Don't send device list updates upon registration (#3307)

Fixes https://github.com/matrix-org/dendrite/issues/3273

As we otherwise send down device list updates which are merely useful
for the user and causes tests to be flakey:

```
❌ TestPushSync/Adding_a_push_rule_wakes_up_an_incremental_/sync (10ms)
      push_test.go:57: no pushrules found in sync response: {"next_batch":"s0_0_0_0_0_1_1_0_1","device_lists":{"changed":["@user-1:hs1"]}}
```

What this does: If a `PerformDeviceCreation` request is coming from
registering an account, it does **not** send device list updates, as
they are merely useful (no joined rooms, no one to inform) . In all
other cases, the behavior is unchanged and device list updates are sent
as usual.

1 files changed, 9 insertions, 3 deletions

diff --git a/userapi/internal/key_api.go b/userapi/internal/key_api.go
index 786a2dcd..422898c7 100644
--- a/userapi/internal/key_api.go
+++ b/userapi/internal/key_api.go
@@ -711,9 +711,15 @@ func (a *UserInternalAPI) uploadLocalDeviceKeys(ctx context.Context, req *api.Pe
 		}
 		return
 	}
-	err = emitDeviceKeyChanges(a.KeyChangeProducer, existingKeys, keysToStore, req.OnlyDisplayNameUpdates)
-	if err != nil {
-		util.GetLogger(ctx).Errorf("Failed to emitDeviceKeyChanges: %s", err)
+
+	// If the request does _not_ come right after registering an account
+	// inform downstream components. However, we're fine with just creating the
+	// database entries above in other cases.
+	if !req.FromRegistration {
+		err = emitDeviceKeyChanges(a.KeyChangeProducer, existingKeys, keysToStore, req.OnlyDisplayNameUpdates)
+		if err != nil {
+			util.GetLogger(ctx).Errorf("Failed to emitDeviceKeyChanges: %s", err)
+		}
 	}
 }