Sliding Sync: Make PerConnectionState immutable

[erikjohnston]

This is so that we can cache it.

We also move the sliding sync types to synapse/types/handlers/sliding_sync.py. This is mainly in-prep for #17599 to avoid circular imports.

The only change in behaviour is that RoomSyncConfig.combine_sync_config(..) now returns a new room sync config rather than mutating in-place.

Reviewable commit-by-commit.

[MadLittleMods]

We could check if the enum member values are all immutable to be sure.

[erikjohnston]

If I could figure out how I would, but I couldn't :(

[MadLittleMods]

Does this work?

        elif rt.type.is_enum:
            for enum_member_name in rt.get_enum_values():
                enum_member_type = rt.type.names[enum_member_name].type
                # Enum members should have types
                assert enum_member_type is not None
                ok, note = is_cacheable(enum_member_type, signature, verbose)
                if not ok:
                    return (
                        False,
                        f"enum member {enum_member_name} value of type {enum_member_type} not cacheable: {note}",
                    )

            # All Enum member values are immutable
            return True, None

It hasn't been tested as this code doesn't seem to matter anymore in this PR? I commented this whole enum logic branch out and mypy still passes.

[erikjohnston]

I had no idea that get enum values was a thing. I'll have a try and see if it works! Very good catch either way

Yeah, turns out this matters for the next PR where we add a cache on this value. Not sure why I added this to this pr.

[MadLittleMods]

Since this is immutable now, I don't think we need this function anymore