Fixed presence results not returning offline users on initial sync (#17231)

This is to address an issue in which `m.presence` results on initial
sync are not returning entries of users who are currently offline.

The original behaviour was from
https://github.com/element-hq/synapse/issues/1535

This change is useful for applications that use the
presence system for tracking user profile information/updates (e.g.
https://github.com/element-hq/synapse/pull/16992 or for profile status
messages).

This is gated behind a new configuration option to avoid performance
impact for applications that don't need this, as a pragmatic solution
for now.

changelog.d/17231.bugfix

@@ -0,0 +1 @@
+Added configurable option to always include offline users in presence sync results. Contributed by @Michael-Hollister.

docs/usage/configuration/config_documentation.md

@@ -246,6 +246,7 @@ Example configuration:
 ```yaml
 presence:
   enabled: false
+  include_offline_users_on_sync: false
 ```
 
 `enabled` can also be set to a special value of "untracked" which ignores updates
@@ -254,6 +255,10 @@ received via clients and federation, while still accepting updates from the
 
 *The "untracked" option was added in Synapse 1.96.0.*
 
+When clients perform an initial or `full_state` sync, presence results for offline users are
+not included by default. Setting `include_offline_users_on_sync` to `true` will always include
+offline users in the results. Defaults to false.
+
 ---
 ### `require_auth_for_profile_requests`
 

synapse/config/server.py

@@ -384,6 +384,11 @@ class ServerConfig(Config):
         # Whether to internally track presence, requires that presence is enabled,
         self.track_presence = self.presence_enabled and presence_enabled != "untracked"
 
+        # Determines if presence results for offline users are included on initial/full sync
+        self.presence_include_offline_users_on_sync = presence_config.get(
+            "include_offline_users_on_sync", False
+        )
+
         # Custom presence router module
         # This is the legacy way of configuring it (the config should now be put in the modules section)
         self.presence_router_module_class = None

synapse/handlers/sync.py

@@ -2270,7 +2270,11 @@ class SyncHandler:
             user=user,
             from_key=presence_key,
             is_guest=sync_config.is_guest,
-            include_offline=include_offline,
+            include_offline=(
+                True
+                if self.hs_config.server.presence_include_offline_users_on_sync
+                else include_offline
+            ),
         )
         assert presence_key
         sync_result_builder.now_token = now_token.copy_and_replace(