This patch aims to:
* Unify connection & querying logic of ZenithPagerserver and Postgres.
* Mitigate changes to transaction machinery introduced in `psycopg2 >= 2.9`.
Now it's possible to acquire db connection using the corresponding
method:
```python
pg = postgres.create_start('main')
conn = pg.connect()
...
conn.close()
```
This pattern can be further improved with the help of `closing`:
```python
from contextlib import closing
pg = postgres.create_start('main')
with closing(pg.connect()) as conn:
...
```
All connections produced by this method will have autocommit
enabled by default.
Previously, transaction commit could happen regardless of whether
pageserver has caught up or not. This patch aims to fix that.
There are two notable changes:
1. ComputeControlPlane::new_node() now sets the
`synchronous_standby_names = 'pageserver'` parameter to delay
transaction commit until pageserver acting as a standby has
fetched and ack'd a relevant portion of WAL.
2. pageserver now has to:
- Specify the `application_name = pageserver` which matches the
one in `synchronous_standby_names`.
- Properly reply with the ack'd LSNs.
This means that some tests don't need sleeps anymore.
TODO: We should probably make this behavior configurable.
Fixes#187.
