Log unknown field tags for sync messages we havent handled

ts/textsecure/MessageReceiver.ts

@@ -123,6 +123,7 @@ import { getStoriesBlocked } from '../util/stories';
 import { isNotNil } from '../util/isNotNil';
 import { chunk } from '../util/iterables';
 import { isOlderThan } from '../util/timestamp';
+import { inspectUnknownFieldTags } from '../util/inspectProtobufs';
 
 const GROUPV1_ID_LENGTH = 16;
 const GROUPV2_ID_LENGTH = 32;
@@ -3068,8 +3069,10 @@ export default class MessageReceiver
     }
 
     this.removeFromCache(envelope);
+    const envelopeId = getEnvelopeId(envelope);
+    const unknownFieldTags = inspectUnknownFieldTags(syncMessage).join(',');
     log.warn(
-      `handleSyncMessage/${getEnvelopeId(envelope)}: Got empty SyncMessage`
+      `handleSyncMessage/${envelopeId}: Got unknown SyncMessage (Unknown field tags: ${unknownFieldTags})`
     );
   }
 

ts/util/inspectProtobufs.ts

@@ -0,0 +1,46 @@
+// Copyright 2023 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { Reader } from 'protobufjs';
+
+type MessageWithUnknownFields = {
+  __unknownFields?: ReadonlyArray<Uint8Array>;
+};
+
+/**
+ * Returns an array of the tags of unknown fields in a protobuf message.
+ *
+ * Clients may use slightly different definitions of our protos, in cases where
+ * we don't recognize a field, we store it in `__unknownFields`.
+ *
+ * For example:
+ *
+ * ```proto
+ * // Our proto definition
+ * message Foo {
+ *   optional string bar = 1;
+ * }
+ *
+ * // Their proto definition
+ * message Foo {
+ *   optional string bar = 1;
+ *   optional string baz = 2;
+ * }
+ * ```
+ *
+ * If we receive a message with `baz` set, we'll store it in `__unknownFields`.
+ *
+ * This function will then return `[2]`.
+ */
+export function inspectUnknownFieldTags(
+  message: MessageWithUnknownFields
+): Array<number> {
+  return (
+    message.__unknownFields?.map(field => {
+      // https://protobuf.dev/programming-guides/encoding/
+      // The first byte of a field is a varint encoding the tag bit-shifted << 3
+      // eslint-disable-next-line no-bitwise
+      return new Reader(field).uint32() >>> 3;
+    }) ?? []
+  );
+}