user_directory_search_enabled is not spec compliant

[Johennes]

Description

As discussed in matrix-org/matrix-spec-proposals#4170 (comment), when the user_directory_search_enabled config setting is disabled, Synapse responds with HTTP 200 and an empty results array on all requests to POST /_matrix/client/v3/user_directory/search. I believe this is incompatible with the spec which requires that at least users from shared and public rooms are searchable:

Performs a search for users. The homeserver may determine which subset of users are searched, however the homeserver MUST at a minimum consider the users the requesting user shares a room with and those who reside in public rooms (known to the homeserver). The search MUST consider local users to the homeserver, and SHOULD query remote users as part of the search.

Excluding these users from search results also doesn't seem to have any benefit because the requester has access to the returned information anyway through room membership.

Steps to reproduce

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Homeserver

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Synapse Version

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Installation Method

I don't know

Database

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Workers

I don't know

Platform

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Configuration

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Relevant log output

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Anything else that would be useful to know?

No response