store: Better error message on handleEvent failure

Author: gnprice

assets/l10n/app_en.arb

@@ -188,6 +188,19 @@
       "error": {"type": "String", "example": "Invalid format"}
     }
   },
+  "errorHandlingEventTitle": "Error handling a Zulip event. Retrying connection…",
+  "@errorHandlingEventTitle": {
+    "description": "Error title on failing to handle a Zulip server event."
+  },
+  "errorHandlingEventDetails": "Error handling a Zulip event from {serverUrl}; will retry.\n\nError: {error}\n\nEvent: {event}",
+  "@errorHandlingEventDetails": {
+    "description": "Error details on failing to handle a Zulip server event.",
+    "placeholders": {
+      "serverUrl": {"type":  "String", "example":  "https://chat.example.com"},
+      "error": {"type": "String", "example": "Unexpected null value"},
+      "event": {"type": "String", "example": "UpdateMessageEvent(id: 123, messageIds: [2345, 3456], newTopic: 'dinner')"}
+    }
+  },
   "errorSharingFailed": "Sharing failed",
   "@errorSharingFailed": {
     "description": "Error message when sharing a message failed."

lib/generated/l10n/zulip_localizations.dart

@@ -343,6 +343,18 @@ abstract class ZulipLocalizations {
   /// **'Error connecting to Zulip at {serverUrl}. Will retry:\n\n{error}'**
   String errorConnectingToServerDetails(String serverUrl, String error);
 
+  /// Error title on failing to handle a Zulip server event.
+  ///
+  /// In en, this message translates to:
+  /// **'Error handling a Zulip event. Retrying connection…'**
+  String get errorHandlingEventTitle;
+
+  /// Error details on failing to handle a Zulip server event.
+  ///
+  /// In en, this message translates to:
+  /// **'Error handling a Zulip event from {serverUrl}; will retry.\n\nError: {error}\n\nEvent: {event}'**
+  String errorHandlingEventDetails(String serverUrl, String error, String event);
+
   /// Error message when sharing a message failed.
   ///
   /// In en, this message translates to:

lib/generated/l10n/zulip_localizations_ar.dart

@@ -157,6 +157,14 @@ class ZulipLocalizationsAr extends ZulipLocalizations {
     return 'Error connecting to Zulip at $serverUrl. Will retry:\n\n$error';
   }
 
+  @override
+  String get errorHandlingEventTitle => 'Error handling a Zulip event. Retrying connection…';
+
+  @override
+  String errorHandlingEventDetails(String serverUrl, String error, String event) {
+    return 'Error handling a Zulip event from $serverUrl; will retry.\n\nError: $error\n\nEvent: $event';
+  }
+
   @override
   String get errorSharingFailed => 'Sharing failed';
 

lib/generated/l10n/zulip_localizations_en.dart

@@ -157,6 +157,14 @@ class ZulipLocalizationsEn extends ZulipLocalizations {
     return 'Error connecting to Zulip at $serverUrl. Will retry:\n\n$error';
   }
 
+  @override
+  String get errorHandlingEventTitle => 'Error handling a Zulip event. Retrying connection…';
+
+  @override
+  String errorHandlingEventDetails(String serverUrl, String error, String event) {
+    return 'Error handling a Zulip event from $serverUrl; will retry.\n\nError: $error\n\nEvent: $event';
+  }
+
   @override
   String get errorSharingFailed => 'Sharing failed';
 

lib/generated/l10n/zulip_localizations_ja.dart

@@ -157,6 +157,14 @@ class ZulipLocalizationsJa extends ZulipLocalizations {
     return 'Error connecting to Zulip at $serverUrl. Will retry:\n\n$error';
   }
 
+  @override
+  String get errorHandlingEventTitle => 'Error handling a Zulip event. Retrying connection…';
+
+  @override
+  String errorHandlingEventDetails(String serverUrl, String error, String event) {
+    return 'Error handling a Zulip event from $serverUrl; will retry.\n\nError: $error\n\nEvent: $event';
+  }
+
   @override
   String get errorSharingFailed => 'Sharing failed';
 

lib/model/store.dart

@@ -1117,8 +1117,14 @@ class UpdateMachine {
 
         final events = result.events;
         for (final event in events) {
-          await store.handleEvent(event);
-          if (_disposed) return;
+          try {
+            await store.handleEvent(event);
+            if (_disposed) return;
+          } catch (e, stackTrace) {
+            if (_disposed) return;
+            Error.throwWithStackTrace(
+              _EventHandlingException(cause: e, event: event), stackTrace);
+          }
         }
         if (events.isNotEmpty) {
           lastEventId = events.last.id;
@@ -1142,10 +1148,14 @@ class UpdateMachine {
           // The old event queue is gone, so we need a new one.  This is normal.
           isUnexpected = false;
 
-        default:
-          assert(debugLog('BUG: Unexpected error in event polling: $e\n' // TODO(log)
+        case _EventHandlingException(:final cause, :final event):
+          assert(debugLog('BUG: Error handling an event: $cause\n' // TODO(log)
+            '  event: $event\n'
             'Replacing event queue…'));
-          _reportToUserErrorConnectingToServer(e);
+          reportErrorToUserBriefly(
+            GlobalLocalizations.zulipLocalizations.errorHandlingEventTitle,
+            details: GlobalLocalizations.zulipLocalizations.errorHandlingEventDetails(
+              store.realmUrl.toString(), cause.toString(), event.toString()));
           // We can't just continue with the next event, because our state
           // may be garbled due to failing to apply this one (and worse,
           // any invariants that were left in a broken state from where
@@ -1154,6 +1164,14 @@ class UpdateMachine {
           // the *change* in state represented by the event, and when we get the
           // new state in a fresh InitialSnapshot we'll handle that just fine.
           isUnexpected = true;
+
+        default:
+          assert(debugLog('BUG: Unexpected error in event polling: $e\n' // TODO(log)
+            'Replacing event queue…'));
+          _reportToUserErrorConnectingToServer(e);
+          // Similar story to the _EventHandlingException case;
+          // separate only so that that other case can print more context.
+          isUnexpected = true;
       }
 
       if (isUnexpected) {
@@ -1257,3 +1275,10 @@ class UpdateMachine {
   @override
   String toString() => '${objectRuntimeType(this, 'UpdateMachine')}#${shortHash(this)}';
 }
+
+class _EventHandlingException implements Exception {
+  final Object cause;
+  final Event event;
+
+  _EventHandlingException({required this.cause, required this.event});
+}

test/model/store_test.dart

@@ -982,10 +982,13 @@ void main() {
         checkNotReported(prepareExpiredEventQueue);
       });
 
-      test('report handleEvent error', () {
-        checkReported(prepareHandleEventError).startsWith(
-          "Error connecting to Zulip. Retrying…\n"
-          "Error connecting to Zulip at");
+      test('nicely report handleEvent error', () {
+        checkReported(prepareHandleEventError).matchesPattern(RegExp(
+          r"Error handling a Zulip event\. Retrying connection…\n"
+          r"Error handling a Zulip event from \S+; will retry\.\n"
+          r"\n"
+          r"Error: .*channel\.dart.. Failed assertion.*"
+        ));
       });
     });
   });