Peeking via MSC2753 (#1370)

Initial implementation of MSC2753, as tested by https://github.com/matrix-org/sytest/pull/944.
Doesn't yet handle unpeeks, peeked EDUs, or history viz changing during a peek - these will follow.
https://github.com/matrix-org/dendrite/pull/1370 has full details.

README.md

@@ -81,6 +81,18 @@ matrixdotorg/sytest-dendrite:latest tests/50federation/40devicelists.pl
 ```
 See [sytest.md](docs/sytest.md) for the full description of these flags.
 
+You can try running sytest outside of docker for faster runs, but the dependencies can be temperamental
+and we recommend using docker where possible.
+```
+cd sytest
+export PERL5LIB=$HOME/lib/perl5
+export PERL_MB_OPT=--install_base=$HOME
+export PERL_MM_OPT=INSTALL_BASE=$HOME
+./install-deps.pl
+
+./run-tests.pl -I Dendrite::Monolith -d $PATH_TO_DENDRITE_BINARIES
+```
+
 Sometimes Sytest is testing the wrong thing or is flakey, so it will need to be patched.
 Ask on `#dendrite-dev:matrix.org` if you think this is the case for you and we'll be happy to help.
 

clientapi/routing/joinroom.go

@@ -52,7 +52,7 @@ func JoinRoomByIDOrAlias(
 		}
 	}
 
-	// If content was provided in the request then incude that
+	// If content was provided in the request then include that
 	// in the request. It'll get used as a part of the membership
 	// event content.
 	_ = httputil.UnmarshalJSONRequest(req, &joinReq.Content)

clientapi/routing/peekroom.go

@@ -0,0 +1,79 @@
+// Copyright 2020 New Vector Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package routing
+
+import (
+	"net/http"
+
+	roomserverAPI "github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/dendrite/userapi/api"
+	"github.com/matrix-org/dendrite/userapi/storage/accounts"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
+)
+
+func PeekRoomByIDOrAlias(
+	req *http.Request,
+	device *api.Device,
+	rsAPI roomserverAPI.RoomserverInternalAPI,
+	accountDB accounts.Database,
+	roomIDOrAlias string,
+) util.JSONResponse {
+	// if this is a remote roomIDOrAlias, we have to ask the roomserver (or federation sender?) to
+	// to call /peek and /state on the remote server.
+	// TODO: in future we could skip this if we know we're already participating in the room,
+	// but this is fiddly in case we stop participating in the room.
+
+	// then we create a local peek.
+	peekReq := roomserverAPI.PerformPeekRequest{
+		RoomIDOrAlias: roomIDOrAlias,
+		UserID:        device.UserID,
+		DeviceID:      device.ID,
+	}
+	peekRes := roomserverAPI.PerformPeekResponse{}
+
+	// Check to see if any ?server_name= query parameters were
+	// given in the request.
+	if serverNames, ok := req.URL.Query()["server_name"]; ok {
+		for _, serverName := range serverNames {
+			peekReq.ServerNames = append(
+				peekReq.ServerNames,
+				gomatrixserverlib.ServerName(serverName),
+			)
+		}
+	}
+
+	// Ask the roomserver to perform the peek.
+	rsAPI.PerformPeek(req.Context(), &peekReq, &peekRes)
+	if peekRes.Error != nil {
+		return peekRes.Error.JSONResponse()
+	}
+
+	// if this user is already joined to the room, we let them peek anyway
+	// (given they might be about to part the room, and it makes things less fiddly)
+
+	// Peeking stops if none of the devices who started peeking have been
+	// /syncing for a while, or if everyone who was peeking calls /leave
+	// (or /unpeek with a server_name param? or DELETE /peek?)
+	// on the peeked room.
+
+	return util.JSONResponse{
+		Code: http.StatusOK,
+		// TODO: Put the response struct somewhere internal.
+		JSON: struct {
+			RoomID string `json:"room_id"`
+		}{peekRes.RoomID},
+	}
+}

clientapi/routing/routing.go

@@ -103,6 +103,17 @@ func Setup(
 			)
 		}),
 	).Methods(http.MethodPost, http.MethodOptions)
+	r0mux.Handle("/peek/{roomIDOrAlias}",
+		httputil.MakeAuthAPI(gomatrixserverlib.Peek, userAPI, func(req *http.Request, device *userapi.Device) util.JSONResponse {
+			vars, err := httputil.URLDecodeMapValues(mux.Vars(req))
+			if err != nil {
+				return util.ErrorResponse(err)
+			}
+			return PeekRoomByIDOrAlias(
+				req, device, rsAPI, accountDB, vars["roomIDOrAlias"],
+			)
+		}),
+	).Methods(http.MethodPost, http.MethodOptions)
 	r0mux.Handle("/joined_rooms",
 		httputil.MakeAuthAPI("joined_rooms", userAPI, func(req *http.Request, device *userapi.Device) util.JSONResponse {
 			return GetJoinedRooms(req, device, rsAPI)

docs/peeking.md

@@ -0,0 +1,19 @@
+## Peeking
+
+Peeking is implemented as per [MSC2753](https://github.com/matrix-org/matrix-doc/pull/2753).
+
+Implementationwise, this means:
+ * Users call `/peek` and `/unpeek` on the clientapi from a given device.
+ * The clientapi delegates these via HTTP to the roomserver, which coordinates peeking in general for a given room
+ * The roomserver writes an NewPeek event into the kafka log headed to the syncserver
+ * The syncserver tracks the existence of the local peek in its DB, and then starts waking up the peeking devices for the room in question, putting it in the `peek` section of the /sync response.
+
+Questions (given this is [my](https://github.com/ara4n) first time hacking on Dendrite):
+ * The whole clientapi -> roomserver -> syncapi flow to initiate a peek seems very indirect.  Is there a reason not to just let syncapi itself host the implementation of `/peek`?
+
+In future, peeking over federation will be added as per [MSC2444](https://github.com/matrix-org/matrix-doc/pull/2444).
+ * The `roomserver` will kick the `federationsender` much as it does for a federated `/join` in order to trigger a federated `/peek`
+ * The `federationsender` tracks the existence of the remote peek in question
+ * The `federationsender` regularly renews the remote peek as long as there are still peeking devices syncing for it.
+  * TBD: how do we tell if there are no devices currently syncing for a given peeked room?  The syncserver needs to tell the roomserver
+    somehow who then needs to warn the federationsender.

federationapi/routing/send_test.go

@@ -112,6 +112,13 @@ func (t *testRoomserverAPI) PerformJoin(
 ) {
 }
 
+func (t *testRoomserverAPI) PerformPeek(
+	ctx context.Context,
+	req *api.PerformPeekRequest,
+	res *api.PerformPeekResponse,
+) {
+}
+
 func (t *testRoomserverAPI) PerformPublish(
 	ctx context.Context,
 	req *api.PerformPublishRequest,

federationsender/storage/shared/storage.go

@@ -138,7 +138,7 @@ func (d *Database) StoreJSON(
 	var err error
 	_ = d.Writer.Do(d.DB, nil, func(txn *sql.Tx) error {
 		nid, err = d.FederationSenderQueueJSON.InsertQueueJSON(ctx, txn, js)
-		return nil
+		return err
 	})
 	if err != nil {
 		return nil, fmt.Errorf("d.insertQueueJSON: %w", err)

internal/sqlutil/trace.go

@@ -74,7 +74,7 @@ func (in *traceInterceptor) RowsNext(c context.Context, rows driver.Rows, dest [
 
 	b := strings.Builder{}
 	for i, val := range dest {
-		b.WriteString(fmt.Sprintf("%v", val))
+		b.WriteString(fmt.Sprintf("%q", val))
 		if i+1 <= len(dest)-1 {
 			b.WriteString(" | ")
 		}

keyserver/storage/shared/storage.go

@@ -41,7 +41,7 @@ func (d *Database) ExistingOneTimeKeys(ctx context.Context, userID, deviceID str
 func (d *Database) StoreOneTimeKeys(ctx context.Context, keys api.OneTimeKeys) (counts *api.OneTimeKeysCount, err error) {
 	_ = d.Writer.Do(d.DB, nil, func(txn *sql.Tx) error {
 		counts, err = d.OneTimeKeysTable.InsertOneTimeKeys(ctx, txn, keys)
-		return nil
+		return err
 	})
 	return
 }

roomserver/api/api.go

@@ -36,6 +36,12 @@ type RoomserverInternalAPI interface {
 		res *PerformLeaveResponse,
 	) error
 
+	PerformPeek(
+		ctx context.Context,
+		req *PerformPeekRequest,
+		res *PerformPeekResponse,
+	)
+
 	PerformPublish(
 		ctx context.Context,
 		req *PerformPublishRequest,

roomserver/api/api_trace.go

@@ -38,6 +38,15 @@ func (t *RoomserverInternalAPITrace) PerformInvite(
 	return t.Impl.PerformInvite(ctx, req, res)
 }
 
+func (t *RoomserverInternalAPITrace) PerformPeek(
+	ctx context.Context,
+	req *PerformPeekRequest,
+	res *PerformPeekResponse,
+) {
+	t.Impl.PerformPeek(ctx, req, res)
+	util.GetLogger(ctx).Infof("PerformPeek req=%+v res=%+v", js(req), js(res))
+}
+
 func (t *RoomserverInternalAPITrace) PerformJoin(
 	ctx context.Context,
 	req *PerformJoinRequest,

roomserver/api/output.go

@@ -46,6 +46,9 @@ const (
 	// - Redact the event and set the corresponding `unsigned` fields to indicate it as redacted.
 	// - Replace the event in the database.
 	OutputTypeRedactedEvent OutputType = "redacted_event"
+
+	// OutputTypeNewPeek indicates that the kafka event is an OutputNewPeek
+	OutputTypeNewPeek OutputType = "new_peek"
 )
 
 // An OutputEvent is an entry in the roomserver output kafka log.
@@ -61,6 +64,8 @@ type OutputEvent struct {
 	RetireInviteEvent *OutputRetireInviteEvent `json:"retire_invite_event,omitempty"`
 	// The content of event with type OutputTypeRedactedEvent
 	RedactedEvent *OutputRedactedEvent `json:"redacted_event,omitempty"`
+	// The content of event with type OutputTypeNewPeek
+	NewPeek *OutputNewPeek `json:"new_peek,omitempty"`
 }
 
 // An OutputNewRoomEvent is written when the roomserver receives a new event.
@@ -195,3 +200,11 @@ type OutputRedactedEvent struct {
 	// The value of `unsigned.redacted_because` - the redaction event itself
 	RedactedBecause gomatrixserverlib.HeaderedEvent
 }
+
+// An OutputNewPeek is written whenever a user starts peeking into a room
+// using a given device.
+type OutputNewPeek struct {
+	RoomID   string
+	UserID   string
+	DeviceID string
+}

roomserver/api/perform.go

@@ -108,6 +108,20 @@ type PerformInviteResponse struct {
 	Error *PerformError
 }
 
+type PerformPeekRequest struct {
+	RoomIDOrAlias string                         `json:"room_id_or_alias"`
+	UserID        string                         `json:"user_id"`
+	DeviceID      string                         `json:"device_id"`
+	ServerNames   []gomatrixserverlib.ServerName `json:"server_names"`
+}
+
+type PerformPeekResponse struct {
+	// The room ID, populated on success.
+	RoomID string `json:"room_id"`
+	// If non-nil, the join request failed. Contains more information why it failed.
+	Error *PerformError
+}
+
 // PerformBackfillRequest is a request to PerformBackfill.
 type PerformBackfillRequest struct {
 	// The room to backfill

roomserver/internal/api.go

@@ -22,6 +22,7 @@ type RoomserverInternalAPI struct {
 	*query.Queryer
 	*perform.Inviter
 	*perform.Joiner
+	*perform.Peeker
 	*perform.Leaver
 	*perform.Publisher
 	*perform.Backfiller
@@ -83,6 +84,13 @@ func (r *RoomserverInternalAPI) SetFederationSenderAPI(fsAPI fsAPI.FederationSen
 		FSAPI:      r.fsAPI,
 		Inputer:    r.Inputer,
 	}
+	r.Peeker = &perform.Peeker{
+		ServerName: r.Cfg.Matrix.ServerName,
+		Cfg:        r.Cfg,
+		DB:         r.DB,
+		FSAPI:      r.fsAPI,
+		Inputer:    r.Inputer,
+	}
 	r.Leaver = &perform.Leaver{
 		Cfg:     r.Cfg,
 		DB:      r.DB,

roomserver/internal/perform/perform_peek.go

@@ -0,0 +1,206 @@
+// Copyright 2020 New Vector Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package perform
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"strings"
+
+	fsAPI "github.com/matrix-org/dendrite/federationsender/api"
+	"github.com/matrix-org/dendrite/internal/config"
+	"github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/dendrite/roomserver/internal/input"
+	"github.com/matrix-org/dendrite/roomserver/storage"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
+	"github.com/sirupsen/logrus"
+)
+
+type Peeker struct {
+	ServerName gomatrixserverlib.ServerName
+	Cfg        *config.RoomServer
+	FSAPI      fsAPI.FederationSenderInternalAPI
+	DB         storage.Database
+
+	Inputer *input.Inputer
+}
+
+// PerformPeek handles peeking into matrix rooms, including over federation by talking to the federationsender.
+func (r *Peeker) PerformPeek(
+	ctx context.Context,
+	req *api.PerformPeekRequest,
+	res *api.PerformPeekResponse,
+) {
+	roomID, err := r.performPeek(ctx, req)
+	if err != nil {
+		perr, ok := err.(*api.PerformError)
+		if ok {
+			res.Error = perr
+		} else {
+			res.Error = &api.PerformError{
+				Msg: err.Error(),
+			}
+		}
+	}
+	res.RoomID = roomID
+}
+
+func (r *Peeker) performPeek(
+	ctx context.Context,
+	req *api.PerformPeekRequest,
+) (string, error) {
+	// FIXME: there's way too much duplication with performJoin
+	_, domain, err := gomatrixserverlib.SplitID('@', req.UserID)
+	if err != nil {
+		return "", &api.PerformError{
+			Code: api.PerformErrorBadRequest,
+			Msg:  fmt.Sprintf("Supplied user ID %q in incorrect format", req.UserID),
+		}
+	}
+	if domain != r.Cfg.Matrix.ServerName {
+		return "", &api.PerformError{
+			Code: api.PerformErrorBadRequest,
+			Msg:  fmt.Sprintf("User %q does not belong to this homeserver", req.UserID),
+		}
+	}
+	if strings.HasPrefix(req.RoomIDOrAlias, "!") {
+		return r.performPeekRoomByID(ctx, req)
+	}
+	if strings.HasPrefix(req.RoomIDOrAlias, "#") {
+		return r.performPeekRoomByAlias(ctx, req)
+	}
+	return "", &api.PerformError{
+		Code: api.PerformErrorBadRequest,
+		Msg:  fmt.Sprintf("Room ID or alias %q is invalid", req.RoomIDOrAlias),
+	}
+}
+
+func (r *Peeker) performPeekRoomByAlias(
+	ctx context.Context,
+	req *api.PerformPeekRequest,
+) (string, error) {
+	// Get the domain part of the room alias.
+	_, domain, err := gomatrixserverlib.SplitID('#', req.RoomIDOrAlias)
+	if err != nil {
+		return "", fmt.Errorf("Alias %q is not in the correct format", req.RoomIDOrAlias)
+	}
+	req.ServerNames = append(req.ServerNames, domain)
+
+	// Check if this alias matches our own server configuration. If it
+	// doesn't then we'll need to try a federated peek.
+	var roomID string
+	if domain != r.Cfg.Matrix.ServerName {
+		// The alias isn't owned by us, so we will need to try peeking using
+		// a remote server.
+		dirReq := fsAPI.PerformDirectoryLookupRequest{
+			RoomAlias:  req.RoomIDOrAlias, // the room alias to lookup
+			ServerName: domain,            // the server to ask
+		}
+		dirRes := fsAPI.PerformDirectoryLookupResponse{}
+		err = r.FSAPI.PerformDirectoryLookup(ctx, &dirReq, &dirRes)
+		if err != nil {
+			logrus.WithError(err).Errorf("error looking up alias %q", req.RoomIDOrAlias)
+			return "", fmt.Errorf("Looking up alias %q over federation failed: %w", req.RoomIDOrAlias, err)
+		}
+		roomID = dirRes.RoomID
+		req.ServerNames = append(req.ServerNames, dirRes.ServerNames...)
+	} else {
+		// Otherwise, look up if we know this room alias locally.
+		roomID, err = r.DB.GetRoomIDForAlias(ctx, req.RoomIDOrAlias)
+		if err != nil {
+			return "", fmt.Errorf("Lookup room alias %q failed: %w", req.RoomIDOrAlias, err)
+		}
+	}
+
+	// If the room ID is empty then we failed to look up the alias.
+	if roomID == "" {
+		return "", fmt.Errorf("Alias %q not found", req.RoomIDOrAlias)
+	}
+
+	// If we do, then pluck out the room ID and continue the peek.
+	req.RoomIDOrAlias = roomID
+	return r.performPeekRoomByID(ctx, req)
+}
+
+func (r *Peeker) performPeekRoomByID(
+	ctx context.Context,
+	req *api.PerformPeekRequest,
+) (roomID string, err error) {
+	roomID = req.RoomIDOrAlias
+
+	// Get the domain part of the room ID.
+	_, domain, err := gomatrixserverlib.SplitID('!', roomID)
+	if err != nil {
+		return "", &api.PerformError{
+			Code: api.PerformErrorBadRequest,
+			Msg:  fmt.Sprintf("Room ID %q is invalid: %s", roomID, err),
+		}
+	}
+
+	// If the server name in the room ID isn't ours then it's a
+	// possible candidate for finding the room via federation. Add
+	// it to the list of servers to try.
+	if domain != r.Cfg.Matrix.ServerName {
+		req.ServerNames = append(req.ServerNames, domain)
+	}
+
+	// If this room isn't world_readable, we reject.
+	// XXX: would be nicer to call this with NIDs
+	// XXX: we should probably factor out history_visibility checks into a common utility method somewhere
+	// which handles the default value etc.
+	var worldReadable = false
+	ev, _ := r.DB.GetStateEvent(ctx, roomID, "m.room.history_visibility", "")
+	if ev != nil {
+		content := map[string]string{}
+		if err = json.Unmarshal(ev.Content(), &content); err != nil {
+			util.GetLogger(ctx).WithError(err).Error("json.Unmarshal for history visibility failed")
+			return
+		}
+		if visibility, ok := content["history_visibility"]; ok {
+			worldReadable = visibility == "world_readable"
+		}
+	}
+
+	if !worldReadable {
+		return "", &api.PerformError{
+			Code: api.PerformErrorNotAllowed,
+			Msg:  "Room is not world-readable",
+		}
+	}
+
+	// TODO: handle federated peeks
+
+	err = r.Inputer.WriteOutputEvents(roomID, []api.OutputEvent{
+		{
+			Type: api.OutputTypeNewPeek,
+			NewPeek: &api.OutputNewPeek{
+				RoomID:   roomID,
+				UserID:   req.UserID,
+				DeviceID: req.DeviceID,
+			},
+		},
+	})
+	if err != nil {
+		return
+	}
+
+	// By this point, if req.RoomIDOrAlias contained an alias, then
+	// it will have been overwritten with a room ID by performPeekRoomByAlias.
+	// We should now include this in the response so that the CS API can
+	// return the right room ID.
+	return roomID, nil
+}

roomserver/inthttp/client.go

@@ -26,6 +26,7 @@ const (
 
 	// Perform operations
 	RoomserverPerformInvitePath   = "/roomserver/performInvite"
+	RoomserverPerformPeekPath     = "/roomserver/performPeek"
 	RoomserverPerformJoinPath     = "/roomserver/performJoin"
 	RoomserverPerformLeavePath    = "/roomserver/performLeave"
 	RoomserverPerformBackfillPath = "/roomserver/performBackfill"
@@ -185,6 +186,23 @@ func (h *httpRoomserverInternalAPI) PerformJoin(
 	}
 }
 
+func (h *httpRoomserverInternalAPI) PerformPeek(
+	ctx context.Context,
+	request *api.PerformPeekRequest,
+	response *api.PerformPeekResponse,
+) {
+	span, ctx := opentracing.StartSpanFromContext(ctx, "PerformPeek")
+	defer span.Finish()
+
+	apiURL := h.roomserverURL + RoomserverPerformPeekPath
+	err := httputil.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
+	if err != nil {
+		response.Error = &api.PerformError{
+			Msg: fmt.Sprintf("failed to communicate with roomserver: %s", err),
+		}
+	}
+}
+
 func (h *httpRoomserverInternalAPI) PerformLeave(
 	ctx context.Context,
 	request *api.PerformLeaveRequest,

roomserver/inthttp/server.go

@@ -63,6 +63,17 @@ func AddRoutes(r api.RoomserverInternalAPI, internalAPIMux *mux.Router) {
 			return util.JSONResponse{Code: http.StatusOK, JSON: &response}
 		}),
 	)
+	internalAPIMux.Handle(RoomserverPerformPeekPath,
+		httputil.MakeInternalAPI("performPeek", func(req *http.Request) util.JSONResponse {
+			var request api.PerformPeekRequest
+			var response api.PerformPeekResponse
+			if err := json.NewDecoder(req.Body).Decode(&request); err != nil {
+				return util.MessageResponse(http.StatusBadRequest, err.Error())
+			}
+			r.PerformPeek(req.Context(), &request, &response)
+			return util.JSONResponse{Code: http.StatusOK, JSON: &response}
+		}),
+	)
 	internalAPIMux.Handle(RoomserverPerformPublishPath,
 		httputil.MakeInternalAPI("performPublish", func(req *http.Request) util.JSONResponse {
 			var request api.PerformPublishRequest

roomserver/storage/shared/storage.go

@@ -359,7 +359,7 @@ func (d *Database) MembershipUpdater(
 	var updater *MembershipUpdater
 	_ = d.Writer.Do(d.DB, txn, func(txn *sql.Tx) error {
 		updater, err = NewMembershipUpdater(ctx, d, txn, roomID, targetUserID, targetLocal, roomVersion)
-		return nil
+		return err
 	})
 	return updater, err
 }
@@ -374,7 +374,7 @@ func (d *Database) GetLatestEventsForUpdate(
 	var updater *LatestEventsUpdater
 	_ = d.Writer.Do(d.DB, txn, func(txn *sql.Tx) error {
 		updater, err = NewLatestEventsUpdater(ctx, d, txn, roomInfo)
-		return nil
+		return err
 	})
 	return updater, err
 }

syncapi/consumers/roomserver.go

@@ -17,6 +17,7 @@ package consumers
 import (
 	"context"
 	"encoding/json"
+	"fmt"
 
 	"github.com/Shopify/sarama"
 	"github.com/matrix-org/dendrite/internal"
@@ -26,11 +27,13 @@ import (
 	"github.com/matrix-org/dendrite/syncapi/sync"
 	"github.com/matrix-org/dendrite/syncapi/types"
 	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/sirupsen/logrus"
 	log "github.com/sirupsen/logrus"
 )
 
 // OutputRoomEventConsumer consumes events that originated in the room server.
 type OutputRoomEventConsumer struct {
+	cfg        *config.SyncAPI
 	rsAPI      api.RoomserverInternalAPI
 	rsConsumer *internal.ContinualConsumer
 	db         storage.Database
@@ -55,6 +58,7 @@ func NewOutputRoomEventConsumer(
 		PartitionStore: store,
 	}
 	s := &OutputRoomEventConsumer{
+		cfg:        cfg,
 		rsConsumer: &consumer,
 		db:         store,
 		notifier:   n,
@@ -100,6 +104,8 @@ func (s *OutputRoomEventConsumer) onMessage(msg *sarama.ConsumerMessage) error {
 		return s.onNewInviteEvent(context.TODO(), *output.NewInviteEvent)
 	case api.OutputTypeRetireInviteEvent:
 		return s.onRetireInviteEvent(context.TODO(), *output.RetireInviteEvent)
+	case api.OutputTypeNewPeek:
+		return s.onNewPeek(context.TODO(), *output.NewPeek)
 	case api.OutputTypeRedactedEvent:
 		return s.onRedactEvent(context.TODO(), *output.RedactedEvent)
 	default:
@@ -162,6 +168,12 @@ func (s *OutputRoomEventConsumer) onNewRoomEvent(
 		}).Panicf("roomserver output log: write event failure")
 		return nil
 	}
+
+	if pduPos, err = s.notifyJoinedPeeks(ctx, &ev, pduPos); err != nil {
+		logrus.WithError(err).Errorf("Failed to notifyJoinedPeeks for PDU pos %d", pduPos)
+		return err
+	}
+
 	s.notifier.OnNewEvent(&ev, "", nil, types.NewStreamToken(pduPos, 0, nil))
 
 	s.notifyKeyChanges(&ev)
@@ -184,6 +196,37 @@ func (s *OutputRoomEventConsumer) notifyKeyChanges(ev *gomatrixserverlib.Headere
 	}
 }
 
+func (s *OutputRoomEventConsumer) notifyJoinedPeeks(ctx context.Context, ev *gomatrixserverlib.HeaderedEvent, sp types.StreamPosition) (types.StreamPosition, error) {
+	if ev.Type() != gomatrixserverlib.MRoomMember {
+		return sp, nil
+	}
+	membership, err := ev.Membership()
+	if err != nil {
+		return sp, fmt.Errorf("ev.Membership: %w", err)
+	}
+	// TODO: check that it's a join and not a profile change (means unmarshalling prev_content)
+	if membership == gomatrixserverlib.Join {
+		// check it's a local join
+		_, domain, err := gomatrixserverlib.SplitID('@', *ev.StateKey())
+		if err != nil {
+			return sp, fmt.Errorf("gomatrixserverlib.SplitID: %w", err)
+		}
+		if domain != s.cfg.Matrix.ServerName {
+			return sp, nil
+		}
+
+		// cancel any peeks for it
+		peekSP, peekErr := s.db.DeletePeeks(ctx, ev.RoomID(), *ev.StateKey())
+		if peekErr != nil {
+			return sp, fmt.Errorf("s.db.DeletePeeks: %w", peekErr)
+		}
+		if peekSP > 0 {
+			sp = peekSP
+		}
+	}
+	return sp, nil
+}
+
 func (s *OutputRoomEventConsumer) onNewInviteEvent(
 	ctx context.Context, msg api.OutputNewInviteEvent,
 ) error {
@@ -219,6 +262,26 @@ func (s *OutputRoomEventConsumer) onRetireInviteEvent(
 	return nil
 }
 
+func (s *OutputRoomEventConsumer) onNewPeek(
+	ctx context.Context, msg api.OutputNewPeek,
+) error {
+	sp, err := s.db.AddPeek(ctx, msg.RoomID, msg.UserID, msg.DeviceID)
+	if err != nil {
+		// panic rather than continue with an inconsistent database
+		log.WithFields(log.Fields{
+			log.ErrorKey: err,
+		}).Panicf("roomserver output log: write peek failure")
+		return nil
+	}
+	// tell the notifier about the new peek so it knows to wake up new devices
+	s.notifier.OnNewPeek(msg.RoomID, msg.UserID, msg.DeviceID)
+
+	// we need to wake up the users who might need to now be peeking into this room,
+	// so we send in a dummy event to trigger a wakeup
+	s.notifier.OnNewEvent(nil, msg.RoomID, nil, types.NewStreamToken(sp, 0, nil))
+	return nil
+}
+
 func (s *OutputRoomEventConsumer) updateStateEvent(event gomatrixserverlib.HeaderedEvent) (gomatrixserverlib.HeaderedEvent, error) {
 	if event.StateKey() == nil {
 		return event, nil

syncapi/storage/interface.go

@@ -30,6 +30,8 @@ type Database interface {
 	internal.PartitionStorer
 	// AllJoinedUsersInRooms returns a map of room ID to a list of all joined user IDs.
 	AllJoinedUsersInRooms(ctx context.Context) (map[string][]string, error)
+	// AllPeekingDevicesInRooms returns a map of room ID to a list of all peeking devices.
+	AllPeekingDevicesInRooms(ctx context.Context) (map[string][]types.PeekingDevice, error)
 	// Events lookups a list of event by their event ID.
 	// Returns a list of events matching the requested IDs found in the database.
 	// If an event is not found in the database then it will be omitted from the list.
@@ -81,6 +83,12 @@ type Database interface {
 	// RetireInviteEvent removes an old invite event from the database. Returns the new position of the retired invite.
 	// Returns an error if there was a problem communicating with the database.
 	RetireInviteEvent(ctx context.Context, inviteEventID string) (types.StreamPosition, error)
+	// AddPeek adds a new peek to our DB for a given room by a given user's device.
+	// Returns an error if there was a problem communicating with the database.
+	AddPeek(ctx context.Context, RoomID, UserID, DeviceID string) (types.StreamPosition, error)
+	// DeletePeek deletes all peeks for a given room by a given user
+	// Returns an error if there was a problem communicating with the database.
+	DeletePeeks(ctx context.Context, RoomID, UserID string) (types.StreamPosition, error)
 	// SetTypingTimeoutCallback sets a callback function that is called right after
 	// a user is removed from the typing user list due to timeout.
 	SetTypingTimeoutCallback(fn cache.TimeoutCallbackFn)

syncapi/storage/postgres/peeks_table.go

@@ -0,0 +1,186 @@
+// Copyright 2020 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package postgres
+
+import (
+	"context"
+	"database/sql"
+	"time"
+
+	"github.com/matrix-org/dendrite/internal"
+	"github.com/matrix-org/dendrite/internal/sqlutil"
+	"github.com/matrix-org/dendrite/syncapi/storage/tables"
+	"github.com/matrix-org/dendrite/syncapi/types"
+)
+
+const peeksSchema = `
+CREATE TABLE IF NOT EXISTS syncapi_peeks (
+	id BIGINT DEFAULT nextval('syncapi_stream_id'),
+	room_id TEXT NOT NULL,
+	user_id TEXT NOT NULL,
+	device_id TEXT NOT NULL,
+	deleted BOOL NOT NULL DEFAULT false,
+    -- When the peek was created in UNIX epoch ms.
+    creation_ts BIGINT NOT NULL,
+    UNIQUE(room_id, user_id, device_id)
+);
+
+CREATE INDEX IF NOT EXISTS syncapi_peeks_room_id_idx ON syncapi_peeks(room_id);
+CREATE INDEX IF NOT EXISTS syncapi_peeks_user_id_device_id_idx ON syncapi_peeks(user_id, device_id);
+`
+
+const insertPeekSQL = "" +
+	"INSERT INTO syncapi_peeks" +
+	" (room_id, user_id, device_id, creation_ts)" +
+	" VALUES ($1, $2, $3, $4)" +
+	" ON CONFLICT (room_id, user_id, device_id) DO UPDATE SET deleted=false, creation_ts=$4" +
+	" RETURNING id"
+
+const deletePeekSQL = "" +
+	"UPDATE syncapi_peeks SET deleted=true, id=nextval('syncapi_stream_id') WHERE room_id = $1 AND user_id = $2 AND device_id = $3 RETURNING id"
+
+const deletePeeksSQL = "" +
+	"UPDATE syncapi_peeks SET deleted=true, id=nextval('syncapi_stream_id') WHERE room_id = $1 AND user_id = $2 RETURNING id"
+
+// we care about all the peeks which were created in this range, deleted in this range,
+// or were created before this range but haven't been deleted yet.
+const selectPeeksInRangeSQL = "" +
+	"SELECT room_id, deleted, (id > $3 AND id <= $4) AS changed FROM syncapi_peeks WHERE user_id = $1 AND device_id = $2 AND ((id <= $3 AND NOT deleted) OR (id > $3 AND id <= $4))"
+
+const selectPeekingDevicesSQL = "" +
+	"SELECT room_id, user_id, device_id FROM syncapi_peeks WHERE deleted=false"
+
+const selectMaxPeekIDSQL = "" +
+	"SELECT MAX(id) FROM syncapi_peeks"
+
+type peekStatements struct {
+	db                       *sql.DB
+	insertPeekStmt           *sql.Stmt
+	deletePeekStmt           *sql.Stmt
+	deletePeeksStmt          *sql.Stmt
+	selectPeeksInRangeStmt   *sql.Stmt
+	selectPeekingDevicesStmt *sql.Stmt
+	selectMaxPeekIDStmt      *sql.Stmt
+}
+
+func NewPostgresPeeksTable(db *sql.DB) (tables.Peeks, error) {
+	_, err := db.Exec(peeksSchema)
+	if err != nil {
+		return nil, err
+	}
+	s := &peekStatements{
+		db: db,
+	}
+	if s.insertPeekStmt, err = db.Prepare(insertPeekSQL); err != nil {
+		return nil, err
+	}
+	if s.deletePeekStmt, err = db.Prepare(deletePeekSQL); err != nil {
+		return nil, err
+	}
+	if s.deletePeeksStmt, err = db.Prepare(deletePeeksSQL); err != nil {
+		return nil, err
+	}
+	if s.selectPeeksInRangeStmt, err = db.Prepare(selectPeeksInRangeSQL); err != nil {
+		return nil, err
+	}
+	if s.selectPeekingDevicesStmt, err = db.Prepare(selectPeekingDevicesSQL); err != nil {
+		return nil, err
+	}
+	if s.selectMaxPeekIDStmt, err = db.Prepare(selectMaxPeekIDSQL); err != nil {
+		return nil, err
+	}
+	return s, nil
+}
+
+func (s *peekStatements) InsertPeek(
+	ctx context.Context, txn *sql.Tx, roomID, userID, deviceID string,
+) (streamPos types.StreamPosition, err error) {
+	nowMilli := time.Now().UnixNano() / int64(time.Millisecond)
+	stmt := sqlutil.TxStmt(txn, s.insertPeekStmt)
+	err = stmt.QueryRowContext(ctx, roomID, userID, deviceID, nowMilli).Scan(&streamPos)
+	return
+}
+
+func (s *peekStatements) DeletePeek(
+	ctx context.Context, txn *sql.Tx, roomID, userID, deviceID string,
+) (streamPos types.StreamPosition, err error) {
+	stmt := sqlutil.TxStmt(txn, s.deletePeekStmt)
+	err = stmt.QueryRowContext(ctx, roomID, userID, deviceID).Scan(&streamPos)
+	return
+}
+
+func (s *peekStatements) DeletePeeks(
+	ctx context.Context, txn *sql.Tx, roomID, userID string,
+) (streamPos types.StreamPosition, err error) {
+	stmt := sqlutil.TxStmt(txn, s.deletePeeksStmt)
+	err = stmt.QueryRowContext(ctx, roomID, userID).Scan(&streamPos)
+	return
+}
+
+func (s *peekStatements) SelectPeeksInRange(
+	ctx context.Context, txn *sql.Tx, userID, deviceID string, r types.Range,
+) (peeks []types.Peek, err error) {
+	rows, err := sqlutil.TxStmt(txn, s.selectPeeksInRangeStmt).QueryContext(ctx, userID, deviceID, r.Low(), r.High())
+	if err != nil {
+		return
+	}
+	defer internal.CloseAndLogIfError(ctx, rows, "SelectPeeksInRange: rows.close() failed")
+
+	for rows.Next() {
+		peek := types.Peek{}
+		var changed bool
+		if err = rows.Scan(&peek.RoomID, &peek.Deleted, &changed); err != nil {
+			return
+		}
+		peek.New = changed && !peek.Deleted
+		peeks = append(peeks, peek)
+	}
+
+	return peeks, rows.Err()
+}
+
+func (s *peekStatements) SelectPeekingDevices(
+	ctx context.Context,
+) (peekingDevices map[string][]types.PeekingDevice, err error) {
+	rows, err := s.selectPeekingDevicesStmt.QueryContext(ctx)
+	if err != nil {
+		return nil, err
+	}
+	defer internal.CloseAndLogIfError(ctx, rows, "SelectPeekingDevices: rows.close() failed")
+
+	result := make(map[string][]types.PeekingDevice)
+	for rows.Next() {
+		var roomID, userID, deviceID string
+		if err := rows.Scan(&roomID, &userID, &deviceID); err != nil {
+			return nil, err
+		}
+		devices := result[roomID]
+		devices = append(devices, types.PeekingDevice{UserID: userID, DeviceID: deviceID})
+		result[roomID] = devices
+	}
+	return result, nil
+}
+
+func (s *peekStatements) SelectMaxPeekID(
+	ctx context.Context, txn *sql.Tx,
+) (id int64, err error) {
+	var nullableID sql.NullInt64
+	stmt := sqlutil.TxStmt(txn, s.selectMaxPeekIDStmt)
+	err = stmt.QueryRowContext(ctx).Scan(&nullableID)
+	if nullableID.Valid {
+		id = nullableID.Int64
+	}
+	return
+}

syncapi/storage/postgres/syncserver.go

@@ -62,6 +62,10 @@ func NewDatabase(dbProperties *config.DatabaseOptions) (*SyncServerDatasource, e
 	if err != nil {
 		return nil, err
 	}
+	peeks, err := NewPostgresPeeksTable(d.db)
+	if err != nil {
+		return nil, err
+	}
 	topology, err := NewPostgresTopologyTable(d.db)
 	if err != nil {
 		return nil, err
@@ -82,6 +86,7 @@ func NewDatabase(dbProperties *config.DatabaseOptions) (*SyncServerDatasource, e
 		DB:                  d.db,
 		Writer:              d.writer,
 		Invites:             invites,
+		Peeks:               peeks,
 		AccountData:         accountData,
 		OutputEvents:        events,
 		Topology:            topology,

syncapi/storage/shared/syncserver.go

@@ -30,7 +30,7 @@ import (
 	"github.com/matrix-org/dendrite/syncapi/storage/tables"
 	"github.com/matrix-org/dendrite/syncapi/types"
 	"github.com/matrix-org/gomatrixserverlib"
-	"github.com/sirupsen/logrus"
+	log "github.com/sirupsen/logrus"
 )
 
 // Database is a temporary struct until we have made syncserver.go the same for both pq/sqlite
@@ -39,6 +39,7 @@ type Database struct {
 	DB                  *sql.DB
 	Writer              sqlutil.Writer
 	Invites             tables.Invites
+	Peeks               tables.Peeks
 	AccountData         tables.AccountData
 	OutputEvents        tables.Events
 	Topology            tables.Topology
@@ -120,6 +121,10 @@ func (d *Database) AllJoinedUsersInRooms(ctx context.Context) (map[string][]stri
 	return d.CurrentRoomState.SelectJoinedUsers(ctx)
 }
 
+func (d *Database) AllPeekingDevicesInRooms(ctx context.Context) (map[string][]types.PeekingDevice, error) {
+	return d.Peeks.SelectPeekingDevices(ctx)
+}
+
 func (d *Database) GetStateEvent(
 	ctx context.Context, roomID, evType, stateKey string,
 ) (*gomatrixserverlib.HeaderedEvent, error) {
@@ -141,7 +146,7 @@ func (d *Database) AddInviteEvent(
 ) (sp types.StreamPosition, err error) {
 	_ = d.Writer.Do(d.DB, nil, func(txn *sql.Tx) error {
 		sp, err = d.Invites.InsertInviteEvent(ctx, txn, inviteEvent)
-		return nil
+		return err
 	})
 	return
 }
@@ -153,11 +158,41 @@ func (d *Database) RetireInviteEvent(
 ) (sp types.StreamPosition, err error) {
 	_ = d.Writer.Do(d.DB, nil, func(txn *sql.Tx) error {
 		sp, err = d.Invites.DeleteInviteEvent(ctx, txn, inviteEventID)
-		return nil
+		return err
 	})
 	return
 }
 
+// AddPeek tracks the fact that a user has started peeking.
+// If the peek was successfully stored this returns the stream ID it was stored at.
+// Returns an error if there was a problem communicating with the database.
+func (d *Database) AddPeek(
+	ctx context.Context, roomID, userID, deviceID string,
+) (sp types.StreamPosition, err error) {
+	err = d.Writer.Do(d.DB, nil, func(txn *sql.Tx) error {
+		sp, err = d.Peeks.InsertPeek(ctx, txn, roomID, userID, deviceID)
+		return err
+	})
+	return
+}
+
+// DeletePeeks tracks the fact that a user has stopped peeking from all devices
+// If the peeks was successfully deleted this returns the stream ID it was stored at.
+// Returns an error if there was a problem communicating with the database.
+func (d *Database) DeletePeeks(
+	ctx context.Context, roomID, userID string,
+) (sp types.StreamPosition, err error) {
+	err = d.Writer.Do(d.DB, nil, func(txn *sql.Tx) error {
+		sp, err = d.Peeks.DeletePeeks(ctx, txn, roomID, userID)
+		return err
+	})
+	if err == sql.ErrNoRows {
+		sp = 0
+		err = nil
+	}
+	return
+}
+
 // GetAccountDataInRange returns all account data for a given user inserted or
 // updated between two given positions
 // Returns a map following the format data[roomID] = []dataTypes
@@ -196,7 +231,7 @@ func (d *Database) StreamEventsToEvents(device *userapi.Device, in []types.Strea
 					"transaction_id", in[i].TransactionID.TransactionID,
 				)
 				if err != nil {
-					logrus.WithFields(logrus.Fields{
+					log.WithFields(log.Fields{
 						"event_id": out[i].EventID(),
 					}).WithError(err).Warnf("Failed to add transaction ID to event")
 				}
@@ -389,7 +424,6 @@ func (d *Database) EventPositionInTopology(
 func (d *Database) syncPositionTx(
 	ctx context.Context, txn *sql.Tx,
 ) (sp types.StreamingToken, err error) {
-
 	maxEventID, err := d.OutputEvents.SelectMaxEventID(ctx, txn)
 	if err != nil {
 		return sp, err
@@ -408,6 +442,13 @@ func (d *Database) syncPositionTx(
 	if maxInviteID > maxEventID {
 		maxEventID = maxInviteID
 	}
+	maxPeekID, err := d.Peeks.SelectMaxPeekID(ctx, txn)
+	if err != nil {
+		return sp, err
+	}
+	if maxPeekID > maxEventID {
+		maxEventID = maxPeekID
+	}
 	sp = types.NewStreamToken(types.StreamPosition(maxEventID), types.StreamPosition(d.EDUCache.GetLatestSyncPosition()), nil)
 	return
 }
@@ -566,6 +607,8 @@ func (d *Database) IncrementalSync(
 		}
 	}
 
+	// TODO: handle EDUs in peeked rooms
+
 	err = d.addEDUDeltaToResponse(
 		fromPos, toPos, joinedRoomIDs, res,
 	)
@@ -582,7 +625,7 @@ func (d *Database) RedactEvent(ctx context.Context, redactedEventID string, reda
 		return err
 	}
 	if len(redactedEvents) == 0 {
-		logrus.WithField("event_id", redactedEventID).WithField("redaction_event", redactedBecause.EventID()).Warnf("missing redacted event for redaction")
+		log.WithField("event_id", redactedEventID).WithField("redaction_event", redactedBecause.EventID()).Warnf("missing redacted event for redaction")
 		return nil
 	}
 	eventToRedact := redactedEvents[0].Unwrap()
@@ -604,7 +647,7 @@ func (d *Database) RedactEvent(ctx context.Context, redactedEventID string, reda
 // nolint:nakedret
 func (d *Database) getResponseWithPDUsForCompleteSync(
 	ctx context.Context, res *types.Response,
-	userID string,
+	userID string, deviceID string,
 	numRecentEventsPerRoom int,
 ) (
 	toPos types.StreamingToken,
@@ -644,8 +687,51 @@ func (d *Database) getResponseWithPDUsForCompleteSync(
 
 	// Build up a /sync response. Add joined rooms.
 	for _, roomID := range joinedRoomIDs {
+		var jr *types.JoinResponse
+		jr, err = d.getJoinResponseForCompleteSync(
+			ctx, txn, roomID, r, &stateFilter, numRecentEventsPerRoom,
+		)
+		if err != nil {
+			return
+		}
+		res.Rooms.Join[roomID] = *jr
+	}
+
+	// Add peeked rooms.
+	peeks, err := d.Peeks.SelectPeeksInRange(ctx, txn, userID, deviceID, r)
+	if err != nil {
+		return
+	}
+	for _, peek := range peeks {
+		if !peek.Deleted {
+			var jr *types.JoinResponse
+			jr, err = d.getJoinResponseForCompleteSync(
+				ctx, txn, peek.RoomID, r, &stateFilter, numRecentEventsPerRoom,
+			)
+			if err != nil {
+				return
+			}
+			res.Rooms.Peek[peek.RoomID] = *jr
+		}
+	}
+
+	if err = d.addInvitesToResponse(ctx, txn, userID, r, res); err != nil {
+		return
+	}
+
+	succeeded = true
+	return //res, toPos, joinedRoomIDs, err
+}
+
+func (d *Database) getJoinResponseForCompleteSync(
+	ctx context.Context, txn *sql.Tx,
+	roomID string,
+	r types.Range,
+	stateFilter *gomatrixserverlib.StateFilter,
+	numRecentEventsPerRoom int,
+) (jr *types.JoinResponse, err error) {
 	var stateEvents []gomatrixserverlib.HeaderedEvent
-		stateEvents, err = d.CurrentRoomState.SelectCurrentState(ctx, txn, roomID, &stateFilter)
+	stateEvents, err = d.CurrentRoomState.SelectCurrentState(ctx, txn, roomID, stateFilter)
 	if err != nil {
 		return
 	}
@@ -678,20 +764,12 @@ func (d *Database) getResponseWithPDUsForCompleteSync(
 	// transaction IDs for complete syncs
 	recentEvents := d.StreamEventsToEvents(nil, recentStreamEvents)
 	stateEvents = removeDuplicates(stateEvents, recentEvents)
-		jr := types.NewJoinResponse()
+	jr = types.NewJoinResponse()
 	jr.Timeline.PrevBatch = prevBatchStr
 	jr.Timeline.Events = gomatrixserverlib.HeaderedToClientEvents(recentEvents, gomatrixserverlib.FormatSync)
 	jr.Timeline.Limited = limited
 	jr.State.Events = gomatrixserverlib.HeaderedToClientEvents(stateEvents, gomatrixserverlib.FormatSync)
-		res.Rooms.Join[roomID] = *jr
+	return jr, nil
-	}
-
-	if err = d.addInvitesToResponse(ctx, txn, userID, r, res); err != nil {
-		return
-	}
-
-	succeeded = true
-	return //res, toPos, joinedRoomIDs, err
 }
 
 func (d *Database) CompleteSync(
@@ -699,12 +777,14 @@ func (d *Database) CompleteSync(
 	device userapi.Device, numRecentEventsPerRoom int,
 ) (*types.Response, error) {
 	toPos, joinedRoomIDs, err := d.getResponseWithPDUsForCompleteSync(
-		ctx, res, device.UserID, numRecentEventsPerRoom,
+		ctx, res, device.UserID, device.ID, numRecentEventsPerRoom,
 	)
 	if err != nil {
 		return nil, fmt.Errorf("d.getResponseWithPDUsForCompleteSync: %w", err)
 	}
 
+	// TODO: handle EDUs in peeked rooms
+
 	// Use a zero value SyncPosition for fromPos so all EDU states are added.
 	err = d.addEDUDeltaToResponse(
 		types.NewStreamToken(0, 0, nil), toPos, joinedRoomIDs, res,
@@ -803,6 +883,12 @@ func (d *Database) addRoomDeltaToResponse(
 		return err
 	}
 
+	// XXX: should we ever get this far if we have no recent events or state in this room?
+	// in practice we do for peeks, but possibly not joins?
+	if len(recentEvents) == 0 && len(delta.stateEvents) == 0 {
+		return nil
+	}
+
 	switch delta.membership {
 	case gomatrixserverlib.Join:
 		jr := types.NewJoinResponse()
@@ -812,6 +898,14 @@ func (d *Database) addRoomDeltaToResponse(
 		jr.Timeline.Limited = limited
 		jr.State.Events = gomatrixserverlib.HeaderedToClientEvents(delta.stateEvents, gomatrixserverlib.FormatSync)
 		res.Rooms.Join[delta.roomID] = *jr
+	case gomatrixserverlib.Peek:
+		jr := types.NewJoinResponse()
+
+		jr.Timeline.PrevBatch = prevBatch.String()
+		jr.Timeline.Events = gomatrixserverlib.HeaderedToClientEvents(recentEvents, gomatrixserverlib.FormatSync)
+		jr.Timeline.Limited = limited
+		jr.State.Events = gomatrixserverlib.HeaderedToClientEvents(delta.stateEvents, gomatrixserverlib.FormatSync)
+		res.Rooms.Peek[delta.roomID] = *jr
 	case gomatrixserverlib.Leave:
 		fallthrough // transitions to leave are the same as ban
 	case gomatrixserverlib.Ban:
@@ -918,6 +1012,7 @@ func (d *Database) fetchMissingStateEvents(
 // exclusive of oldPos, inclusive of newPos, for the rooms in which
 // the user has new membership events.
 // A list of joined room IDs is also returned in case the caller needs it.
+// nolint:gocyclo
 func (d *Database) getStateDeltas(
 	ctx context.Context, device *userapi.Device, txn *sql.Tx,
 	r types.Range, userID string,
@@ -933,7 +1028,7 @@ func (d *Database) getStateDeltas(
 	// - Get all CURRENTLY joined rooms, and add them to 'joined' block.
 	var deltas []stateDelta
 
-	// get all the state events ever between these two positions
+	// get all the state events ever (i.e. for all available rooms) between these two positions
 	stateNeeded, eventMap, err := d.OutputEvents.SelectStateInRange(ctx, txn, r, stateFilter)
 	if err != nil {
 		return nil, nil, err
@@ -943,6 +1038,34 @@ func (d *Database) getStateDeltas(
 		return nil, nil, err
 	}
 
+	// find out which rooms this user is peeking, if any.
+	// We do this before joins so any peeks get overwritten
+	peeks, err := d.Peeks.SelectPeeksInRange(ctx, txn, userID, device.ID, r)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// add peek blocks
+	for _, peek := range peeks {
+		if peek.New {
+			// send full room state down instead of a delta
+			var s []types.StreamEvent
+			s, err = d.currentStateStreamEventsForRoom(ctx, txn, peek.RoomID, stateFilter)
+			if err != nil {
+				return nil, nil, err
+			}
+			state[peek.RoomID] = s
+		}
+		if !peek.Deleted {
+			deltas = append(deltas, stateDelta{
+				membership:  gomatrixserverlib.Peek,
+				stateEvents: d.StreamEventsToEvents(device, state[peek.RoomID]),
+				roomID:      peek.RoomID,
+			})
+		}
+	}
+
+	// handle newly joined rooms and non-joined rooms
 	for roomID, stateStreamEvents := range state {
 		for _, ev := range stateStreamEvents {
 			// TODO: Currently this will incorrectly add rooms which were ALREADY joined but they sent another no-op join event.
@@ -993,6 +1116,7 @@ func (d *Database) getStateDeltas(
 // requests with full_state=true.
 // Fetches full state for all joined rooms and uses selectStateInRange to get
 // updates for other rooms.
+// nolint:gocyclo
 func (d *Database) getStateDeltasForFullStateSync(
 	ctx context.Context, device *userapi.Device, txn *sql.Tx,
 	r types.Range, userID string,
@@ -1001,6 +1125,26 @@ func (d *Database) getStateDeltasForFullStateSync(
 	// Use a reasonable initial capacity
 	deltas := make(map[string]stateDelta)
 
+	peeks, err := d.Peeks.SelectPeeksInRange(ctx, txn, userID, device.ID, r)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// Add full states for all peeking rooms
+	for _, peek := range peeks {
+		if !peek.Deleted {
+			s, stateErr := d.currentStateStreamEventsForRoom(ctx, txn, peek.RoomID, stateFilter)
+			if stateErr != nil {
+				return nil, nil, stateErr
+			}
+			deltas[peek.RoomID] = stateDelta{
+				membership:  gomatrixserverlib.Peek,
+				stateEvents: d.StreamEventsToEvents(device, s),
+				roomID:      peek.RoomID,
+			}
+		}
+	}
+
 	// Get all the state events ever between these two positions
 	stateNeeded, eventMap, err := d.OutputEvents.SelectStateInRange(ctx, txn, r, stateFilter)
 	if err != nil {

syncapi/storage/sqlite3/peeks_table.go

@@ -0,0 +1,206 @@
+// Copyright 2020 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package sqlite3
+
+import (
+	"context"
+	"database/sql"
+	"time"
+
+	"github.com/matrix-org/dendrite/internal"
+	"github.com/matrix-org/dendrite/internal/sqlutil"
+	"github.com/matrix-org/dendrite/syncapi/storage/tables"
+	"github.com/matrix-org/dendrite/syncapi/types"
+)
+
+const peeksSchema = `
+CREATE TABLE IF NOT EXISTS syncapi_peeks (
+	id INTEGER,
+	room_id TEXT NOT NULL,
+	user_id TEXT NOT NULL,
+	device_id TEXT NOT NULL,
+	deleted BOOL NOT NULL DEFAULT false,
+    -- When the peek was created in UNIX epoch ms.
+    creation_ts INTEGER NOT NULL,
+    UNIQUE(room_id, user_id, device_id)
+);
+
+CREATE INDEX IF NOT EXISTS syncapi_peeks_room_id_idx ON syncapi_peeks(room_id);
+CREATE INDEX IF NOT EXISTS syncapi_peeks_user_id_device_id_idx ON syncapi_peeks(user_id, device_id);
+`
+
+const insertPeekSQL = "" +
+	"INSERT OR REPLACE INTO syncapi_peeks" +
+	" (id, room_id, user_id, device_id, creation_ts, deleted)" +
+	" VALUES ($1, $2, $3, $4, $5, false)"
+
+const deletePeekSQL = "" +
+	"UPDATE syncapi_peeks SET deleted=true, id=$1 WHERE room_id = $2 AND user_id = $3 AND device_id = $4"
+
+const deletePeeksSQL = "" +
+	"UPDATE syncapi_peeks SET deleted=true, id=$1 WHERE room_id = $2 AND user_id = $3"
+
+// we care about all the peeks which were created in this range, deleted in this range,
+// or were created before this range but haven't been deleted yet.
+// BEWARE: sqlite chokes on out of order substitution strings.
+const selectPeeksInRangeSQL = "" +
+	"SELECT id, room_id, deleted FROM syncapi_peeks WHERE user_id = $1 AND device_id = $2 AND ((id <= $3 AND NOT deleted=true) OR (id > $3 AND id <= $4))"
+
+const selectPeekingDevicesSQL = "" +
+	"SELECT room_id, user_id, device_id FROM syncapi_peeks WHERE deleted=false"
+
+const selectMaxPeekIDSQL = "" +
+	"SELECT MAX(id) FROM syncapi_peeks"
+
+type peekStatements struct {
+	db                       *sql.DB
+	streamIDStatements       *streamIDStatements
+	insertPeekStmt           *sql.Stmt
+	deletePeekStmt           *sql.Stmt
+	deletePeeksStmt          *sql.Stmt
+	selectPeeksInRangeStmt   *sql.Stmt
+	selectPeekingDevicesStmt *sql.Stmt
+	selectMaxPeekIDStmt      *sql.Stmt
+}
+
+func NewSqlitePeeksTable(db *sql.DB, streamID *streamIDStatements) (tables.Peeks, error) {
+	_, err := db.Exec(peeksSchema)
+	if err != nil {
+		return nil, err
+	}
+	s := &peekStatements{
+		db:                 db,
+		streamIDStatements: streamID,
+	}
+	if s.insertPeekStmt, err = db.Prepare(insertPeekSQL); err != nil {
+		return nil, err
+	}
+	if s.deletePeekStmt, err = db.Prepare(deletePeekSQL); err != nil {
+		return nil, err
+	}
+	if s.deletePeeksStmt, err = db.Prepare(deletePeeksSQL); err != nil {
+		return nil, err
+	}
+	if s.selectPeeksInRangeStmt, err = db.Prepare(selectPeeksInRangeSQL); err != nil {
+		return nil, err
+	}
+	if s.selectPeekingDevicesStmt, err = db.Prepare(selectPeekingDevicesSQL); err != nil {
+		return nil, err
+	}
+	if s.selectMaxPeekIDStmt, err = db.Prepare(selectMaxPeekIDSQL); err != nil {
+		return nil, err
+	}
+	return s, nil
+}
+
+func (s *peekStatements) InsertPeek(
+	ctx context.Context, txn *sql.Tx, roomID, userID, deviceID string,
+) (streamPos types.StreamPosition, err error) {
+	streamPos, err = s.streamIDStatements.nextStreamID(ctx, txn)
+	if err != nil {
+		return
+	}
+	nowMilli := time.Now().UnixNano() / int64(time.Millisecond)
+	_, err = sqlutil.TxStmt(txn, s.insertPeekStmt).ExecContext(ctx, streamPos, roomID, userID, deviceID, nowMilli)
+	return
+}
+
+func (s *peekStatements) DeletePeek(
+	ctx context.Context, txn *sql.Tx, roomID, userID, deviceID string,
+) (streamPos types.StreamPosition, err error) {
+	streamPos, err = s.streamIDStatements.nextStreamID(ctx, txn)
+	if err != nil {
+		return
+	}
+	_, err = sqlutil.TxStmt(txn, s.deletePeekStmt).ExecContext(ctx, streamPos, roomID, userID, deviceID)
+	return
+}
+
+func (s *peekStatements) DeletePeeks(
+	ctx context.Context, txn *sql.Tx, roomID, userID string,
+) (types.StreamPosition, error) {
+	streamPos, err := s.streamIDStatements.nextStreamID(ctx, txn)
+	if err != nil {
+		return 0, err
+	}
+	result, err := sqlutil.TxStmt(txn, s.deletePeeksStmt).ExecContext(ctx, streamPos, roomID, userID)
+	if err != nil {
+		return 0, err
+	}
+	numAffected, err := result.RowsAffected()
+	if err != nil {
+		return 0, err
+	}
+	if numAffected == 0 {
+		return 0, sql.ErrNoRows
+	}
+	return streamPos, nil
+}
+
+func (s *peekStatements) SelectPeeksInRange(
+	ctx context.Context, txn *sql.Tx, userID, deviceID string, r types.Range,
+) (peeks []types.Peek, err error) {
+	rows, err := sqlutil.TxStmt(txn, s.selectPeeksInRangeStmt).QueryContext(ctx, userID, deviceID, r.Low(), r.High())
+	if err != nil {
+		return
+	}
+	defer internal.CloseAndLogIfError(ctx, rows, "SelectPeeksInRange: rows.close() failed")
+
+	for rows.Next() {
+		peek := types.Peek{}
+		var id types.StreamPosition
+		if err = rows.Scan(&id, &peek.RoomID, &peek.Deleted); err != nil {
+			return
+		}
+		peek.New = (id > r.Low() && id <= r.High()) && !peek.Deleted
+		peeks = append(peeks, peek)
+	}
+
+	return peeks, rows.Err()
+}
+
+func (s *peekStatements) SelectPeekingDevices(
+	ctx context.Context,
+) (peekingDevices map[string][]types.PeekingDevice, err error) {
+	rows, err := s.selectPeekingDevicesStmt.QueryContext(ctx)
+	if err != nil {
+		return nil, err
+	}
+	defer internal.CloseAndLogIfError(ctx, rows, "SelectPeekingDevices: rows.close() failed")
+
+	result := make(map[string][]types.PeekingDevice)
+	for rows.Next() {
+		var roomID, userID, deviceID string
+		if err := rows.Scan(&roomID, &userID, &deviceID); err != nil {
+			return nil, err
+		}
+		devices := result[roomID]
+		devices = append(devices, types.PeekingDevice{UserID: userID, DeviceID: deviceID})
+		result[roomID] = devices
+	}
+	return result, nil
+}
+
+func (s *peekStatements) SelectMaxPeekID(
+	ctx context.Context, txn *sql.Tx,
+) (id int64, err error) {
+	var nullableID sql.NullInt64
+	stmt := sqlutil.TxStmt(txn, s.selectMaxPeekIDStmt)
+	err = stmt.QueryRowContext(ctx).Scan(&nullableID)
+	if nullableID.Valid {
+		id = nullableID.Int64
+	}
+	return
+}

syncapi/storage/sqlite3/syncserver.go

@@ -75,6 +75,10 @@ func (d *SyncServerDatasource) prepare() (err error) {
 	if err != nil {
 		return err
 	}
+	peeks, err := NewSqlitePeeksTable(d.db, &d.streamID)
+	if err != nil {
+		return err
+	}
 	topology, err := NewSqliteTopologyTable(d.db)
 	if err != nil {
 		return err
@@ -95,6 +99,7 @@ func (d *SyncServerDatasource) prepare() (err error) {
 		DB:                  d.db,
 		Writer:              d.writer,
 		Invites:             invites,
+		Peeks:               peeks,
 		AccountData:         accountData,
 		OutputEvents:        events,
 		BackwardExtremities: bwExtrem,

syncapi/storage/tables/interface.go

@@ -39,6 +39,15 @@ type Invites interface {
 	SelectMaxInviteID(ctx context.Context, txn *sql.Tx) (id int64, err error)
 }
 
+type Peeks interface {
+	InsertPeek(ctx context.Context, txn *sql.Tx, roomID, userID, deviceID string) (streamPos types.StreamPosition, err error)
+	DeletePeek(ctx context.Context, txn *sql.Tx, roomID, userID, deviceID string) (streamPos types.StreamPosition, err error)
+	DeletePeeks(ctx context.Context, txn *sql.Tx, roomID, userID string) (streamPos types.StreamPosition, err error)
+	SelectPeeksInRange(ctxt context.Context, txn *sql.Tx, userID, deviceID string, r types.Range) (peeks []types.Peek, err error)
+	SelectPeekingDevices(ctxt context.Context) (peekingDevices map[string][]types.PeekingDevice, err error)
+	SelectMaxPeekID(ctx context.Context, txn *sql.Tx) (id int64, err error)
+}
+
 type Events interface {
 	SelectStateInRange(ctx context.Context, txn *sql.Tx, r types.Range, stateFilter *gomatrixserverlib.StateFilter) (map[string]map[string]bool, map[string]types.StreamEvent, error)
 	SelectMaxEventID(ctx context.Context, txn *sql.Tx) (id int64, err error)

syncapi/sync/notifier.go

@@ -33,6 +33,8 @@ import (
 type Notifier struct {
 	// A map of RoomID => Set<UserID> : Must only be accessed by the OnNewEvent goroutine
 	roomIDToJoinedUsers map[string]userIDSet
+	// A map of RoomID => Set<UserID> : Must only be accessed by the OnNewEvent goroutine
+	roomIDToPeekingDevices map[string]peekingDeviceSet
 	// Protects currPos and userStreams.
 	streamLock *sync.Mutex
 	// The latest sync position
@@ -50,6 +52,7 @@ func NewNotifier(pos types.StreamingToken) *Notifier {
 	return &Notifier{
 		currPos:                pos,
 		roomIDToJoinedUsers:    make(map[string]userIDSet),
+		roomIDToPeekingDevices: make(map[string]peekingDeviceSet),
 		userDeviceStreams:      make(map[string]map[string]*UserDeviceStream),
 		streamLock:             &sync.Mutex{},
 		lastCleanUpTime:        time.Now(),
@@ -82,6 +85,8 @@ func (n *Notifier) OnNewEvent(
 	if ev != nil {
 		// Map this event's room_id to a list of joined users, and wake them up.
 		usersToNotify := n.joinedUsers(ev.RoomID())
+		// Map this event's room_id to a list of peeking devices, and wake them up.
+		peekingDevicesToNotify := n.PeekingDevices(ev.RoomID())
 		// If this is an invite, also add in the invitee to this list.
 		if ev.Type() == "m.room.member" && ev.StateKey() != nil {
 			targetUserID := *ev.StateKey()
@@ -108,11 +113,11 @@ func (n *Notifier) OnNewEvent(
 			}
 		}
 
-		n.wakeupUsers(usersToNotify, latestPos)
+		n.wakeupUsers(usersToNotify, peekingDevicesToNotify, latestPos)
 	} else if roomID != "" {
-		n.wakeupUsers(n.joinedUsers(roomID), latestPos)
+		n.wakeupUsers(n.joinedUsers(roomID), n.PeekingDevices(roomID), latestPos)
 	} else if len(userIDs) > 0 {
-		n.wakeupUsers(userIDs, latestPos)
+		n.wakeupUsers(userIDs, nil, latestPos)
 	} else {
 		log.WithFields(log.Fields{
 			"posUpdate": posUpdate.String,
@@ -120,6 +125,18 @@ func (n *Notifier) OnNewEvent(
 	}
 }
 
+func (n *Notifier) OnNewPeek(
+	roomID, userID, deviceID string,
+) {
+	n.streamLock.Lock()
+	defer n.streamLock.Unlock()
+
+	n.addPeekingDevice(roomID, userID, deviceID)
+
+	// we don't wake up devices here given the roomserver consumer will do this shortly afterwards
+	// by calling OnNewEvent.
+}
+
 func (n *Notifier) OnNewSendToDevice(
 	userID string, deviceIDs []string,
 	posUpdate types.StreamingToken,
@@ -139,7 +156,7 @@ func (n *Notifier) OnNewKeyChange(
 	defer n.streamLock.Unlock()
 	latestPos := n.currPos.WithUpdates(posUpdate)
 	n.currPos = latestPos
-	n.wakeupUsers([]string{wakeUserID}, latestPos)
+	n.wakeupUsers([]string{wakeUserID}, nil, latestPos)
 }
 
 // GetListener returns a UserStreamListener that can be used to wait for
@@ -169,6 +186,13 @@ func (n *Notifier) Load(ctx context.Context, db storage.Database) error {
 		return err
 	}
 	n.setUsersJoinedToRooms(roomToUsers)
+
+	roomToPeekingDevices, err := db.AllPeekingDevicesInRooms(ctx)
+	if err != nil {
+		return err
+	}
+	n.setPeekingDevices(roomToPeekingDevices)
+
 	return nil
 }
 
@@ -195,9 +219,24 @@ func (n *Notifier) setUsersJoinedToRooms(roomIDToUserIDs map[string][]string) {
 	}
 }
 
+// setPeekingDevices marks the given devices as peeking in the given rooms, such that new events from
+// these rooms will wake the given devices' /sync requests. This should be called prior to ANY calls to
+// OnNewEvent (eg on startup) to prevent racing.
+func (n *Notifier) setPeekingDevices(roomIDToPeekingDevices map[string][]types.PeekingDevice) {
+	// This is just the bulk form of addPeekingDevice
+	for roomID, peekingDevices := range roomIDToPeekingDevices {
+		if _, ok := n.roomIDToPeekingDevices[roomID]; !ok {
+			n.roomIDToPeekingDevices[roomID] = make(peekingDeviceSet)
+		}
+		for _, peekingDevice := range peekingDevices {
+			n.roomIDToPeekingDevices[roomID].add(peekingDevice)
+		}
+	}
+}
+
 // wakeupUsers will wake up the sync strems for all of the devices for all of the
-// specified user IDs.
+// specified user IDs, and also the specified peekingDevices
-func (n *Notifier) wakeupUsers(userIDs []string, newPos types.StreamingToken) {
+func (n *Notifier) wakeupUsers(userIDs []string, peekingDevices []types.PeekingDevice, newPos types.StreamingToken) {
 	for _, userID := range userIDs {
 		for _, stream := range n.fetchUserStreams(userID) {
 			if stream == nil {
@@ -206,6 +245,13 @@ func (n *Notifier) wakeupUsers(userIDs []string, newPos types.StreamingToken) {
 			stream.Broadcast(newPos) // wake up all goroutines Wait()ing on this stream
 		}
 	}
+
+	for _, peekingDevice := range peekingDevices {
+		// TODO: don't bother waking up for devices whose users we already woke up
+		if stream := n.fetchUserDeviceStream(peekingDevice.UserID, peekingDevice.DeviceID, false); stream != nil {
+			stream.Broadcast(newPos) // wake up all goroutines Wait()ing on this stream
+		}
+	}
 }
 
 // wakeupUserDevice will wake up the sync stream for a specific user device. Other
@@ -284,6 +330,32 @@ func (n *Notifier) joinedUsers(roomID string) (userIDs []string) {
 	return n.roomIDToJoinedUsers[roomID].values()
 }
 
+// Not thread-safe: must be called on the OnNewEvent goroutine only
+func (n *Notifier) addPeekingDevice(roomID, userID, deviceID string) {
+	if _, ok := n.roomIDToPeekingDevices[roomID]; !ok {
+		n.roomIDToPeekingDevices[roomID] = make(peekingDeviceSet)
+	}
+	n.roomIDToPeekingDevices[roomID].add(types.PeekingDevice{UserID: userID, DeviceID: deviceID})
+}
+
+// Not thread-safe: must be called on the OnNewEvent goroutine only
+// nolint:unused
+func (n *Notifier) removePeekingDevice(roomID, userID, deviceID string) {
+	if _, ok := n.roomIDToPeekingDevices[roomID]; !ok {
+		n.roomIDToPeekingDevices[roomID] = make(peekingDeviceSet)
+	}
+	// XXX: is this going to work as a key?
+	n.roomIDToPeekingDevices[roomID].remove(types.PeekingDevice{UserID: userID, DeviceID: deviceID})
+}
+
+// Not thread-safe: must be called on the OnNewEvent goroutine only
+func (n *Notifier) PeekingDevices(roomID string) (peekingDevices []types.PeekingDevice) {
+	if _, ok := n.roomIDToPeekingDevices[roomID]; !ok {
+		return
+	}
+	return n.roomIDToPeekingDevices[roomID].values()
+}
+
 // removeEmptyUserStreams iterates through the user stream map and removes any
 // that have been empty for a certain amount of time. This is a crude way of
 // ensuring that the userStreams map doesn't grow forver.
@@ -329,3 +401,23 @@ func (s userIDSet) values() (vals []string) {
 	}
 	return
 }
+
+// A set of PeekingDevices, similar to userIDSet
+
+type peekingDeviceSet map[types.PeekingDevice]bool
+
+func (s peekingDeviceSet) add(d types.PeekingDevice) {
+	s[d] = true
+}
+
+// nolint:unused
+func (s peekingDeviceSet) remove(d types.PeekingDevice) {
+	delete(s, d)
+}
+
+func (s peekingDeviceSet) values() (vals []types.PeekingDevice) {
+	for d := range s {
+		vals = append(vals, d)
+	}
+	return
+}

syncapi/types/types.go

@@ -388,6 +388,7 @@ type Response struct {
 	} `json:"presence,omitempty"`
 	Rooms struct {
 		Join   map[string]JoinResponse   `json:"join"`
+		Peek   map[string]JoinResponse   `json:"peek"`
 		Invite map[string]InviteResponse `json:"invite"`
 		Leave  map[string]LeaveResponse  `json:"leave"`
 	} `json:"rooms"`
@@ -407,6 +408,7 @@ func NewResponse() *Response {
 	// Pre-initialise the maps. Synapse will return {} even if there are no rooms under a specific section,
 	// so let's do the same thing. Bonus: this means we can't get dreaded 'assignment to entry in nil map' errors.
 	res.Rooms.Join = make(map[string]JoinResponse)
+	res.Rooms.Peek = make(map[string]JoinResponse)
 	res.Rooms.Invite = make(map[string]InviteResponse)
 	res.Rooms.Leave = make(map[string]LeaveResponse)
 
@@ -433,7 +435,7 @@ func (r *Response) IsEmpty() bool {
 		len(r.ToDevice.Events) == 0
 }
 
-// JoinResponse represents a /sync response for a room which is under the 'join' key.
+// JoinResponse represents a /sync response for a room which is under the 'join' or 'peek' key.
 type JoinResponse struct {
 	State struct {
 		Events []gomatrixserverlib.ClientEvent `json:"events"`
@@ -507,3 +509,14 @@ type SendToDeviceEvent struct {
 	DeviceID    string
 	SentByToken *StreamingToken
 }
+
+type PeekingDevice struct {
+	UserID   string
+	DeviceID string
+}
+
+type Peek struct {
+	RoomID  string
+	New     bool
+	Deleted bool
+}

sytest-whitelist

@@ -465,3 +465,9 @@ After changing password, can log in with new password
 After changing password, existing session still works
 After changing password, different sessions can optionally be kept
 After changing password, a different session no longer works by default
+Local users can peek into world_readable rooms by room ID
+We can't peek into rooms with shared history_visibility
+We can't peek into rooms with invited history_visibility
+We can't peek into rooms with joined history_visibility
+Local users can peek by room alias
+Peeked rooms only turn up in the sync for the device who peeked them