Add an admin API to run background jobs. (#11352)

Instead of having admins poke into the database directly.

Can currently run jobs to populate stats and to populate
the user directory.

docs/sample_config.yaml

@@ -2360,8 +2360,8 @@ user_directory:
     # indexes were (re)built was before Synapse 1.44, you'll have to
     # rebuild the indexes in order to search through all known users.
     # These indexes are built the first time Synapse starts; admins can
-    # manually trigger a rebuild following the instructions at
-    #     https://matrix-org.github.io/synapse/latest/user_directory.html
+    # manually trigger a rebuild via API following the instructions at
+    #     https://matrix-org.github.io/synapse/latest/usage/administration/admin_api/background_updates.html#run
     #
     # Uncomment to return search results containing all known users, even if that
     # user does not share a room with the requester.

docs/usage/administration/admin_api/background_updates.md

@@ -42,7 +42,6 @@ For each update:
 `average_items_per_ms` how many items are processed per millisecond based on an exponential average.
 
 
-
 ## Enabled
 
 This API allow pausing background updates.
@@ -82,3 +81,29 @@ The API returns the `enabled` param.
 ```
 
 There is also a `GET` version which returns the `enabled` state.
+
+
+## Run
+
+This API schedules a specific background update to run. The job starts immediately after calling the API.
+
+
+The API is:
+
+```
+POST /_synapse/admin/v1/background_updates/start_job
+```
+
+with the following body:
+
+```json
+{
+    "job_name": "populate_stats_process_rooms"
+}
+```
+
+The following JSON body parameters are available:
+
+- `job_name` - A string which job to run. Valid values are:
+  - `populate_stats_process_rooms` - Recalculate the stats for all rooms.
+  - `regenerate_directory` - Recalculate the [user directory](../../../user_directory.md) if it is stale or out of sync.

docs/user_directory.md

@@ -6,9 +6,9 @@ on this particular server - i.e. ones which your account shares a room with, or
 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](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
+DB corruption) get stale or out of sync. If this happens, for now the
+solution to fix it is to use the [admin API](usage/administration/admin_api/background_updates.md#run)
+and execute the job `regenerate_directory`. This should then start a background task to
 flush the current tables and regenerate the directory.
 
 Data model