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.

.github/codecov.yaml

@@ -7,7 +7,7 @@ coverage:
     project:
       default:
         target: auto
-        threshold: 0%
+        threshold: 0.1%
         base: auto
         flags:
           - unittests

clientapi/routing/register.go

@@ -630,6 +630,7 @@ func handleGuestRegistration(
 		AccessToken:       token,
 		IPAddr:            req.RemoteAddr,
 		UserAgent:         req.UserAgent(),
+		FromRegistration:  true,
 	}, &devRes)
 	if err != nil {
 		return util.JSONResponse{
@@ -919,6 +920,7 @@ func completeRegistration(
 		DeviceID:          deviceID,
 		IPAddr:            ipAddr,
 		UserAgent:         userAgent,
+		FromRegistration:  true,
 	}, &devRes)
 	if err != nil {
 		return util.JSONResponse{

userapi/api/api.go

@@ -379,6 +379,10 @@ type PerformDeviceCreationRequest struct {
 	// update for this account. Generally the only reason to do this is if the account
 	// is an appservice account.
 	NoDeviceListUpdate bool
+
+	// FromRegistration determines if this request comes from registering a new account
+	// and is in most cases false.
+	FromRegistration bool
 }
 
 // PerformDeviceCreationResponse is the response for PerformDeviceCreation
@@ -803,6 +807,10 @@ type PerformUploadKeysRequest struct {
 	// itself doesn't change but it's easier to pretend upload new keys and reuse the same code paths.
 	// Without this flag, requests to modify device display names would delete device keys.
 	OnlyDisplayNameUpdates bool
+
+	// FromRegistration is set if this key upload comes right after creating an account
+	// and determines if we need to inform downstream components.
+	FromRegistration bool
 }
 
 // PerformUploadKeysResponse is the response to PerformUploadKeys

userapi/internal/key_api.go

@@ -711,10 +711,16 @@ func (a *UserInternalAPI) uploadLocalDeviceKeys(ctx context.Context, req *api.Pe
 		}
 		return
 	}
+
+	// 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)
 		}
+	}
 }
 
 func (a *UserInternalAPI) uploadOneTimeKeys(ctx context.Context, req *api.PerformUploadKeysRequest, res *api.PerformUploadKeysResponse) {

userapi/internal/user_api.go

@@ -316,7 +316,7 @@ func (a *UserInternalAPI) PerformDeviceCreation(ctx context.Context, req *api.Pe
 		return nil
 	}
 	// create empty device keys and upload them to trigger device list changes
-	return a.deviceListUpdate(dev.UserID, []string{dev.ID})
+	return a.deviceListUpdate(dev.UserID, []string{dev.ID}, req.FromRegistration)
 }
 
 func (a *UserInternalAPI) PerformDeviceDeletion(ctx context.Context, req *api.PerformDeviceDeletionRequest, res *api.PerformDeviceDeletionResponse) error {
@@ -356,10 +356,10 @@ func (a *UserInternalAPI) PerformDeviceDeletion(ctx context.Context, req *api.Pe
 		return fmt.Errorf("a.KeyAPI.PerformDeleteKeys: %w", err)
 	}
 	// create empty device keys and upload them to delete what was once there and trigger device list changes
-	return a.deviceListUpdate(req.UserID, deletedDeviceIDs)
+	return a.deviceListUpdate(req.UserID, deletedDeviceIDs, false)
 }
 
-func (a *UserInternalAPI) deviceListUpdate(userID string, deviceIDs []string) error {
+func (a *UserInternalAPI) deviceListUpdate(userID string, deviceIDs []string, fromRegistration bool) error {
 	deviceKeys := make([]api.DeviceKeys, len(deviceIDs))
 	for i, did := range deviceIDs {
 		deviceKeys[i] = api.DeviceKeys{
@@ -373,6 +373,7 @@ func (a *UserInternalAPI) deviceListUpdate(userID string, deviceIDs []string) er
 	if err := a.PerformUploadKeys(context.Background(), &api.PerformUploadKeysRequest{
 		UserID:           userID,
 		DeviceKeys:       deviceKeys,
+		FromRegistration: fromRegistration,
 	}, &uploadRes); err != nil {
 		return err
 	}