Add `DeviceKeysEqual` (#2219)

* Add `DeviceKeysEqual`

* Update check order

* Fix check

* Tweak conditions again

* One more time

* Single return value

keyserver/api/api.go

@@ -15,6 +15,7 @@
 package api
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"strings"
@@ -73,6 +74,26 @@ type DeviceMessage struct {
 	DeviceChangeID int64
 }
 
+// DeviceKeysEqual returns true if the device keys updates contain the
+// same display name and key JSON. This will return false if either of
+// the updates is not a device keys update, or if the user ID/device ID
+// differ between the two.
+func (m1 *DeviceMessage) DeviceKeysEqual(m2 *DeviceMessage) bool {
+	if m1.DeviceKeys == nil || m2.DeviceKeys == nil {
+		return false
+	}
+	if m1.UserID != m2.UserID || m1.DeviceID != m2.DeviceID {
+		return false
+	}
+	if m1.DisplayName != m2.DisplayName {
+		return false // different display names
+	}
+	if len(m1.KeyJSON) == 0 || len(m2.KeyJSON) == 0 {
+		return false // either is empty
+	}
+	return bytes.Equal(m1.KeyJSON, m2.KeyJSON)
+}
+
 // DeviceKeys represents a set of device keys for a single device
 // https://matrix.org/docs/spec/client_server/r0.6.1#post-matrix-client-r0-keys-upload
 type DeviceKeys struct {

keyserver/internal/internal.go

@@ -718,7 +718,7 @@ func emitDeviceKeyChanges(producer KeyChangeProducer, existing, new []api.Device
 		for _, existingKey := range existing {
 			// Do not treat the absence of keys as equal, or else we will not emit key changes
 			// when users delete devices which never had a key to begin with as both KeyJSONs are nil.
-			if bytes.Equal(existingKey.KeyJSON, newKey.KeyJSON) && len(existingKey.KeyJSON) > 0 {
+			if existingKey.DeviceKeysEqual(&newKey) {
 				exists = true
 				break
 			}