Fix bug in vtgate when running with keyspace filtering

[setassociative]

Description

This checks if a vtgate is currently filtering keyspaces before requesting the TopoServer. This is necessary because a TopoServer can't be accessed in those cases as the filtered Topo in those cases could make it unsafe to make writes since all reads would be returning a subset of the actual topo data.

The only use of the requested topoServer that I found was in the DDL handling path and was introduced in vitessio#6547.

This is deployed on dev but should get testing (endtoend or unit, unclear on best path atm) before going upstream.

[rafael]

This makes sense to me. In other words, if I'm reading this correctly: if you have filters on, you can't safely access the topo because it doesn't have the full view.

For the context of online DDL's, you need the full topo, so you need to make sure that you run them from a vtgate that is not filtering.

[rafael]

one nit - this error might be too cryptic for an end user. I wonder if we should mention that probably they have a keyspace filter and that is incompatible with online DDLs

[setassociative]

Fair enough -- I referenced filtered keyspaces in the error but since it's a bit indirect i'm going to leave the general commentary around why the failure is happening as well.

[setassociative]

For the context of online DDL's, you need the full topo, so you need to make sure that you run them from a vtgate that is not filtering.

More or less. Technically I don't actually think it needs the full topo, the impl for writing the request into topo looks like:

func (onlineDDL *OnlineDDL) WriteTopo(ctx context.Context, conn topo.Conn, basePath string) error {
	if onlineDDL.UUID == "" {
		return fmt.Errorf("onlineDDL UUID not found; keyspace=%s, sql=%s", onlineDDL.Keyspace, onlineDDL.SQL)
	}
	bytes, err := onlineDDL.ToJSON()
	if err != nil {
		return fmt.Errorf("onlineDDL marshall error:%s, keyspace=%s, sql=%s", err.Error(), onlineDDL.Keyspace, onlineDDL.SQL)
	}
	_, err = conn.Create(ctx, fmt.Sprintf("%s/%s", basePath, onlineDDL.UUID), bytes)
	if err != nil {
		return fmt.Errorf("onlineDDL topo create error:%s, keyspace=%s, sql=%s", err.Error(), onlineDDL.Keyspace, onlineDDL.SQL)
	}
	return nil
}

which has a limited impact -- it's writing only to the schema-migration/requests/<uuid> path which shouldn't be filtered... but the actual topo.Server interface doesn't have any kind of guard rails around what's being written when you request a writeable interface so the keyspace filtering stuff blocks access to it.

[setassociative]

Taking your 👍 as a Ship it to merge this branch down, feel free to claw it back if we don't want to roll this out