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

  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 https://github.com/matrix-org/dendrite/blob/4d9f5b2e5787d23e1dbcebfda1c6d99d3498ec7e/clientapi/routing/createroom.go#L473-L482

  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 https://github.com/matrix-org/dendrite/blob/63ec8a48842fdd930bfc8113eab36c4dcd1c43f0/src/github.com/matrix-org/dendrite/clientapi/routing/directory.go#L116-L129

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