Opening or closing a table built every shard and then dropped them

[bjmeetsfo]

Opening a table reads one number out of the answer -- the version a caller must quote to be given a topology. Closing one reads only whether the table could be served at all. Both asked for the whole topology, so both paid to build a shard list that neither looked at, and the cost was the size of the table:

shards	OpenTable	CloseTable
50	28.9us	31.5us
200	114.6us	113.6us
800	434.9us	435.7us

Now 0.5us at every one of them, and flat rather than growing with the table.

Why it is written this way

The cheap answer already existed inside the same call. A caller that is already current is given the status and the table back and stops before the shard list is built, and every missing, dropped and frozen check -- including the namespace ones -- runs before that point.

So this asks for the topology with the version set past anything a table can hold, rather than repeating those checks somewhere new where they could drift from the originals. The status these routes report is the same status because it is the same code producing it.

Nothing observable changes

Opening reads the version, which the early answer carries. Closing reads the status, which is identical in every case. Neither route exposes the flag that says the answer was unchanged.

The route just below these two, which serves topologies to proxies, was already passing the caller's version through and is untouched.

Testing

Neither route had a test. They have one now, covering the version, the refusals for a table that was never created and for one that has been dropped.

There is also a test that the cheap answer agrees with the full one on status and version across a normal table, a frozen table, a dropped table, a table in a frozen namespace, and two that do not exist -- and that it carries no shards while the full answer still carries four. The saving is asserted by counting what was built rather than by timing it, so it holds on a loaded machine.

Both tests were checked against a mutation: made to build the shard list again, and made to report the version as zero. Each fails.

Measurements were taken with the arms interleaved, because the box is shared.

Suites: 327 metadata tests and 48 metaserver binary tests, all green.