Appservice Can't set alias

[nkitsaini]

Same as #661 which is closed but I can still reproduce the issue.

Background information

• dendrite v0.5
• Monolith
• Sqlite3
• In Docker

Description

Appservice can't set alias in it's own namespace.
Request fails with error message: Alias is reserved by an application service

Steps to reproduce

• Start Dendrite with a appservice
• Try to set alias using appservice

Logs

Can't find anything relevant in debug logs.

[kegsay]

× AS can make room aliases

In sytest, which is probably why this isn't possible.

[nkitsaini]

I went through the code and found some relevant stuff (I might be wrong as not familiar with go)

When the Room is created

• We don't check if the alias is exclusive.
• Anyone can use exclusive Alias
  

  dendrite/clientapi/routing/createroom.go

  Lines 473 to 482 in 4d9f5b2

  	if roomAlias != "" {
  	aliasReq := roomserverAPI.SetRoomAliasRequest{
  	Alias: roomAlias,
  	RoomID: roomID,
  	UserID: userID,
  	}
  	var aliasResp roomserverAPI.SetRoomAliasResponse
  	err = rsAPI.SetRoomAlias(req.Context(), &aliasReq, &aliasResp)
  	if err != nil {

While there's a request to set alias after room is created:

• We only check if alias is exclusive.
• Even the Appservice having exclusive access cannot set the alias
  

  dendrite/src/github.com/matrix-org/dendrite/clientapi/routing/directory.go

  Lines 116 to 129 in 63ec8a4

  	// Check that the alias does not fall within an exclusive namespace of an
  	// application service
  	for _, appservice := range cfg.Derived.ApplicationServices {
  	if userNamespaces, ok := appservice.NamespaceMap["users"]; ok {
  	for _, namespace := range userNamespaces {
  	if namespace.Exclusive && namespace.RegexpObject.MatchString(alias) {
  	return util.JSONResponse{
  	Code: http.StatusBadRequest,
  	JSON: jsonerror.ASExclusive("Alias is reserved by an application service"),
  	}
  	}
  	}
  	}
  	}

Psuedo Code for fix:

alias_owner = getExclusiveOwner(alias)
if alias_owner == None:
      proceed and create the alias
else:
      if alias_owner != appservice_who_sent_the_request:
           return AliasIsExclusive() # shouldn't create alias
      else:
           proceed and create the alias

(Similar thing should be confirmed for other namespaces, namely user and room_id