Merge tag 'v1.35.0' into babolivier/dinsic_1.41.0

Synapse 1.35.0 (2021-06-01)
===========================

Note that [the tag](https://github.com/matrix-org/synapse/releases/tag/v1.35.0rc3) and [docker images](https://hub.docker.com/layers/matrixdotorg/synapse/v1.35.0rc3/images/sha256-34ccc87bd99a17e2cbc0902e678b5937d16bdc1991ead097eee6096481ecf2c4?context=explore) for `v1.35.0rc3` were incorrectly built. If you are experiencing issues with either, it is recommended to upgrade to the equivalent tag or docker image for the `v1.35.0` release.

Deprecations and Removals
-------------------------

- The core Synapse development team plan to drop support for the [unstable API of MSC2858](https://github.com/matrix-org/matrix-doc/blob/master/proposals/2858-Multiple-SSO-Identity-Providers.md#unstable-prefix), including the undocumented `experimental.msc2858_enabled` config option, in August 2021. Client authors should ensure that their clients are updated to use the stable API (which has been supported since Synapse 1.30) well before that time, to give their users time to upgrade. ([\#10101](https://github.com/matrix-org/synapse/issues/10101))

Bugfixes
--------

- Fixed a bug causing replication requests to fail when receiving a lot of events via federation. Introduced in v1.33.0. ([\#10082](https://github.com/matrix-org/synapse/issues/10082))
- Fix HTTP response size limit to allow joining very large rooms over federation. Introduced in v1.33.0. ([\#10093](https://github.com/matrix-org/synapse/issues/10093))

Internal Changes
----------------

- Log method and path when dropping request due to size limit. ([\#10091](https://github.com/matrix-org/synapse/issues/10091))

Synapse 1.35.0rc2 (2021-05-27)
==============================

Bugfixes
--------

- Fix a bug introduced in v1.35.0rc1 when calling the spaces summary API via a GET request. ([\#10079](https://github.com/matrix-org/synapse/issues/10079))

Synapse 1.35.0rc1 (2021-05-25)
==============================

Features
--------

- Add experimental support to allow a user who could join a restricted room to view it in the spaces summary. ([\#9922](https://github.com/matrix-org/synapse/issues/9922), [\#10007](https://github.com/matrix-org/synapse/issues/10007), [\#10038](https://github.com/matrix-org/synapse/issues/10038))
- Reduce memory usage when joining very large rooms over federation. ([\#9958](https://github.com/matrix-org/synapse/issues/9958))
- Add a configuration option which allows enabling opentracing by user id. ([\#9978](https://github.com/matrix-org/synapse/issues/9978))
- Enable experimental support for [MSC2946](https://github.com/matrix-org/matrix-doc/pull/2946) (spaces summary API) and [MSC3083](https://github.com/matrix-org/matrix-doc/pull/3083) (restricted join rules) by default. ([\#10011](https://github.com/matrix-org/synapse/issues/10011))

Bugfixes
--------

- Fix a bug introduced in v1.26.0 which meant that `synapse_port_db` would not correctly initialise some postgres sequences, requiring manual updates afterwards. ([\#9991](https://github.com/matrix-org/synapse/issues/9991))
- Fix `synctl`'s `--no-daemonize` parameter to work correctly with worker processes. ([\#9995](https://github.com/matrix-org/synapse/issues/9995))
- Fix a validation bug introduced in v1.34.0 in the ordering of spaces in the space summary API. ([\#10002](https://github.com/matrix-org/synapse/issues/10002))
- Fixed deletion of new presence stream states from database. ([\#10014](https://github.com/matrix-org/synapse/issues/10014), [\#10033](https://github.com/matrix-org/synapse/issues/10033))
- Fixed a bug with very high resolution image uploads throwing internal server errors. ([\#10029](https://github.com/matrix-org/synapse/issues/10029))

Updates to the Docker image
---------------------------

- Fix bug introduced in Synapse 1.33.0 which caused a `Permission denied: '/homeserver.log'` error when starting Synapse with the generated log configuration. Contributed by Sergio Miguéns Iglesias. ([\#10045](https://github.com/matrix-org/synapse/issues/10045))

Improved Documentation
----------------------

- Add hardened systemd files as proposed in [#9760](https://github.com/matrix-org/synapse/issues/9760) and added them to `contrib/`. Change the docs to reflect the presence of these files. ([\#9803](https://github.com/matrix-org/synapse/issues/9803))
- Clarify documentation around SSO mapping providers generating unique IDs and localparts. ([\#9980](https://github.com/matrix-org/synapse/issues/9980))
- Updates to the PostgreSQL documentation (`postgres.md`). ([\#9988](https://github.com/matrix-org/synapse/issues/9988), [\#9989](https://github.com/matrix-org/synapse/issues/9989))
- Fix broken link in user directory documentation. Contributed by @junquera. ([\#10016](https://github.com/matrix-org/synapse/issues/10016))
- Add missing room state entry to the table of contents of room admin API. ([\#10043](https://github.com/matrix-org/synapse/issues/10043))

Deprecations and Removals
-------------------------

- Removed support for the deprecated `tls_fingerprints` configuration setting. Contributed by Jerin J Titus. ([\#9280](https://github.com/matrix-org/synapse/issues/9280))

Internal Changes
----------------

- Allow sending full presence to users via workers other than the one that called `ModuleApi.send_local_online_presence_to`. ([\#9823](https://github.com/matrix-org/synapse/issues/9823))
- Update comments in the space summary handler. ([\#9974](https://github.com/matrix-org/synapse/issues/9974))
- Minor enhancements to the `@cachedList` descriptor. ([\#9975](https://github.com/matrix-org/synapse/issues/9975))
- Split multipart email sending into a dedicated handler. ([\#9977](https://github.com/matrix-org/synapse/issues/9977))
- Run `black` on files in the `scripts` directory. ([\#9981](https://github.com/matrix-org/synapse/issues/9981))
- Add missing type hints to `synapse.util` module. ([\#9982](https://github.com/matrix-org/synapse/issues/9982))
- Simplify a few helper functions. ([\#9984](https://github.com/matrix-org/synapse/issues/9984), [\#9985](https://github.com/matrix-org/synapse/issues/9985), [\#9986](https://github.com/matrix-org/synapse/issues/9986))
- Remove unnecessary property from SQLBaseStore. ([\#9987](https://github.com/matrix-org/synapse/issues/9987))
- Remove `keylen` param on `LruCache`. ([\#9993](https://github.com/matrix-org/synapse/issues/9993))
- Update the Grafana dashboard in `contrib/`. ([\#10001](https://github.com/matrix-org/synapse/issues/10001))
- Add a batching queue implementation. ([\#10017](https://github.com/matrix-org/synapse/issues/10017))
- Reduce memory usage when verifying signatures on large numbers of events at once. ([\#10018](https://github.com/matrix-org/synapse/issues/10018))
- Properly invalidate caches for destination retry timings every (instead of expiring entries every 5 minutes). ([\#10036](https://github.com/matrix-org/synapse/issues/10036))
- Fix running complement tests with Synapse workers. ([\#10039](https://github.com/matrix-org/synapse/issues/10039))
- Fix typo in `get_state_ids_for_event` docstring where the return type was incorrect. ([\#10050](https://github.com/matrix-org/synapse/issues/10050))

8 files changed, 179 insertions, 150 deletions

diff --git a/docs/admin_api/rooms.md b/docs/admin_api/rooms.md
index 01d3882426..5721210fee 100644
--- a/docs/admin_api/rooms.md
+++ b/docs/admin_api/rooms.md
@@ -4,6 +4,7 @@
   * [Usage](#usage)
 - [Room Details API](#room-details-api)
 - [Room Members API](#room-members-api)
+- [Room State API](#room-state-api)
 - [Delete Room API](#delete-room-api)
   * [Parameters](#parameters-1)
   * [Response](#response)
diff --git a/docs/opentracing.md b/docs/opentracing.md
index 4c7a56a5d7..f91362f112 100644
--- a/docs/opentracing.md
+++ b/docs/opentracing.md
@@ -42,17 +42,17 @@ To receive OpenTracing spans, start up a Jaeger server. This can be done
 using docker like so:
 
 ```sh
-docker run -d --name jaeger
+docker run -d --name jaeger \
   -p 6831:6831/udp \
   -p 6832:6832/udp \
   -p 5778:5778 \
   -p 16686:16686 \
   -p 14268:14268 \
-  jaegertracing/all-in-one:1.13
+  jaegertracing/all-in-one:1
 ```
 
 Latest documentation is probably at
-<https://www.jaegertracing.io/docs/1.13/getting-started/>
+https://www.jaegertracing.io/docs/latest/getting-started.
 
 ## Enable OpenTracing in Synapse
 
@@ -62,7 +62,7 @@ as shown in the [sample config](./sample_config.yaml). For example:
 
 ```yaml
 opentracing:
-  tracer_enabled: true
+  enabled: true
   homeserver_whitelist:
     - "mytrustedhomeserver.org"
     - "*.myotherhomeservers.com"
@@ -90,4 +90,4 @@ to two problems, namely:
 ## Configuring Jaeger
 
 Sampling strategies can be set as in this document:
-<https://www.jaegertracing.io/docs/1.13/sampling/>
+<https://www.jaegertracing.io/docs/latest/sampling/>.
diff --git a/docs/postgres.md b/docs/postgres.md
index 680685d04e..f83155e52a 100644
--- a/docs/postgres.md
+++ b/docs/postgres.md
@@ -1,6 +1,6 @@
 # Using Postgres
 
-Postgres version 9.5 or later is known to work.
+Synapse supports PostgreSQL versions 9.6 or later.
 
 ## Install postgres client libraries
 
@@ -33,28 +33,15 @@ Assuming your PostgreSQL database user is called `postgres`, first authenticate
     # Or, if your system uses sudo to get administrative rights
     sudo -u postgres bash
 
-Then, create a user ``synapse_user`` with:
+Then, create a postgres user and a database with:
 
+    # this will prompt for a password for the new user
     createuser --pwprompt synapse_user
 
-Before you can authenticate with the `synapse_user`, you must create a
-database that it can access. To create a database, first connect to the
-database with your database user:
+    createdb --encoding=UTF8 --locale=C --template=template0 --owner=synapse_user synapse
 
-    su - postgres # Or: sudo -u postgres bash
-    psql
-
-and then run:
-
-    CREATE DATABASE synapse
-     ENCODING 'UTF8'
-     LC_COLLATE='C'
-     LC_CTYPE='C'
-     template=template0
-     OWNER synapse_user;
-
-This would create an appropriate database named `synapse` owned by the
-`synapse_user` user (which must already have been created as above).
+The above will create a user called `synapse_user`, and a database called
+`synapse`.
 
 Note that the PostgreSQL database *must* have the correct encoding set
 (as shown above), otherwise it will not be able to store UTF8 strings.
@@ -63,79 +50,6 @@ You may need to enable password authentication so `synapse_user` can
 connect to the database. See
 <https://www.postgresql.org/docs/current/auth-pg-hba-conf.html>.
 
-If you get an error along the lines of `FATAL:  Ident authentication failed for
-user "synapse_user"`, you may need to use an authentication method other than
-`ident`:
-
-* If the `synapse_user` user has a password, add the password to the `database:`
-  section of `homeserver.yaml`. Then add the following to `pg_hba.conf`:
-
-  ```
-  host    synapse     synapse_user    ::1/128     md5  # or `scram-sha-256` instead of `md5` if you use that
-  ```
-
-* If the `synapse_user` user does not have a password, then a password doesn't
-  have to be added to `homeserver.yaml`. But the following does need to be added
-  to `pg_hba.conf`:
-
-  ```
-  host    synapse     synapse_user    ::1/128     trust
-  ```
-
-Note that line order matters in `pg_hba.conf`, so make sure that if you do add a
-new line, it is inserted before:
-
-```
-host    all         all             ::1/128     ident
-```
-
-### Fixing incorrect `COLLATE` or `CTYPE`
-
-Synapse will refuse to set up a new database if it has the wrong values of
-`COLLATE` and `CTYPE` set, and will log warnings on existing databases. Using
-different locales can cause issues if the locale library is updated from
-underneath the database, or if a different version of the locale is used on any
-replicas.
-
-The safest way to fix the issue is to take a dump and recreate the database with
-the correct `COLLATE` and `CTYPE` parameters (as shown above). It is also possible to change the
-parameters on a live database and run a `REINDEX` on the entire database,
-however extreme care must be taken to avoid database corruption.
-
-Note that the above may fail with an error about duplicate rows if corruption
-has already occurred, and such duplicate rows will need to be manually removed.
-
-
-## Fixing inconsistent sequences error
-
-Synapse uses Postgres sequences to generate IDs for various tables. A sequence
-and associated table can get out of sync if, for example, Synapse has been
-downgraded and then upgraded again.
-
-To fix the issue shut down Synapse (including any and all workers) and run the
-SQL command included in the error message. Once done Synapse should start
-successfully.
-
-
-## Tuning Postgres
-
-The default settings should be fine for most deployments. For larger
-scale deployments tuning some of the settings is recommended, details of
-which can be found at
-<https://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server>.
-
-In particular, we've found tuning the following values helpful for
-performance:
-
--   `shared_buffers`
--   `effective_cache_size`
--   `work_mem`
--   `maintenance_work_mem`
--   `autovacuum_work_mem`
-
-Note that the appropriate values for those fields depend on the amount
-of free memory the database host has available.
-
 ## Synapse config
 
 When you are ready to start using PostgreSQL, edit the `database`
@@ -165,18 +79,42 @@ may block for an extended period while it waits for a response from the
 database server. Example values might be:
 
 ```yaml
-# seconds of inactivity after which TCP should send a keepalive message to the server
-keepalives_idle: 10
+database:
+  args:
+    # ... as above
+
+    # seconds of inactivity after which TCP should send a keepalive message to the server
+    keepalives_idle: 10
 
-# the number of seconds after which a TCP keepalive message that is not
-# acknowledged by the server should be retransmitted
-keepalives_interval: 10
+    # the number of seconds after which a TCP keepalive message that is not
+    # acknowledged by the server should be retransmitted
+    keepalives_interval: 10
 
-# the number of TCP keepalives that can be lost before the client's connection
-# to the server is considered dead
-keepalives_count: 3
+    # the number of TCP keepalives that can be lost before the client's connection
+    # to the server is considered dead
+    keepalives_count: 3
 ```
 
+## Tuning Postgres
+
+The default settings should be fine for most deployments. For larger
+scale deployments tuning some of the settings is recommended, details of
+which can be found at
+<https://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server>.
+
+In particular, we've found tuning the following values helpful for
+performance:
+
+-   `shared_buffers`
+-   `effective_cache_size`
+-   `work_mem`
+-   `maintenance_work_mem`
+-   `autovacuum_work_mem`
+
+Note that the appropriate values for those fields depend on the amount
+of free memory the database host has available.
+
+
 ## Porting from SQLite
 
 ### Overview
@@ -185,9 +123,8 @@ The script `synapse_port_db` allows porting an existing synapse server
 backed by SQLite to using PostgreSQL. This is done in as a two phase
 process:
 
-1.  Copy the existing SQLite database to a separate location (while the
-    server is down) and running the port script against that offline
-    database.
+1.  Copy the existing SQLite database to a separate location and run
+    the port script against that offline database.
 2.  Shut down the server. Rerun the port script to port any data that
     has come in since taking the first snapshot. Restart server against
     the PostgreSQL database.
@@ -245,3 +182,60 @@ PostgreSQL database configuration file `homeserver-postgres.yaml`:
     ./synctl start
 
 Synapse should now be running against PostgreSQL.
+
+
+## Troubleshooting
+
+### Alternative auth methods
+
+If you get an error along the lines of `FATAL:  Ident authentication failed for
+user "synapse_user"`, you may need to use an authentication method other than
+`ident`:
+
+* If the `synapse_user` user has a password, add the password to the `database:`
+  section of `homeserver.yaml`. Then add the following to `pg_hba.conf`:
+
+  ```
+  host    synapse     synapse_user    ::1/128     md5  # or `scram-sha-256` instead of `md5` if you use that
+  ```
+
+* If the `synapse_user` user does not have a password, then a password doesn't
+  have to be added to `homeserver.yaml`. But the following does need to be added
+  to `pg_hba.conf`:
+
+  ```
+  host    synapse     synapse_user    ::1/128     trust
+  ```
+
+Note that line order matters in `pg_hba.conf`, so make sure that if you do add a
+new line, it is inserted before:
+
+```
+host    all         all             ::1/128     ident
+```
+
+### Fixing incorrect `COLLATE` or `CTYPE`
+
+Synapse will refuse to set up a new database if it has the wrong values of
+`COLLATE` and `CTYPE` set, and will log warnings on existing databases. Using
+different locales can cause issues if the locale library is updated from
+underneath the database, or if a different version of the locale is used on any
+replicas.
+
+The safest way to fix the issue is to dump the database and recreate it with
+the correct locale parameter (as shown above). It is also possible to change the
+parameters on a live database and run a `REINDEX` on the entire database,
+however extreme care must be taken to avoid database corruption.
+
+Note that the above may fail with an error about duplicate rows if corruption
+has already occurred, and such duplicate rows will need to be manually removed.
+
+### Fixing inconsistent sequences error
+
+Synapse uses Postgres sequences to generate IDs for various tables. A sequence
+and associated table can get out of sync if, for example, Synapse has been
+downgraded and then upgraded again.
+
+To fix the issue shut down Synapse (including any and all workers) and run the
+SQL command included in the error message. Once done Synapse should start
+successfully.
diff --git a/docs/presence_router_module.md b/docs/presence_router_module.md
index d6566d978d..d2844915df 100644
--- a/docs/presence_router_module.md
+++ b/docs/presence_router_module.md
@@ -28,7 +28,11 @@ async def ModuleApi.send_local_online_presence_to(users: Iterable[str]) -> None
 which can be given a list of local or remote MXIDs to broadcast known, online user
 presence to (for those users that the receiving user is considered interested in). 
 It does not include state for users who are currently offline, and it can only be
-called on workers that support sending federation.
+called on workers that support sending federation. Additionally, this method must
+only be called from the process that has been configured to write to the
+the [presence stream](https://github.com/matrix-org/synapse/blob/master/docs/workers.md#stream-writers).
+By default, this is the main process, but another worker can be configured to do
+so.
 
 ### Module structure
 
diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index e07fc86935..085053127b 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -751,33 +751,6 @@ acme:
     #
     account_key_file: DATADIR/acme_account.key
 
-# List of allowed TLS fingerprints for this server to publish along
-# with the signing keys for this server. Other matrix servers that
-# make HTTPS requests to this server will check that the TLS
-# certificates returned by this server match one of the fingerprints.
-#
-# Synapse automatically adds the fingerprint of its own certificate
-# to the list. So if federation traffic is handled directly by synapse
-# then no modification to the list is required.
-#
-# If synapse is run behind a load balancer that handles the TLS then it
-# will be necessary to add the fingerprints of the certificates used by
-# the loadbalancers to this list if they are different to the one
-# synapse is using.
-#
-# Homeservers are permitted to cache the list of TLS fingerprints
-# returned in the key responses up to the "valid_until_ts" returned in
-# key. It may be necessary to publish the fingerprints of a new
-# certificate and wait until the "valid_until_ts" of the previous key
-# responses have passed before deploying it.
-#
-# You can calculate a fingerprint from a given TLS listener via:
-# openssl s_client -connect $host:$port < /dev/null 2> /dev/null |
-#   openssl x509 -outform DER | openssl sha256 -binary | base64 | tr -d '='
-# or by checking matrix.org/federationtester/api/report?server_name=$host
-#
-#tls_fingerprints: [{"sha256": "<base64_encoded_sha256_fingerprint>"}]
-
 
 ## Federation ##
 
@@ -3092,7 +3065,8 @@ opentracing:
     #enabled: true
 
     # The list of homeservers we wish to send and receive span contexts and span baggage.
-    # See docs/opentracing.rst
+    # See docs/opentracing.rst.
+    #
     # This is a list of regexes which are matched against the server_name of the
     # homeserver.
     #
@@ -3101,19 +3075,26 @@ opentracing:
     #homeserver_whitelist:
     #  - ".*"
 
+    # A list of the matrix IDs of users whose requests will always be traced,
+    # even if the tracing system would otherwise drop the traces due to
+    # probabilistic sampling.
+    #
+    # By default, the list is empty.
+    #
+    #force_tracing_for_users:
+    #  - "@user1:server_name"
+    #  - "@user2:server_name"
+
     # Jaeger can be configured to sample traces at different rates.
     # All configuration options provided by Jaeger can be set here.
-    # Jaeger's configuration mostly related to trace sampling which
+    # Jaeger's configuration is mostly related to trace sampling which
     # is documented here:
-    # https://www.jaegertracing.io/docs/1.13/sampling/.
+    # https://www.jaegertracing.io/docs/latest/sampling/.
     #
     #jaeger_config:
     #  sampler:
     #    type: const
     #    param: 1
-
-    #  Logging whether spans were started and reported
-    #
     #  logging:
     #    false
 
@@ -3182,3 +3163,18 @@ redis:
   # Optional password if configured on the Redis instance
   #
   #password: <secret_password>
+
+
+# Enable experimental features in Synapse.
+#
+# Experimental features might break or be removed without a deprecation
+# period.
+#
+experimental_features:
+  # Support for Spaces (MSC1772), it enables the following:
+  #
+  # * The Spaces Summary API (MSC2946).
+  # * Restricting room membership based on space membership (MSC3083).
+  #
+  # Uncomment to disable support for Spaces.
+  #spaces_enabled: false
diff --git a/docs/sso_mapping_providers.md b/docs/sso_mapping_providers.md
index 50020d1a4a..6db2dc8be5 100644
--- a/docs/sso_mapping_providers.md
+++ b/docs/sso_mapping_providers.md
@@ -67,8 +67,8 @@ A custom mapping provider must specify the following methods:
     - Arguments:
       - `userinfo` - A `authlib.oidc.core.claims.UserInfo` object to extract user
                      information from.
-    - This method must return a string, which is the unique identifier for the
-      user. Commonly the ``sub`` claim of the response.
+    - This method must return a string, which is the unique, immutable identifier
+      for the user. Commonly the `sub` claim of the response.
 * `map_user_attributes(self, userinfo, token, failures)`
     - This method must be async.
     - Arguments:
@@ -87,7 +87,9 @@ A custom mapping provider must specify the following methods:
                      `localpart` value, such as `john.doe1`.
     - Returns a dictionary with two keys:
       - `localpart`: A string, used to generate the Matrix ID. If this is
-        `None`, the user is prompted to pick their own username.
+        `None`, the user is prompted to pick their own username. This is only used
+        during a user's first login. Once a localpart has been associated with a
+        remote user ID (see `get_remote_user_id`) it cannot be updated.
       - `displayname`: An optional string, the display name for the user.
 * `get_extra_attributes(self, userinfo, token)`
     - This method must be async.
@@ -153,8 +155,8 @@ A custom mapping provider must specify the following methods:
                           information from.
       - `client_redirect_url` - A string, the URL that the client will be
                                 redirected to.
-    - This method must return a string, which is the unique identifier for the
-      user. Commonly the ``uid`` claim of the response.
+    - This method must return a string, which is the unique, immutable identifier
+      for the user. Commonly the `uid` claim of the response.
 * `saml_response_to_user_attributes(self, saml_response, failures, client_redirect_url)`
     - Arguments:
       - `saml_response` - A `saml2.response.AuthnResponse` object to extract user
@@ -172,8 +174,10 @@ A custom mapping provider must specify the following methods:
                                 redirected to.
     - This method must return a dictionary, which will then be used by Synapse
       to build a new user. The following keys are allowed:
-       * `mxid_localpart` - The mxid localpart of the new user.  If this is
-         `None`, the user is prompted to pick their own username.
+       * `mxid_localpart` - A string, the mxid localpart of the new user. If this is
+         `None`, the user is prompted to pick their own username. This is only used
+         during a user's first login. Once a localpart has been associated with a
+         remote user ID (see `get_remote_user_id`) it cannot be updated.
        * `displayname` - The displayname of the new user. If not provided, will default to
                          the value of `mxid_localpart`.
        * `emails` - A list of emails for the new user. If not provided, will
diff --git a/docs/systemd-with-workers/README.md b/docs/systemd-with-workers/README.md
index cfa36be7b4..a1135e9ed5 100644
--- a/docs/systemd-with-workers/README.md
+++ b/docs/systemd-with-workers/README.md
@@ -65,3 +65,33 @@ systemctl restart matrix-synapse-worker@federation_reader.service
 systemctl enable matrix-synapse-worker@federation_writer.service
 systemctl restart matrix-synapse.target
 ```
+
+## Hardening
+
+**Optional:** If further hardening is desired, the file
+`override-hardened.conf` may be copied from
+`contrib/systemd/override-hardened.conf` in this repository to the location
+`/etc/systemd/system/matrix-synapse.service.d/override-hardened.conf` (the
+directory may have to be created). It enables certain sandboxing features in
+systemd to further secure the synapse service. You may read the comments to
+understand what the override file is doing. The same file will need to be copied
+to
+`/etc/systemd/system/matrix-synapse-worker@.service.d/override-hardened-worker.conf`
+(this directory may also have to be created) in order to apply the same
+hardening options to any worker processes.
+
+Once these files have been copied to their appropriate locations, simply reload
+systemd's manager config files and restart all Synapse services to apply the hardening options. They will automatically
+be applied at every restart as long as the override files are present at the
+specified locations.
+
+```sh
+systemctl daemon-reload
+
+# Restart services
+systemctl restart matrix-synapse.target
+```
+
+In order to see their effect, you may run `systemd-analyze security
+matrix-synapse.service` before and after applying the hardening options to see
+the changes being applied at a glance.
diff --git a/docs/user_directory.md b/docs/user_directory.md
index 872fc21979..d4f38d2cf1 100644
--- a/docs/user_directory.md
+++ b/docs/user_directory.md
@@ -7,6 +7,6 @@ who are present in a publicly viewable room present on the server.
 
 The directory info is stored in various tables, which can (typically after
 DB corruption) get stale or out of sync.  If this happens, for now the
-solution to fix it is to execute the SQL [here](../synapse/storage/databases/main/schema/delta/53/user_dir_populate.sql)
+solution to fix it is to execute the SQL [here](https://github.com/matrix-org/synapse/blob/master/synapse/storage/schema/main/delta/53/user_dir_populate.sql)
 and then restart synapse. This should then start a background task to
 flush the current tables and regenerate the directory.