Remove `worker_replication_*` settings (#15491)

* Add master to the instance_map as part of Complement, have ReplicationEndpoint look at instance_map for master.

* Fix typo in drive by.

* Remove unnecessary worker_replication_* bits from unit tests and add master to instance_map(hopefully in the right place)

* Several updates:

1. Switch from master to main for naming the main process in the instance_map. Add useful constants for easier adjustment of names in the future.
2. Add backwards compatibility for worker_replication_* to allow time to transition to new style. Make sure to prioritize declaring main directly on the instance_map.
3. Clean up old comments/commented out code.
4. Adjust unit tests to match with new code.
5. Adjust Complement setup infrastructure to only add main to the instance_map if workers are used and remove now unused options from the worker.yaml template.

* Initial Docs upload

* Changelog

* Missed some commented out code that can go now

* Remove TODO comment that no longer holds true.

* Fix links in docs

* More docs

* Remove debug logging

* Apply suggestions from code review

Co-authored-by: reivilibre <olivier@librepush.net>

* Apply suggestions from code review

Co-authored-by: reivilibre <olivier@librepush.net>

* Update version to latest, include completeish before/after examples in upgrade notes.

* Fix up and docs too

---------

Co-authored-by: reivilibre <olivier@librepush.net>

4 files changed, 121 insertions, 22 deletions

diff --git a/docs/systemd-with-workers/workers/generic_worker.yaml b/docs/systemd-with-workers/workers/generic_worker.yaml
index a858f99ed1..db6436ee6e 100644
--- a/docs/systemd-with-workers/workers/generic_worker.yaml
+++ b/docs/systemd-with-workers/workers/generic_worker.yaml
@@ -1,10 +1,6 @@
 worker_app: synapse.app.generic_worker
 worker_name: generic_worker1
 
-# The replication listener on the main synapse process.
-worker_replication_host: 127.0.0.1
-worker_replication_http_port: 9093
-
 worker_listeners:
   - type: http
     port: 8083
diff --git a/docs/upgrade.md b/docs/upgrade.md
index 0886b03115..0625de8afb 100644
--- a/docs/upgrade.md
+++ b/docs/upgrade.md
@@ -88,6 +88,84 @@ process, for example:
     dpkg -i matrix-synapse-py3_1.3.0+stretch1_amd64.deb
     ```
 
+# Upgrading to v1.84.0
+
+## Deprecation of `worker_replication_*` configuration settings
+
+When using workers, 
+* `worker_replication_host`
+* `worker_replication_http_port`
+* `worker_replication_http_tls`
+ 
+can now be removed from individual worker YAML configuration ***if*** you add the main process to the `instance_map` in the shared YAML configuration,
+using the name `main`.
+
+### Before:
+Shared YAML
+```yaml
+instance_map:
+  generic_worker1:
+    host: localhost
+    port: 5678
+    tls: false
+```
+Worker YAML
+```yaml
+worker_app: synapse.app.generic_worker
+worker_name: generic_worker1
+
+worker_replication_host: localhost
+worker_replication_http_port: 3456
+worker_replication_http_tls: false
+
+worker_listeners:
+  - type: http
+    port: 1234
+    resources:
+      - names: [client, federation]
+  - type: http
+    port: 5678
+    resources:
+      - names: [replication]
+
+worker_log_config: /etc/matrix-synapse/generic-worker-log.yaml
+```
+### After:
+Shared YAML
+```yaml
+instance_map:
+  main:
+    host: localhost
+    port: 3456
+    tls: false
+  generic_worker1:
+    host: localhost
+    port: 5678
+    tls: false
+```
+Worker YAML
+```yaml
+worker_app: synapse.app.generic_worker
+worker_name: generic_worker1
+
+worker_listeners:
+  - type: http
+    port: 1234
+    resources:
+      - names: [client, federation]
+  - type: http
+    port: 5678
+    resources:
+      - names: [replication]
+
+worker_log_config: /etc/matrix-synapse/generic-worker-log.yaml
+
+```
+Notes: 
+* `tls` is optional but mirrors the functionality of `worker_replication_http_tls`
+
+
+
 # Upgrading to v1.81.0
 
 ## Application service path & authentication deprecations
diff --git a/docs/usage/configuration/config_documentation.md b/docs/usage/configuration/config_documentation.md
index 6dd1a639ed..dc965b4119 100644
--- a/docs/usage/configuration/config_documentation.md
+++ b/docs/usage/configuration/config_documentation.md
@@ -3884,15 +3884,20 @@ federation_sender_instances:
 ### `instance_map`
 
 When using workers this should be a map from [`worker_name`](#worker_name) to the
-HTTP replication listener of the worker, if configured.
+HTTP replication listener of the worker, if configured, and to the main process.
 Each worker declared under [`stream_writers`](../../workers.md#stream-writers) needs
 a HTTP replication listener, and that listener should be included in the `instance_map`.
-(The main process also needs an HTTP replication listener, but it should not be
-listed in the `instance_map`.)
+The main process also needs an entry on the `instance_map`, and it should be listed under
+`main` **if even one other worker exists**. Ensure the port matches with what is declared 
+inside the `listener` block for a `replication` listener.
+
 
 Example configuration:
 ```yaml
 instance_map:
+  main:
+    host: localhost
+    port: 8030
   worker1:
     host: localhost
     port: 8034
@@ -4024,6 +4029,7 @@ worker_name: generic_worker1
 ```
 ---
 ### `worker_replication_host`
+*Deprecated as of version 1.84.0. Place `host` under `main` entry on the [`instance_map`](#instance_map) in your shared yaml configuration instead.*
 
 The HTTP replication endpoint that it should talk to on the main Synapse process.
 The main Synapse process defines this with a `replication` resource in
@@ -4035,6 +4041,7 @@ worker_replication_host: 127.0.0.1
 ```
 ---
 ### `worker_replication_http_port`
+*Deprecated as of version 1.84.0. Place `port` under `main` entry on the [`instance_map`](#instance_map) in your shared yaml configuration instead.*
 
 The HTTP replication port that it should talk to on the main Synapse process.
 The main Synapse process defines this with a `replication` resource in
@@ -4046,6 +4053,7 @@ worker_replication_http_port: 9093
 ```
 ---
 ### `worker_replication_http_tls`
+*Deprecated as of version 1.84.0. Place `tls` under `main` entry on the [`instance_map`](#instance_map) in your shared yaml configuration instead.*
 
 Whether TLS should be used for talking to the HTTP replication port on the main
 Synapse process.
@@ -4071,9 +4079,9 @@ A worker can handle HTTP requests. To do so, a `worker_listeners` option
 must be declared, in the same way as the [`listeners` option](#listeners)
 in the shared config.
 
-Workers declared in [`stream_writers`](#stream_writers) will need to include a
-`replication` listener here, in order to accept internal HTTP requests from
-other workers.
+Workers declared in [`stream_writers`](#stream_writers) and [`instance_map`](#instance_map)
+ will need to include a `replication` listener here, in order to accept internal HTTP 
+requests from other workers.
 
 Example configuration:
 ```yaml
diff --git a/docs/workers.md b/docs/workers.md
index 765f03c263..991814c0bc 100644
--- a/docs/workers.md
+++ b/docs/workers.md
@@ -87,12 +87,18 @@ shared configuration file.
 
 ### Shared configuration
 
-Normally, only a couple of changes are needed to make an existing configuration
-file suitable for use with workers. First, you need to enable an
+Normally, only a few changes are needed to make an existing configuration
+file suitable for use with workers:
+* First, you need to enable an
 ["HTTP replication listener"](usage/configuration/config_documentation.md#listeners)
-for the main process; and secondly, you need to enable
-[redis-based replication](usage/configuration/config_documentation.md#redis).
-Optionally, a [shared secret](usage/configuration/config_documentation.md#worker_replication_secret)
+for the main process
+* Secondly, you need to enable
+[redis-based replication](usage/configuration/config_documentation.md#redis)
+* You will need to add an [`instance_map`](usage/configuration/config_documentation.md#instance_map) 
+with the `main` process defined, as well as the relevant connection information from 
+it's HTTP `replication` listener (defined in step 1 above). Note that the `host` defined 
+is the address the worker needs to look for the `main` process at, not necessarily the same address that is bound to.
+* Optionally, a [shared secret](usage/configuration/config_documentation.md#worker_replication_secret)
 can be used to authenticate HTTP traffic between workers. For example:
 
 ```yaml
@@ -111,6 +117,11 @@ worker_replication_secret: ""
 
 redis:
     enabled: true
+
+instance_map:
+    main:
+        host: 'localhost'
+        port: 9093
 ```
 
 See the [configuration manual](usage/configuration/config_documentation.md)
@@ -130,13 +141,13 @@ In the config file for each worker, you must specify:
  * The type of worker ([`worker_app`](usage/configuration/config_documentation.md#worker_app)).
    The currently available worker applications are listed [below](#available-worker-applications).
  * A unique name for the worker ([`worker_name`](usage/configuration/config_documentation.md#worker_name)).
- * The HTTP replication endpoint that it should talk to on the main synapse process
-   ([`worker_replication_host`](usage/configuration/config_documentation.md#worker_replication_host) and
-   [`worker_replication_http_port`](usage/configuration/config_documentation.md#worker_replication_http_port)).
  * If handling HTTP requests, a [`worker_listeners`](usage/configuration/config_documentation.md#worker_listeners) option
    with an `http` listener.
  * **Synapse 1.72 and older:** if handling the `^/_matrix/client/v3/keys/upload` endpoint, the HTTP URI for
    the main process (`worker_main_http_uri`). This config option is no longer required and is ignored when running Synapse 1.73 and newer.
+ * **Synapse 1.83 and older:** The HTTP replication endpoint that the worker should talk to on the main synapse process
+   ([`worker_replication_host`](usage/configuration/config_documentation.md#worker_replication_host) and
+   [`worker_replication_http_port`](usage/configuration/config_documentation.md#worker_replication_http_port)). If using Synapse 1.84 and newer, these are not needed if `main` is defined on the [shared configuration](#shared-configuration) `instance_map`
 
 For example:
 
@@ -417,11 +428,14 @@ effects of bursts of events from that bridge on events sent by normal users.
 Additionally, the writing of specific streams (such as events) can be moved off
 of the main process to a particular worker.
 
-To enable this, the worker must have a
-[HTTP `replication` listener](usage/configuration/config_documentation.md#listeners) configured,
-have a [`worker_name`](usage/configuration/config_documentation.md#worker_name)
+To enable this, the worker must have:
+* An [HTTP `replication` listener](usage/configuration/config_documentation.md#listeners) configured,
+* Have a [`worker_name`](usage/configuration/config_documentation.md#worker_name)
 and be listed in the [`instance_map`](usage/configuration/config_documentation.md#instance_map)
-config. The same worker can handle multiple streams, but unless otherwise documented,
+config. 
+* Have the main process declared on the [`instance_map`](usage/configuration/config_documentation.md#instance_map) as well.
+
+Note: The same worker can handle multiple streams, but unless otherwise documented,
 each stream can only have a single writer.
 
 For example, to move event persistence off to a dedicated worker, the shared
@@ -429,6 +443,9 @@ configuration would include:
 
 ```yaml
 instance_map:
+    main:
+        host: localhost
+        port: 8030
     event_persister1:
         host: localhost
         port: 8034