nil {
		reply.Header().SetGoError(err)
	} else {
		store.ExecuteCmd(method, args, reply)
	}
	reflect.ValueOf(replyChan).Send(reflect.ValueOf(reply))
}

// ReadWriteCmd first consults the response cache to determine whether
// this command has already been sent to the range. If a response is
// found, it's returned immediately and not submitted to raft. Next,
// the timestamp cache is checked to determine if any newer accesses to
// this command's affected keys have been made. If so, this command's
// timestamp is moved forward. Finally the keys affected by this
// command are added as pending writes to the read queue and the
// command is submitted to Raft. Upon completion, the write is removed
// from the read queue and the reply is added to the repsonse cache.
func (r *Range) ReadWriteCmd(method string, args proto.Request, reply proto.Response) error {
	// Check the response cache in case this is a replay. This call
	// may block if the same command is already underway.
	header := args.Header()
	if ok, err := r.respCache.GetResponse(header.CmdID, reply); ok || err != nil {
		if ok { // this is a replay! extract error for return
			return reply.Header().GoError()
		}
		// In this case there was an error reading from the response
		// cache. Instead of failing the request just because we can't
		// decode the reply in the response cache, we proceed as though
		// idempotence has expired.
		log.Errorf("unable to read result for %+v from the response cache: %v", args, err)
	}

	// One of the prime invariants of Cockroach is that a mutating command
	// cannot write a key with an earlier timestamp than the most recent
	// read of the same key. So first order of business here is to check
	// the timestamp cache for reads/writes which are more recent than the
	// timestamp of this write. If more recent, we simply update the
	// write's timestamp before enqueuing it for execution. When the write
	// returns, the updated timestamp will inform the final commit
	// timestamp.
	r.Lock() // Protect access to timestamp cache and read queue.
	if ts := r.tsCache.GetMax(header.Key, header.EndKey); header.Timestamp.Less(ts) {
		if glog.V(1) {
			glog.Infof("Overriding existing timestamp %s with %s", header.Timestamp, ts)
		}
		ts.Logical++ // increment logical component by one to differentiate.
		// Update the request timestamp.
		header.Timestamp = ts
	}
	// Just as for reads, we update the timestamp cache with the
	// timestamp of this write. This ensures a strictly higher timestamp
	// for successive writes to the same key or key range.
	r.tsCache.Add(header.Key, header.EndKey, header.Timestamp)

	// The next step is to add the write to the read queue to inform
	// subsequent reads that there is a pending write. Reads which
	// overlap pending writes must wait for those writes to complete.
	wKey := r.readQ.AddWrite(header.Key, header.EndKey)
	r.Unlock()

	// Create command and enqueue for Raft.
	cmd := &Cmd{
		Method: method,
		Args:   args,
		Reply:  reply,
		done:   make(chan error, 1),
	}
	// This waits for the command to complete.
	err := r.EnqueueCmd(cmd)

	// Now that the command has completed, remove the pending write.
	r.Lock()
	r.readQ.RemoveWrite(wKey)
	r.Unlock()

	return err
}

// usesTimestampCache returns true if the request affects or is
// affected by the timestamp cache.
func usesTimestampCache(r proto.Request) bool {
	m := r.Method()
	if m < 0 || m >= proto.Method(len(tsCacheMethods)) {
		return false
	}
	return tsCacheMethods[m]
}

// MaybeWrap wraps the given argument in a batch, unless it is already one.
func maybeWrap(args proto.Request) (*proto.BatchRequest, func(*proto.BatchResponse) proto.Response) {
	if ba, ok := args.(*proto.BatchRequest); ok {
		return ba, func(br *proto.BatchResponse) proto.Response { return br }
	}
	ba := &proto.BatchRequest{}
	ba.RequestHeader = *(gogoproto.Clone(args.Header()).(*proto.RequestHeader))
	ba.Add(args)
	return ba, func(br *proto.BatchResponse) proto.Response {
		var unwrappedReply proto.Response
		if len(br.Responses) == 0 {
			unwrappedReply = args.CreateReply()
		} else {
			unwrappedReply = br.Responses[0].GetInner()
		}
		// The ReplyTxn is propagated from one response to the next request,
		// and we adopt the mechanism that whenever the Txn changes, it needs
		// to be set in the reply, for example to ratched up the transaction
		// timestamp on writes when necessary.
		// This is internally necessary to sequentially execute the batch,
		// so it makes some sense to take the burden of updating the Txn
		// from TxnCoordSender - it will only need to act on retries/aborts
		// in the future.
		unwrappedReply.Header().Txn = br.Txn
		if unwrappedReply.Header().Error == nil {
			unwrappedReply.Header().Error = br.Error
		}
		return unwrappedReply
	}
}

// updateForBatch updates the first argument (the header of a request contained
// in a batch) from the second one (the batch header), returning an error when
// inconsistencies are found.
// It is checked that the individual call does not have a User, UserPriority
// or Txn set that differs from the batch's.
func updateForBatch(args proto.Request, bHeader proto.RequestHeader) error {
	// Disallow transaction, user and priority on individual calls, unless
	// equal.
	aHeader := args.Header()
	if aHeader.User != "" && aHeader.User != bHeader.User {
		return util.Error("conflicting user on call in batch")
	}
	if aPrio := aHeader.GetUserPriority(); aPrio != proto.Default_RequestHeader_UserPriority && aPrio != bHeader.GetUserPriority() {
		return util.Error("conflicting user priority on call in batch")
	}
	aHeader.User = bHeader.User
	aHeader.UserPriority = bHeader.UserPriority
	// Only allow individual transactions on the requests of a batch if
	// - the batch is non-transactional,
	// - the individual transaction does not write intents, and
	// - the individual transaction is initialized.
	// The main usage of this is to allow mass-resolution of intents, which
	// entails sending a non-txn batch of transactional InternalResolveIntent.
	if aHeader.Txn != nil && !aHeader.Txn.Equal(bHeader.Txn) {
		if len(aHeader.Txn.ID) == 0 || proto.IsTransactionWrite(args) || bHeader.Txn != nil {
			return util.Error("conflicting transaction in transactional batch")
		}
	} else {
		aHeader.Txn = bHeader.Txn
	}
	return nil
}

// ExecuteCmd synchronously runs Store.ExecuteCmd. The store is looked
// up from the store map if specified by header.Replica; otherwise,
// the command is being executed locally, and the replica is
// determined via lookup through each of the stores.
func (kv *LocalKV) ExecuteCmd(method string, args proto.Request, replyChan interface{}) {
	// If the replica isn't specified in the header, look it up.
	var err error
	var store *storage.Store
	// If we aren't given a Replica, then a little bending over
	// backwards here. We need to find the Store, but all we have is the
	// Key. So find its Range locally. This lets us use the same
	// codepath below (store.ExecuteCmd) for both locally and remotely
	// originated commands.
	header := args.Header()
	if header.Replica.StoreID == 0 {
		var repl *proto.Replica
		repl, err = kv.lookupReplica(header.Key, header.EndKey)
		if err == nil {
			header.Replica = *repl
		}
	}
	if err == nil {
		store, err = kv.GetStore(header.Replica.StoreID)
	}
	reply := reflect.New(reflect.TypeOf(replyChan).Elem().Elem()).Interface().(proto.Response)
	if err != nil {
		reply.Header().SetGoError(err)
	} else {
		store.ExecuteCmd(method, args, reply)
		if err := reply.Verify(args); err != nil {
			reply.Header().SetGoError(err)
		}
	}
	reflect.ValueOf(replyChan).Send(reflect.ValueOf(reply))
}

// executeCmd looks up the store specified by header.Replica, and runs
// Store.ExecuteCmd.
func (n *Node) executeCmd(method string, args proto.Request, reply proto.Response) error {
	store, err := n.localKV.GetStore(&args.Header().Replica)
	if err != nil {
		return err
	}
	store.ExecuteCmd(method, args, reply)
	return nil
}

// endCmd removes a pending command from the command queue.
func (r *Range) endCmd(cmdKey interface{}, args proto.Request, err error, readOnly bool) {
	r.Lock()
	if err == nil && usesTimestampCache(args) {
		header := args.Header()
		r.tsCache.Add(header.Key, header.EndKey, header.Timestamp, header.Txn.GetID(), readOnly)
	}
	r.cmdQ.Remove(cmdKey)
	r.Unlock()
}

// applyRaftCommand applies a raft command from the replicated log to the
// underlying state machine (i.e. the engine).
// When certain critical operations fail, a replicaCorruptionError may be
// returned and must be handled by the caller.
func (r *Range) applyRaftCommand(ctx context.Context, index uint64, originNode proto.RaftNodeID, args proto.Request, reply proto.Response) error {
	if index <= 0 {
		log.Fatalc(ctx, "raft command index is <= 0")
	}

	// If we have an out of order index, there's corruption. No sense in trying
	// to update anything or run the command. Simply return a corruption error.
	if oldIndex := atomic.LoadUint64(&r.appliedIndex); oldIndex >= index {
		return newReplicaCorruptionError(util.Errorf("applied index moved backwards: %d >= %d", oldIndex, index))
	}

	// Call the helper, which returns a batch containing data written
	// during command execution and any associated error.
	ms := engine.MVCCStats{}
	batch, rErr := r.applyRaftCommandInBatch(ctx, index, originNode, args, reply, &ms)
	// ALWAYS set the reply header error to the error returned by the
	// helper. This is the definitive result of the execution. The
	// error must be set before saving to the response cache.
	// TODO(tschottdorf,tamird) For #1400, want to refactor executeCmd to not
	// touch the reply header's error field.
	reply.Header().SetGoError(rErr)
	defer batch.Close()

	// Advance the last applied index and commit the batch.
	if err := setAppliedIndex(batch, r.Desc().RaftID, index); err != nil {
		log.Fatalc(ctx, "setting applied index in a batch should never fail: %s", err)
	}
	if err := batch.Commit(); err != nil {
		rErr = newReplicaCorruptionError(util.Errorf("could not commit batch"), err, rErr)
	} else {
		// Update cached appliedIndex if we were able to set the applied index on disk.
		atomic.StoreUint64(&r.appliedIndex, index)
	}

	// On successful write commands, flush to event feed, and handle other
	// write-related triggers including splitting and config gossip updates.
	if rErr == nil && proto.IsWrite(args) {
		// Publish update to event feed.
		r.rm.EventFeed().updateRange(r, args.Method(), &ms)
		// If the commit succeeded, potentially add range to split queue.
		r.maybeAddToSplitQueue()
		// Maybe update gossip configs on a put.
		switch args.(type) {
		case *proto.PutRequest, *proto.DeleteRequest, *proto.DeleteRangeRequest:
			if key := args.Header().Key; key.Less(keys.SystemMax) {
				// We hold the lock already.
				r.maybeGossipConfigsLocked(func(configPrefix proto.Key) bool {
					return bytes.HasPrefix(key, configPrefix)
				})
			}
		}
	}

	return rErr
}

// UpdateForBatch updates the first argument (the header of a request contained
// in a batch) from the second one (the batch header), returning an error when
// inconsistencies are found.
// It is checked that the individual call does not have a UserPriority
// or Txn set that differs from the batch's.
// TODO(tschottdorf): will go with #2143.
func updateForBatch(args proto.Request, bHeader proto.RequestHeader) error {
	// Disallow transaction, user and priority on individual calls, unless
	// equal.
	aHeader := args.Header()
	if aPrio := aHeader.GetUserPriority(); aPrio != proto.Default_RequestHeader_UserPriority && aPrio != bHeader.GetUserPriority() {
		return util.Errorf("conflicting user priority on call in batch")
	}
	aHeader.UserPriority = bHeader.UserPriority
	aHeader.Txn = bHeader.Txn // reqs always take Txn from batch
	return nil
}

// addReadOnlyCmd updates the read timestamp cache and waits for any
// overlapping writes currently processing through Raft ahead of us to
// clear via the read queue.
func (r *Range) addReadOnlyCmd(ctx context.Context, args proto.Request, reply proto.Response) error {
	header := args.Header()

	if err := r.checkCmdHeader(header); err != nil {
		reply.Header().SetGoError(err)
		return err
	}

	// If read-consistency is set to INCONSISTENT, run directly.
	if header.ReadConsistency == proto.INCONSISTENT {
		// But disallow any inconsistent reads within txns.
		if header.Txn != nil {
			reply.Header().SetGoError(util.Error("cannot allow inconsistent reads within a transaction"))
			return reply.Header().GoError()
		}
		if header.Timestamp.Equal(proto.ZeroTimestamp) {
			header.Timestamp = r.rm.Clock().Now()
		}
		intents, err := r.executeCmd(r.rm.Engine(), nil, args, reply)
		if err == nil {
			r.handleSkippedIntents(args, intents)
		}
		return err
	} else if header.ReadConsistency == proto.CONSENSUS {
		reply.Header().SetGoError(util.Error("consensus reads not implemented"))
		return reply.Header().GoError()
	}

	// Add the read to the command queue to gate subsequent
	// overlapping commands until this command completes.
	cmdKey := r.beginCmd(header, true)

	// This replica must have leader lease to process a consistent read.
	if err := r.redirectOnOrAcquireLeaderLease(tracer.FromCtx(ctx), header.Timestamp); err != nil {
		r.endCmd(cmdKey, args, err, true /* readOnly */)
		reply.Header().SetGoError(err)
		return err
	}

	// Execute read-only command.
	intents, err := r.executeCmd(r.rm.Engine(), nil, args, reply)

	// Only update the timestamp cache if the command succeeded.
	r.endCmd(cmdKey, args, err, true /* readOnly */)

	if err == nil {
		r.handleSkippedIntents(args, intents)
	}
	return err
}

// sendAttempt is invoked by Send. It temporarily truncates the arguments to
// match the descriptor's EndKey (if necessary) and gathers and rearranges the
// replicas before making a single attempt at sending the request. It returns
// the result of sending the RPC; a potential error contained in the reply has
// to be handled separately by the caller.
func (ds *DistSender) sendAttempt(trace *tracer.Trace, args proto.Request, reply proto.Response, desc *proto.RangeDescriptor) error {
	defer trace.Epoch("sending RPC")()
	// Truncate the request to our current range, making sure not to
	// touch it unless we have to (it is illegal to send EndKey on
	// commands which do not operate on ranges).
	if endKey := args.Header().EndKey; endKey != nil && !endKey.Less(desc.EndKey) {
		defer func(k proto.Key) { args.Header().EndKey = k }(endKey)
		args.Header().EndKey = desc.EndKey
	}
	leader := ds.leaderCache.Lookup(proto.RaftID(desc.RaftID))

	// Try to send the call.
	replicas := newReplicaSlice(ds.gossip, desc)

	// Rearrange the replicas so that those replicas with long common
	// prefix of attributes end up first. If there's no prefix, this is a
	// no-op.
	order := ds.optimizeReplicaOrder(replicas)

	// If this request needs to go to a leader and we know who that is, move
	// it to the front.
	if !(proto.IsRead(args) && args.Header().ReadConsistency == proto.INCONSISTENT) &&
		leader.StoreID > 0 {
		if i := replicas.FindReplica(leader.StoreID); i >= 0 {
			replicas.MoveToFront(i)
			order = rpc.OrderStable
		}
	}

	return ds.sendRPC(trace, desc.RaftID, replicas, order, args, reply)
}

// CallComplete is called by a node whenever it completes a request. This will
// publish an appropriate event to the feed based on the results of the call.
func (nef NodeEventFeed) CallComplete(args proto.Request, reply proto.Response) {
	if err := reply.Header().Error; err != nil &&
		err.CanRestartTransaction() == proto.TransactionRestart_ABORT {
		nef.f.Publish(&CallErrorEvent{
			NodeID: nef.id,
			Method: args.Method(),
		})
	} else {
		nef.f.Publish(&CallSuccessEvent{
			NodeID: nef.id,
			Method: args.Method(),
		})
	}
}

// addAdminCmd executes the command directly. There is no interaction
// with the command queue or the timestamp cache, as admin commands
// are not meant to consistently access or modify the underlying data.
// Admin commands must run on the leader replica.
func (r *Range) addAdminCmd(ctx context.Context, args proto.Request, reply proto.Response) error {
	// Admin commands always require the leader lease.
	if err := r.redirectOnOrAcquireLeaderLease(args.Header().Timestamp); err != nil {
		reply.Header().SetGoError(err)
		return err
	}

	switch args.(type) {
	case *proto.AdminSplitRequest:
		r.AdminSplit(args.(*proto.AdminSplitRequest), reply.(*proto.AdminSplitResponse))
	case *proto.AdminMergeRequest:
		r.AdminMerge(args.(*proto.AdminMergeRequest), reply.(*proto.AdminMergeResponse))
	default:
		return util.Error("unrecognized admin command")
	}
	return reply.Header().GoError()
}

// executeCmd creates a proto.Call struct and sends it via our local sender.
func (n *nodeServer) executeCmd(args proto.Request, reply proto.Response) error {
	// TODO(tschottdorf) get a hold of the client's ID, add it to the
	// context before dispatching, and create an ID for tracing the request.
	header := args.Header()
	header.CmdID = header.GetOrCreateCmdID(n.ctx.Clock.PhysicalNow())
	trace := n.ctx.Tracer.NewTrace(header)
	defer trace.Finalize()
	defer trace.Epoch("node")()
	ctx := tracer.ToCtx((*Node)(n).context(), trace)

	n.lSender.Send(ctx, proto.Call{Args: args, Reply: reply})
	n.feed.CallComplete(args, reply)
	if err := reply.Header().GoError(); err != nil {
		trace.Event(fmt.Sprintf("error: %T", err))
	}
	return nil
}

// CallComplete is called by a node whenever it completes a request. This will
// publish an appropriate event to the feed based on the results of the call.
// TODO(tschottdorf): move to batch, account for multiple methods per batch.
// In particular, on error want an error position to identify the failed
// request.
func (nef NodeEventFeed) CallComplete(args proto.Request, reply proto.Response) {
	method := args.Method()
	if ba, ok := args.(*proto.BatchRequest); ok && len(ba.Requests) > 0 {
		method = ba.Requests[0].GetInner().Method()
	}
	if err := reply.Header().Error; err != nil &&
		err.TransactionRestart == proto.TransactionRestart_ABORT {
		nef.f.Publish(&CallErrorEvent{
			NodeID: nef.id,
			Method: method,
		})
	} else {
		nef.f.Publish(&CallSuccessEvent{
			NodeID: nef.id,
			Method: method,
		})
	}
}

// ExecuteCmd fetches a range based on the header's replica, assembles
// method, args & reply into a Raft Cmd struct and executes the
// command using the fetched range.
func (s *Store) ExecuteCmd(method string, args proto.Request, reply proto.Response) error {
	// If the request has a zero timestamp, initialize to this node's clock.
	header := args.Header()
	if header.Timestamp.WallTime == 0 && header.Timestamp.Logical == 0 {
		// Update both incoming and outgoing timestamps.
		now := s.clock.Now()
		args.Header().Timestamp = now
		reply.Header().Timestamp = now
	} else {
		// Otherwise, update our clock with the incoming request. This
		// advances the local node's clock to a high water mark from
		// amongst all nodes with which it has interacted. The update is
		// bounded by the max clock drift.
		_, err := s.clock.Update(header.Timestamp)
		if err != nil {
			return err
		}
	}

	// Verify specified range contains the command's implicated keys.
	rng, err := s.GetRange(header.Replica.RangeID)
	if err != nil {
		return err
	}
	if !rng.ContainsKeyRange(header.Key, header.EndKey) {
		return proto.NewRangeKeyMismatchError(header.Key, header.EndKey, rng.Meta)
	}
	if !rng.IsLeader() {
		// TODO(spencer): when we happen to know the leader, fill it in here via replica.
		return &proto.NotLeaderError{}
	}

	// Differentiate between read-only and read-write.
	if IsReadOnly(method) {
		return rng.ReadOnlyCmd(method, args, reply)
	}

	return rng.ReadWriteCmd(method, args, reply)
}

// proposeRaftCommand prepares necessary pending command struct and
// initializes a client command ID if one hasn't been. It then
// proposes the command to Raft and returns the error channel and
// pending command struct for receiving.
func (r *Range) proposeRaftCommand(ctx context.Context, args proto.Request) (<-chan error, *pendingCmd) {
	pendingCmd := &pendingCmd{
		ctx:  ctx,
		done: make(chan responseWithErr, 1),
	}
	raftCmd := proto.InternalRaftCommand{
		RaftID:       r.Desc().RaftID,
		OriginNodeID: r.rm.RaftNodeID(),
	}
	cmdID := args.Header().GetOrCreateCmdID(r.rm.Clock().PhysicalNow())
	ok := raftCmd.Cmd.SetValue(args)
	if !ok {
		log.Fatalc(ctx, "unknown command type %T", args)
	}
	idKey := makeCmdIDKey(cmdID)
	r.Lock()
	r.pendingCmds[idKey] = pendingCmd
	r.Unlock()
	errChan := r.rm.ProposeRaftCommand(idKey, raftCmd)

	return errChan, pendingCmd
}

// addAdminCmd executes the command directly. There is no interaction
// with the command queue or the timestamp cache, as admin commands
// are not meant to consistently access or modify the underlying data.
// Admin commands must run on the leader replica.
func (r *Range) addAdminCmd(ctx context.Context, args proto.Request) (proto.Response, error) {
	header := args.Header()

	if err := r.checkCmdHeader(header); err != nil {
		return nil, err
	}

	// Admin commands always require the leader lease.
	if err := r.redirectOnOrAcquireLeaderLease(tracer.FromCtx(ctx), header.Timestamp); err != nil {
		return nil, err
	}

	switch tArgs := args.(type) {
	case *proto.AdminSplitRequest:
		resp, err := r.AdminSplit(tArgs)
		return &resp, err
	case *proto.AdminMergeRequest:
		resp, err := r.AdminMerge(tArgs)
		return &resp, err
	default:
		return nil, util.Error("unrecognized admin command")
	}
}