Support VStream with keyspaces_to_watch

[mattlord]

Description

In order to safely support the VStream api against VTGates running with the -keyspaces_to_watch flag we needed to add a read-only VTTopo server interface to ensure that no changes are made to the topology from the (potentially) explicitly limited view of the topology provided via this vtgate.

Related Issue(s)

• VStream does not work when keyspaces_to_watch is set on vtgate #8772

Checklist

• Should this PR be backported? No. IMO this should bake in 13.0-snapshot
• Tests were added
• Documentation is not required (we never documented that these two features did NOT work together)

[mattlord]

The new vstream_with_keyspaces_to_watch test is failing as expected: https://github.com/vitessio/vitess/pull/8988/checks?check_run_id=3879443572

...
I1013 07:14:19.558124   18763 vstream_test.go:125] 2021-10-13 07:14:19.558123471 +0000 UTC m=+39.257232583:: remote error: Code: UNKNOWN
173
rpc error: code = Unknown desc = cannot access underlying topology server when keyspace filtering is enabled
...

[sougou]

I think it's important that the we don't have dummy implementations of SetReadOnly in the servers. This can become a source of confusion in the future if someone accidentally uses one of these connections as an interface, and the implementation doesn't enforce it.

If explicit casting or the interface approach is harder, I'd recommend changing SetReadOnly to return an error, and we can have these underlying implementations fail with a "not supported" error.

[mattlord]

I think it's important that the we don't have dummy implementations of SetReadOnly in the servers. This can become a source of confusion in the future if someone accidentally uses one of these connections as an interface, and the implementation doesn't enforce it.

If explicit casting or the interface approach is harder, I'd recommend changing SetReadOnly to return an error, and we can have these underlying implementations fail with a "not supported" error.

Good points. I'll look into this next. Thanks!

[mattlord]

@sougou done: https://github.com/vitessio/vitess/pull/8988/files 🙇

[sougou]

This is great!

[deepthi]

LGTM too. Nice work!