Implement `/context` (#2207)

* Add QueryEventsAfter

* Add /context

* Make all tests pass on sqlite

* Add queries to get the events for /context requests

* Move /context to the syncapi

* Revert "Add QueryEventsAfter"

This reverts commit 440a771d10632622e8c65d35fe90f0804bc98862.

* Simplify getting the required events

* Apply RoomEventFilter when getting events

* Add passing tests

* Remove logging

* Remove unused SQL statements
Update comments & add TODO

3 files changed, 273 insertions, 0 deletions

diff --git a/syncapi/routing/context.go b/syncapi/routing/context.go
new file mode 100644
index 00000000..709f6291
--- /dev/null
+++ b/syncapi/routing/context.go
@@ -0,0 +1,190 @@
+// Copyright 2022 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 routing
+
+import (
+	"database/sql"
+	"encoding/json"
+	"net/http"
+	"strconv"
+
+	"github.com/matrix-org/dendrite/clientapi/jsonerror"
+	roomserver "github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/dendrite/syncapi/storage"
+	userapi "github.com/matrix-org/dendrite/userapi/api"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
+	"github.com/sirupsen/logrus"
+)
+
+type ContextRespsonse struct {
+	End          string                          `json:"end"`
+	Event        gomatrixserverlib.ClientEvent   `json:"event"`
+	EventsAfter  []gomatrixserverlib.ClientEvent `json:"events_after,omitempty"`
+	EventsBefore []gomatrixserverlib.ClientEvent `json:"events_before,omitempty"`
+	Start        string                          `json:"start"`
+	State        []gomatrixserverlib.ClientEvent `json:"state"`
+}
+
+func Context(
+	req *http.Request, device *userapi.Device,
+	rsAPI roomserver.RoomserverInternalAPI,
+	syncDB storage.Database,
+	roomID, eventID string,
+) util.JSONResponse {
+	filter, err := parseContextParams(req)
+	if err != nil {
+		errMsg := ""
+		switch err.(type) {
+		case *json.InvalidUnmarshalError:
+			errMsg = "unable to parse filter"
+		case *strconv.NumError:
+			errMsg = "unable to parse limit"
+		}
+		return util.JSONResponse{
+			Code:    http.StatusBadRequest,
+			JSON:    jsonerror.InvalidParam(errMsg),
+			Headers: nil,
+		}
+	}
+	filter.Rooms = append(filter.Rooms, roomID)
+
+	ctx := req.Context()
+	membershipRes := roomserver.QueryMembershipForUserResponse{}
+	membershipReq := roomserver.QueryMembershipForUserRequest{UserID: device.UserID, RoomID: roomID}
+	if err := rsAPI.QueryMembershipForUser(ctx, &membershipReq, &membershipRes); err != nil {
+		logrus.WithError(err).Error("unable to fo membership")
+		return jsonerror.InternalServerError()
+	}
+
+	stateFilter := gomatrixserverlib.StateFilter{
+		Limit:                   100,
+		NotSenders:              filter.NotSenders,
+		NotTypes:                filter.NotTypes,
+		Senders:                 filter.Senders,
+		Types:                   filter.Types,
+		LazyLoadMembers:         filter.LazyLoadMembers,
+		IncludeRedundantMembers: filter.IncludeRedundantMembers,
+		NotRooms:                filter.NotRooms,
+		Rooms:                   filter.Rooms,
+		ContainsURL:             filter.ContainsURL,
+	}
+
+	// TODO: Get the actual state at the last event returned by SelectContextAfterEvent
+	state, _ := syncDB.CurrentState(ctx, roomID, &stateFilter, nil)
+	// verify the user is allowed to see the context for this room/event
+	for _, x := range state {
+		hisVis, err := x.HistoryVisibility()
+		if err != nil {
+			continue
+		}
+		allowed := hisVis != "world_readable" && membershipRes.Membership == "join"
+		if !allowed {
+			return util.JSONResponse{
+				Code: http.StatusForbidden,
+				JSON: jsonerror.Forbidden("User is not allowed to query context"),
+			}
+		}
+	}
+
+	id, requestedEvent, err := syncDB.SelectContextEvent(ctx, roomID, eventID)
+	if err != nil {
+		logrus.WithError(err).WithField("eventID", eventID).Error("unable to find requested event")
+		return jsonerror.InternalServerError()
+	}
+
+	eventsBefore, err := syncDB.SelectContextBeforeEvent(ctx, id, roomID, filter)
+	if err != nil && err != sql.ErrNoRows {
+		logrus.WithError(err).Error("unable to fetch before events")
+		return jsonerror.InternalServerError()
+	}
+
+	_, eventsAfter, err := syncDB.SelectContextAfterEvent(ctx, id, roomID, filter)
+	if err != nil && err != sql.ErrNoRows {
+		logrus.WithError(err).Error("unable to fetch after events")
+		return jsonerror.InternalServerError()
+	}
+
+	eventsBeforeClient := gomatrixserverlib.HeaderedToClientEvents(eventsBefore, gomatrixserverlib.FormatAll)
+	eventsAfterClient := gomatrixserverlib.HeaderedToClientEvents(eventsAfter, gomatrixserverlib.FormatAll)
+	newState := applyLazyLoadMembers(filter, eventsAfterClient, eventsBeforeClient, state)
+
+	response := ContextRespsonse{
+		Event:        gomatrixserverlib.HeaderedToClientEvent(&requestedEvent, gomatrixserverlib.FormatAll),
+		EventsAfter:  eventsAfterClient,
+		EventsBefore: eventsBeforeClient,
+		State:        gomatrixserverlib.HeaderedToClientEvents(newState, gomatrixserverlib.FormatAll),
+	}
+
+	if len(response.State) > filter.Limit {
+		response.State = response.State[len(response.State)-filter.Limit:]
+	}
+
+	return util.JSONResponse{
+		Code: http.StatusOK,
+		JSON: response,
+	}
+}
+
+func applyLazyLoadMembers(filter *gomatrixserverlib.RoomEventFilter, eventsAfter, eventsBefore []gomatrixserverlib.ClientEvent, state []*gomatrixserverlib.HeaderedEvent) []*gomatrixserverlib.HeaderedEvent {
+	if filter == nil || !filter.LazyLoadMembers {
+		return state
+	}
+	allEvents := append(eventsBefore, eventsAfter...)
+	x := make(map[string]bool)
+	// get members who actually send an event
+	for _, e := range allEvents {
+		x[e.Sender] = true
+	}
+
+	newState := []*gomatrixserverlib.HeaderedEvent{}
+	for _, event := range state {
+		if event.Type() != gomatrixserverlib.MRoomMember {
+			newState = append(newState, event)
+		} else {
+			// did the user send an event?
+			if x[event.Sender()] {
+				newState = append(newState, event)
+			}
+		}
+	}
+	return newState
+}
+
+func parseContextParams(req *http.Request) (*gomatrixserverlib.RoomEventFilter, error) {
+	// Default room filter
+	filter := &gomatrixserverlib.RoomEventFilter{Limit: 10}
+
+	l := req.URL.Query().Get("limit")
+	f := req.URL.Query().Get("filter")
+	if l != "" {
+		limit, err := strconv.Atoi(l)
+		if err != nil {
+			return nil, err
+		}
+		// NOTSPEC: feels like a good idea to have an upper bound limit
+		if limit > 100 {
+			limit = 100
+		}
+		filter.Limit = limit
+	}
+	if f != "" {
+		if err := json.Unmarshal([]byte(f), &filter); err != nil {
+			return nil, err
+		}
+	}
+
+	return filter, nil
+}
diff --git a/syncapi/routing/context_test.go b/syncapi/routing/context_test.go
new file mode 100644
index 00000000..1b430d83
--- /dev/null
+++ b/syncapi/routing/context_test.go
@@ -0,0 +1,68 @@
+package routing
+
+import (
+	"net/http"
+	"reflect"
+	"testing"
+
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+func Test_parseContextParams(t *testing.T) {
+
+	noParamsReq, _ := http.NewRequest("GET", "https://localhost:8800/_matrix/client/r0/rooms/!hyi4UaxS9mUXpSG9:localhost:8800/context/%24um_T82QqAXN8PayGiBW7j9WExpqTIQ7-JRq-Q6xpIf8?access_token=5dMB0z4tiulyBvCaIKgyjuWG71ybDiYIwNJVJ2UmxRI", nil)
+	limit2Req, _ := http.NewRequest("GET", "https://localhost:8800/_matrix/client/r0/rooms/!hyi4UaxS9mUXpSG9:localhost:8800/context/%24um_T82QqAXN8PayGiBW7j9WExpqTIQ7-JRq-Q6xpIf8?access_token=5dMB0z4tiulyBvCaIKgyjuWG71ybDiYIwNJVJ2UmxRI&limit=2", nil)
+	limit10000Req, _ := http.NewRequest("GET", "https://localhost:8800/_matrix/client/r0/rooms/!hyi4UaxS9mUXpSG9:localhost:8800/context/%24um_T82QqAXN8PayGiBW7j9WExpqTIQ7-JRq-Q6xpIf8?access_token=5dMB0z4tiulyBvCaIKgyjuWG71ybDiYIwNJVJ2UmxRI&limit=10000", nil)
+	invalidLimitReq, _ := http.NewRequest("GET", "https://localhost:8800/_matrix/client/r0/rooms/!hyi4UaxS9mUXpSG9:localhost:8800/context/%24um_T82QqAXN8PayGiBW7j9WExpqTIQ7-JRq-Q6xpIf8?access_token=5dMB0z4tiulyBvCaIKgyjuWG71ybDiYIwNJVJ2UmxRI&limit=100as", nil)
+	lazyLoadReq, _ := http.NewRequest("GET", "https://localhost:8800//_matrix/client/r0/rooms/!kvEtX3rFamfwKHO3:localhost:8800/context/%24GjmkRbajRHy8_cxcSbUU4qF_njV8yHeLphI2azTrPaI?limit=2&filter=%7B+%22lazy_load_members%22+%3A+true+%7D&access_token=t1Njzm74w3G40CJ5xrlf1V2haXom0z0Iq1qyyVWhbVo", nil)
+	invalidFilterReq, _ := http.NewRequest("GET", "https://localhost:8800//_matrix/client/r0/rooms/!kvEtX3rFamfwKHO3:localhost:8800/context/%24GjmkRbajRHy8_cxcSbUU4qF_njV8yHeLphI2azTrPaI?limit=2&filter=%7B+%22lazy_load_members%22+%3A+true&access_token=t1Njzm74w3G40CJ5xrlf1V2haXom0z0Iq1qyyVWhbVo", nil)
+	tests := []struct {
+		name       string
+		req        *http.Request
+		wantFilter *gomatrixserverlib.RoomEventFilter
+		wantErr    bool
+	}{
+		{
+			name:       "no params set",
+			req:        noParamsReq,
+			wantFilter: &gomatrixserverlib.RoomEventFilter{Limit: 10},
+		},
+		{
+			name:       "limit 2 param set",
+			req:        limit2Req,
+			wantFilter: &gomatrixserverlib.RoomEventFilter{Limit: 2},
+		},
+		{
+			name:       "limit 10000 param set",
+			req:        limit10000Req,
+			wantFilter: &gomatrixserverlib.RoomEventFilter{Limit: 100},
+		},
+		{
+			name:       "filter lazy_load_members param set",
+			req:        lazyLoadReq,
+			wantFilter: &gomatrixserverlib.RoomEventFilter{Limit: 2, LazyLoadMembers: true},
+		},
+		{
+			name:    "invalid limit req",
+			req:     invalidLimitReq,
+			wantErr: true,
+		},
+		{
+			name:    "invalid filter req",
+			req:     invalidFilterReq,
+			wantErr: true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			gotFilter, err := parseContextParams(tt.req)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("parseContextParams() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if !reflect.DeepEqual(gotFilter, tt.wantFilter) {
+				t.Errorf("parseContextParams() gotFilter = %v, want %v", gotFilter, tt.wantFilter)
+			}
+		})
+	}
+}
diff --git a/syncapi/routing/routing.go b/syncapi/routing/routing.go
index 005a3355..be366ba1 100644
--- a/syncapi/routing/routing.go
+++ b/syncapi/routing/routing.go
@@ -77,4 +77,19 @@ func Setup(
 	v3mux.Handle("/keys/changes", httputil.MakeAuthAPI("keys_changes", userAPI, func(req *http.Request, device *userapi.Device) util.JSONResponse {
 		return srp.OnIncomingKeyChangeRequest(req, device)
 	})).Methods(http.MethodGet, http.MethodOptions)
+
+	v3mux.Handle("/rooms/{roomId}/context/{eventId}",
+		httputil.MakeAuthAPI(gomatrixserverlib.Join, 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 Context(
+				req, device,
+				rsAPI, syncDB,
+				vars["roomId"], vars["eventId"],
+			)
+		}),
+	).Methods(http.MethodGet, http.MethodOptions)
 }