log: On malformed event, include stack trace in log and alert dialog

(The comment about log entries being truncated at 1 kiB is copied
from a similar debugLog call.)

Issue zulip#1083 isn't completely fixed here; we'd still like to show
stack traces for malformed `/register` responses.

Fixes-partly: zulip#1083

Author: chrisbobbe

lib/model/store.dart

@@ -1339,9 +1339,9 @@ class UpdateMachine {
           lastEventId = events.last.id;
         }
       }
-    } catch (e) {
+    } catch (e, stackTrace) {
       if (_disposed) return;
-      await _handlePollError(e);
+      await _handlePollError(e, stackTrace);
       assert(_disposed);
       return;
     }
@@ -1458,7 +1458,7 @@ class UpdateMachine {
   /// See also:
   ///  * [_handlePollRequestError], which handles certain errors
   ///    and causes them not to reach this method.
-  Future<void> _handlePollError(Object error) async {
+  Future<void> _handlePollError(Object error, StackTrace stackTrace) async {
     // An error occurred, other than the transient request errors we retry on.
     // This means either a lost/expired event queue on the server (which is
     // normal after the app is offline for a period like 10 minutes),
@@ -1493,9 +1493,12 @@ class UpdateMachine {
         isUnexpected = true;
 
       default:
-        assert(debugLog('BUG: Unexpected error in event polling: $error\n' // TODO(log)
-          'Replacing event queue…'));
-        _reportToUserErrorConnectingToServer(error);
+        assert(debugLog('BUG: Unexpected error in event polling: $error')); // TODO(log)
+        // Print stack trace in its own log entry; log entries are truncated
+        // at 1 kiB (at least on Android), and stack can be longer than that.
+        assert(debugLog('Stack trace:\n$stackTrace'));
+        assert(debugLog('Replacing event queue…'));
+        _reportToUserErrorConnectingToServer(error, stackTrace);
         // Similar story to the _EventHandlingException case;
         // separate only so that that other case can print more context.
         // The bug here could be in the server if it's an ApiRequestException,
@@ -1530,12 +1533,18 @@ class UpdateMachine {
     }
   }
 
-  void _reportToUserErrorConnectingToServer(Object error) {
+  void _reportToUserErrorConnectingToServer(Object error, [StackTrace? stackTrace]) {
     final zulipLocalizations = GlobalLocalizations.zulipLocalizations;
+
+    final details = StringBuffer()..write(error.toString());
+    if (stackTrace != null) {
+      details.write('\nStack:\n$stackTrace');
+    }
+
     reportErrorToUserBriefly(
       zulipLocalizations.errorConnectingToServerShort,
       details: zulipLocalizations.errorConnectingToServerDetails(
-        store.realmUrl.toString(), error.toString()));
+        store.realmUrl.toString(), details.toString()));
   }
 
   /// Cleans up resources and tells the instance not to make new API requests.