message [nfc]: In moves, give newStreamId/newTopic their fallback values sooner

To see that this is NFC, first observe that the only possible
behavior change this could cause is to change some event from being
ignored (hitting a `return` in this section of code) to being
processed by the code below, or vice versa.

(It might also change what particular message gets printed to the
debug log; but that doesn't count as a behavior change.)

Then both before and after this change, the events that this overall
set of conditions accepts are those that (a) have all of origTopic,
origStreamId, and propagateMode non-null, and (b) have at least one
of newTopic or newStreamId with a non-null value different from
origTopic or origStreamId respectively.

This change is useful because it brings this logic closer to how it
will look when we move it into the API parsing code.

While here, we add a debug log message on unexpected propagateMode,
to further bring this closer to how it will look in the API parsing
code.

Co-authored-by: Zixuan James Li <[email protected]>

Author: gnprice

lib/model/message.dart

@@ -173,43 +173,39 @@ class MessageStoreImpl with MessageStore {
     // For reference, see: https://zulip.com/api/get-events#update_message
 
     final origStreamId = event.origStreamId;
+    final newStreamId = event.newStreamId ?? origStreamId;
     final origTopic = event.origTopic;
+    final newTopic = event.newTopic ?? origTopic;
     final propagateMode = event.propagateMode;
 
-    if (origTopic == null) {
+    if (newStreamId == origStreamId && newTopic == origTopic) {
       // There was no move.
-      assert(() {
-        if (event.newStreamId != null && origStreamId != null
-            && event.newStreamId != origStreamId) {
-          // This should be impossible; `orig_subject` (aka origTopic) is
-          // documented to be present when either the stream or topic changed.
-          debugLog('Malformed UpdateMessageEvent: stream move but no origTopic'); // TODO(log)
-        }
-        return true;
-      }());
+      if (propagateMode != null) {
+        assert(debugLog(
+          'Malformed UpdateMessageEvent: incoherent message-move fields; '
+          'propagate_mode present but no new channel or topic')); // TODO(log)
+      }
       return;
     }
 
-    if (origStreamId == null) {
+    if (origStreamId == null || newStreamId == null) {
       // The `stream_id` field (aka origStreamId) is documented to be present on moves.
+      // newStreamId should not be null either because it falls back to origStreamId.
       assert(debugLog('Malformed UpdateMessageEvent: move but no origStreamId')); // TODO(log)
       return;
     }
+    if (origTopic == null || newTopic == null) {
+      // The `orig_subject` field (aka origTopic) is documented to be present on moves.
+      // newTopic should not be null either because it falls back to origTopic.
+      assert(debugLog('Malformed UpdateMessageEvent: move but no origTopic')); // TODO(log)
+      return;
+    }
     if (propagateMode == null) {
       // The `propagate_mode` field (aka propagateMode) is documented to be present on moves.
       assert(debugLog('Malformed UpdateMessageEvent: move but no propagateMode')); // TODO(log)
       return;
     }
 
-    final newStreamId = event.newStreamId ?? origStreamId;
-    final newTopic = event.newTopic ?? origTopic;
-    if (newStreamId == origStreamId && newTopic == origTopic) {
-      // If neither the channel nor topic name changed, nothing moved.
-      // In that case `orig_subject` (aka origTopic) should have been null.
-      assert(debugLog('Malformed UpdateMessageEvent: move but no newStreamId or newTopic')); // TODO(log)
-      return;
-    }
-
     final wasResolveOrUnresolve = newStreamId == origStreamId
       && MessageEditState.topicMoveWasResolveOrUnresolve(origTopic, newTopic);