Merge branch 'develop' into dmr/pyproject-poetry

Signed-off-by: Dan Callahan <danc@element.io>

docs/development/contributing_guide.md

@@ -201,9 +201,10 @@ To do so, [configure Postgres](../postgres.md) and run `trial` with the
 following environment variables matching your configuration:
 
 - `SYNAPSE_POSTGRES` to anything nonempty
-- `SYNAPSE_POSTGRES_HOST`
-- `SYNAPSE_POSTGRES_USER`
-- `SYNAPSE_POSTGRES_PASSWORD`
+- `SYNAPSE_POSTGRES_HOST` (optional if it's the default: UNIX socket)
+- `SYNAPSE_POSTGRES_PORT` (optional if it's the default: 5432)
+- `SYNAPSE_POSTGRES_USER` (optional if using a UNIX socket)
+- `SYNAPSE_POSTGRES_PASSWORD` (optional if using a UNIX socket)
 
 For example:
 
@@ -215,6 +216,13 @@ export SYNAPSE_POSTGRES_PASSWORD=mydevenvpassword
 trial
 ```
 
+You don't need to specify the host, user, port or password if your Postgres
+server is set to authenticate you over the UNIX socket (i.e. if the `psql` command
+works without further arguments).
+
+Your Postgres account needs to be able to create databases.
+
+
 ## Run the integration tests ([Sytest](https://github.com/matrix-org/sytest)).
 
 The integration tests are a more comprehensive suite of tests. They

docs/development/room-dag-concepts.md

@@ -39,7 +39,8 @@ yet correlated to the DAG.
 Outliers typically arise when we fetch the auth chain or state for a given
 event. When that happens, we just grab the events in the state/auth chain,
 without calculating the state at those events, or backfilling their
-`prev_events`.
+`prev_events`. Since we don't have the state at any events fetched in that
+way, we mark them as outliers.
 
 So, typically, we won't have the `prev_events` of an `outlier` in the database,
 (though it's entirely possible that we *might* have them for some other