## Problem
When picking locations for a shard, we should use a ScheduleContext that
includes all the other shards in the tenant, so that we apply proper
anti-affinity between shards. If we don't do this, then it can lead to
unstable scheduling, where we place a shard somewhere that the optimizer
will then immediately move it away from.
We didn't always do this, because it was a bit awkward to accumulate the
context for a tenant rather than just walking tenants.
This was a TODO in `handle_node_availability_transition`:
```
// TODO: populate a ScheduleContext including all shards in the same tenant_id (only matters
// for tenants without secondary locations: if they have a secondary location, then this
// schedule() call is just promoting an existing secondary)
```
This is a precursor to https://github.com/neondatabase/neon/issues/8264,
where the current imperfect scheduling during node evacuation hampers
testing.
## Summary of changes
- Add an iterator type that yields each shard along with a
schedulecontext that includes all the other shards from the same tenant
- Use the iterator to replace hand-crafted logic in optimize_all_plan
(functionally identical)
- Use the iterator in `handle_node_availability_transition` to apply
proper anti-affinity during node evacuation.
## Problem
We may sometimes use scheduling modes like `Pause` to pin a tenant in
its current location for operational reasons. It is undesirable for the
chaos task to make any changes to such projects.
## Summary of changes
- Add a check for scheduling mode
- Add a log line when we do choose to do a chaos action for a tenant:
this will help us understand which operations originate from the chaos
task.
Chaos injection bridges the gap between automated testing (where we do
lots of different things with small, short-lived tenants), and staging
(where we do many fewer things, but with larger, long-lived tenants).
This PR adds a first type of chaos which isn't really very chaotic: it's
live migration of tenants between healthy pageservers. This nevertheless
provides continuous checks that things like clean, prompt shutdown of
tenants works for realistically deployed pageservers with realistically
large tenants.
