Move /event to the SyncAPI (#2782)

This allows us to apply history visibility without having to recalculate
it in the roomserver.
Unblocks https://github.com/matrix-org/complement/pull/495, fix missing
part of https://github.com/matrix-org/dendrite/issues/617

3 files changed, 117 insertions, 3 deletions

diff --git a/syncapi/internal/history_visibility.go b/syncapi/internal/history_visibility.go
index bbfe19f4..71d7ddd1 100644
--- a/syncapi/internal/history_visibility.go
+++ b/syncapi/internal/history_visibility.go
@@ -19,11 +19,13 @@ import (
 	"math"
 	"time"
 
-	"github.com/matrix-org/dendrite/roomserver/api"
-	"github.com/matrix-org/dendrite/syncapi/storage"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/sirupsen/logrus"
 	"github.com/tidwall/gjson"
+
+	"github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/dendrite/syncapi/storage"
 )
 
 func init() {
@@ -189,7 +191,7 @@ func visibilityForEvents(
 		UserID:   userID,
 	}, membershipResp)
 	if err != nil {
-		return result, err
+		logrus.WithError(err).Error("visibilityForEvents: failed to fetch membership at event, defaulting to 'leave'")
 	}
 
 	// Create a map from eventID -> eventVisibility
diff --git a/syncapi/routing/getevent.go b/syncapi/routing/getevent.go
new file mode 100644
index 00000000..d2cdc1b5
--- /dev/null
+++ b/syncapi/routing/getevent.go
@@ -0,0 +1,102 @@
+// 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 (
+	"net/http"
+
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
+	"github.com/sirupsen/logrus"
+
+	"github.com/matrix-org/dendrite/clientapi/jsonerror"
+	"github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/dendrite/setup/config"
+	"github.com/matrix-org/dendrite/syncapi/internal"
+	"github.com/matrix-org/dendrite/syncapi/storage"
+	userapi "github.com/matrix-org/dendrite/userapi/api"
+)
+
+// GetEvent implements
+//
+//	GET /_matrix/client/r0/rooms/{roomId}/event/{eventId}
+//
+// https://spec.matrix.org/v1.4/client-server-api/#get_matrixclientv3roomsroomideventeventid
+func GetEvent(
+	req *http.Request,
+	device *userapi.Device,
+	roomID string,
+	eventID string,
+	cfg *config.SyncAPI,
+	syncDB storage.Database,
+	rsAPI api.SyncRoomserverAPI,
+) util.JSONResponse {
+	ctx := req.Context()
+	db, err := syncDB.NewDatabaseTransaction(ctx)
+	logger := util.GetLogger(ctx).WithFields(logrus.Fields{
+		"event_id": eventID,
+		"room_id":  roomID,
+	})
+	if err != nil {
+		logger.WithError(err).Error("GetEvent: syncDB.NewDatabaseTransaction failed")
+		return jsonerror.InternalServerError()
+	}
+
+	events, err := db.Events(ctx, []string{eventID})
+	if err != nil {
+		logger.WithError(err).Error("GetEvent: syncDB.Events failed")
+		return jsonerror.InternalServerError()
+	}
+
+	// The requested event does not exist in our database
+	if len(events) == 0 {
+		logger.Debugf("GetEvent: requested event doesn't exist locally")
+		return util.JSONResponse{
+			Code: http.StatusNotFound,
+			JSON: jsonerror.NotFound("The event was not found or you do not have permission to read this event"),
+		}
+	}
+
+	// If the request is coming from an appservice, get the user from the request
+	userID := device.UserID
+	if asUserID := req.FormValue("user_id"); device.AppserviceID != "" && asUserID != "" {
+		userID = asUserID
+	}
+
+	// Apply history visibility to determine if the user is allowed to view the event
+	events, err = internal.ApplyHistoryVisibilityFilter(ctx, db, rsAPI, events, nil, userID, "event")
+	if err != nil {
+		logger.WithError(err).Error("GetEvent: internal.ApplyHistoryVisibilityFilter failed")
+		return util.JSONResponse{
+			Code: http.StatusInternalServerError,
+			JSON: jsonerror.InternalServerError(),
+		}
+	}
+
+	// We only ever expect there to be one event
+	if len(events) != 1 {
+		// 0 events -> not allowed to view event; > 1 events -> something that shouldn't happen
+		logger.WithField("event_count", len(events)).Debug("GetEvent: can't return the requested event")
+		return util.JSONResponse{
+			Code: http.StatusNotFound,
+			JSON: jsonerror.NotFound("The event was not found or you do not have permission to read this event"),
+		}
+	}
+
+	return util.JSONResponse{
+		Code: http.StatusOK,
+		JSON: gomatrixserverlib.HeaderedToClientEvent(events[0], gomatrixserverlib.FormatAll),
+	}
+}
diff --git a/syncapi/routing/routing.go b/syncapi/routing/routing.go
index 8f84a134..069dee81 100644
--- a/syncapi/routing/routing.go
+++ b/syncapi/routing/routing.go
@@ -60,6 +60,16 @@ func Setup(
 		return OnIncomingMessagesRequest(req, syncDB, vars["roomID"], device, rsAPI, cfg, srp, lazyLoadCache)
 	})).Methods(http.MethodGet, http.MethodOptions)
 
+	v3mux.Handle("/rooms/{roomID}/event/{eventID}",
+		httputil.MakeAuthAPI("rooms_get_event", 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 GetEvent(req, device, vars["roomID"], vars["eventID"], cfg, syncDB, rsAPI)
+		}),
+	).Methods(http.MethodGet, http.MethodOptions)
+
 	v3mux.Handle("/user/{userId}/filter",
 		httputil.MakeAuthAPI("put_filter", userAPI, func(req *http.Request, device *userapi.Device) util.JSONResponse {
 			vars, err := httputil.URLDecodeMapValues(mux.Vars(req))