🐛(global) handle non-serializable celery task errors and stop infinite polling (#633)

When a Celery worker crashes (e.g. OOM during large imports), the task
result contains a raw exception object (WorkerLostError) that is not
JSON-serializable, causing a 500 on the task status endpoint. The
frontend polling hooks never received a FAILURE status and kept polling
indefinitely.

Backend: convert exception objects in task results to serializable
strings instead of letting DRF fail on serialization.
Frontend: stop polling on API errors and surface the failure state to
consumers so they can display appropriate error UI immediately.

src/backend/core/api/viewsets/task.py

@@ -103,6 +103,16 @@ class TaskDetailView(APIView):
             result_data["status"] = task_result.result["status"]
             result_data["result"] = task_result.result["result"]
             result_data["error"] = task_result.result["error"]
+        elif isinstance(task_result.result, Exception):
+            logger.exception(
+                "Task %s failed with unhandled exception", task_id,
+                exc_info=task_result.result,
+            )
+            result_data["status"] = "FAILURE"
+            result_data["result"] = None
+            result_data["error"] = (
+                str(task_result.result) or type(task_result.result).__name__
+            )
         else:
             result_data["result"] = task_result.result
         if task_result.state == "PROGRESS" and task_result.info:

src/backend/core/tests/api/test_import_file_upload.py

@@ -81,6 +81,56 @@ class TestTaskDetailViewPermissions:
             assert response.data["result"]["imported"] == 42
 
 
+    def test_api_task_detail_worker_crash_exception_should_be_serialized(self):
+        """Test that a task with an exception result (e.g. WorkerLostError)
+        returns a properly serialized error instead of a 500."""
+        user = factories.UserFactory()
+        client = APIClient()
+        client.force_authenticate(user=user)
+
+        task_id = "test-task-crashed-worker"
+        register_task_owner(task_id, user.id)
+        url = reverse("task-detail", kwargs={"task_id": task_id})
+
+        with mock.patch("core.api.viewsets.task.AsyncResult") as mock_async_result:
+            mock_result = mock.MagicMock()
+            mock_result.status = "FAILURE"
+            mock_result.state = "FAILURE"
+            mock_result.result = Exception("Worker lost")
+            mock_result.info = None
+            mock_async_result.return_value = mock_result
+
+            response = client.get(url)
+            assert response.status_code == status.HTTP_200_OK
+            assert response.data["status"] == "FAILURE"
+            assert response.data["result"] is None
+            assert response.data["error"] == "Worker lost"
+
+    def test_api_task_detail_exception_without_message_should_use_class_name(self):
+        """Test that an exception with an empty str() uses the class name."""
+        user = factories.UserFactory()
+        client = APIClient()
+        client.force_authenticate(user=user)
+
+        task_id = "test-task-empty-exception"
+        register_task_owner(task_id, user.id)
+        url = reverse("task-detail", kwargs={"task_id": task_id})
+
+        with mock.patch("core.api.viewsets.task.AsyncResult") as mock_async_result:
+            mock_result = mock.MagicMock()
+            mock_result.status = "FAILURE"
+            mock_result.state = "FAILURE"
+            mock_result.result = ConnectionResetError()
+            mock_result.info = None
+            mock_async_result.return_value = mock_result
+
+            response = client.get(url)
+            assert response.status_code == status.HTTP_200_OK
+            assert response.data["status"] == "FAILURE"
+            assert response.data["result"] is None
+            assert response.data["error"] == "ConnectionResetError"
+
+
 class TestImportViewSetPermissions:
     """Test that ImportViewSet enforces proper role checks."""
 

src/frontend/public/locales/common/en-US.json

@@ -25,6 +25,8 @@
   "{{count}} messages mentioning you_other": "{{count}} messages mentioning you",
   "{{count}} messages of this thread have been deleted._one": "{{count}} message of this thread has been deleted.",
   "{{count}} messages of this thread have been deleted._other": "{{count}} messages of this thread have been deleted.",
+  "{{count}} messages were imported before the error._one": "{{count}} message were imported before the error.",
+  "{{count}} messages were imported before the error._other": "{{count}} messages were imported before the error.",
   "{{count}} minutes ago_one": "{{count}} minute ago",
   "{{count}} minutes ago_other": "{{count}} minutes ago",
   "{{count}} months ago_one": "{{count}} month ago",
@@ -673,6 +675,7 @@
   "You are the last editor of this thread, you cannot therefore modify your access.": "You are the last editor of this thread, you cannot therefore modify your access.",
   "You can close this window and continue using the app.": "You can close this window and continue using the app.",
   "You can now inform the person that their mailbox is ready to be used and communicate the instructions for authentication.": "You can now inform the person that their mailbox is ready to be used and communicate the instructions for authentication.",
+  "You can safely retry the import — messages already imported will not be duplicated.": "You can safely retry the import — messages already imported will not be duplicated.",
   "You cannot delete the last editor of this thread": "You cannot delete the last editor of this thread",
   "You cannot modify it.": "You cannot modify it.",
   "You have {{count}} recipients, which exceeds the maximum of {{max}} recipients per message. The message cannot be sent until you reduce the number of recipients._one": "You have {{count}} recipient, which exceeds the maximum of {{max}} recipients per message. The message cannot be sent until you reduce the number of recipients.",

src/frontend/public/locales/common/fr-FR.json

@@ -38,6 +38,9 @@
   "{{count}} messages of this thread have been deleted._one": "{{count}} message de cette conversation a été supprimé.",
   "{{count}} messages of this thread have been deleted._many": "{{count}} messages de cette conversation ont été supprimés.",
   "{{count}} messages of this thread have been deleted._other": "{{count}} messages de cette conversation ont été supprimés.",
+  "{{count}} messages were imported before the error._one": "{{count}} message a été importé avant l'erreur.",
+  "{{count}} messages were imported before the error._many": "{{count}} messages ont été importés avant l'erreur.",
+  "{{count}} messages were imported before the error._other": "{{count}} messages ont été importés avant l'erreur.",
   "{{count}} minutes ago_one": "il y a {{count}} minute",
   "{{count}} minutes ago_many": "il y a {{count}} minutes",
   "{{count}} minutes ago_other": "il y a {{count}} minutes",
@@ -734,6 +737,7 @@
   "You are the last editor of this thread, you cannot therefore modify your access.": "Vous êtes le dernier éditeur de cette conversation, vous ne pouvez donc pas modifier votre accès.",
   "You can close this window and continue using the app.": "Vous pouvez fermer cette fenêtre et continuer à utiliser l'application.",
   "You can now inform the person that their mailbox is ready to be used and communicate the instructions for authentication.": "Vous pouvez désormais prévenir la personne que sa boîte aux lettres est prête à être utilisée et lui communiquer les instructions pour s'authentifier.",
+  "You can safely retry the import — messages already imported will not be duplicated.": "Vous pouvez relancer l'import en toute sécurité — les messages déjà importés ne seront pas dupliqués.",
   "You cannot delete the last editor of this thread": "Vous ne pouvez pas supprimer le dernier éditeur de cette conversation",
   "You cannot modify it.": "Vous ne pouvez pas la modifier.",
   "You have {{count}} recipients, which exceeds the maximum of {{max}} recipients per message. The message cannot be sent until you reduce the number of recipients._one": "Vous avez {{count}} destinataire, ce qui dépasse le maximum de {{max}} destinataires autorisés par message. Le message ne peut pas être envoyé tant que vous n'avez pas réduit le nombre de destinataires.",

src/frontend/src/features/controlled-modals/message-importer/step-loader.tsx

@@ -49,14 +49,23 @@ export const StepLoader = ({ taskId, onComplete, onError }: StepLoaderProps) =>
             onCompleteRef.current();
         } else if (importStatus?.state === StatusEnum.FAILURE) {
             const error = importStatus?.error || '';
-            let errorKey = t('An error occurred while importing messages.');
-            if (
+            const isAuthError =
                 error.includes("AUTHENTICATIONFAILED") ||
-                error.includes("IMAP authentication failed")
-            ) {
-                errorKey = t('Authentication failed. Please check your credentials and ensure you have enabled IMAP connections in your account.');
+                error.includes("IMAP authentication failed");
+
+            const parts: string[] = [];
+
+            if (isAuthError) {
+                parts.push(t('Authentication failed. Please check your credentials and ensure you have enabled IMAP connections in your account.'));
+            } else {
+                parts.push(t('An error occurred while importing messages.'));
+                if (importStatus.successCount > 0) {
+                    parts.push(t('{{count}} messages were imported before the error.', { count: importStatus.successCount }));
+                }
+                parts.push(t('You can safely retry the import — messages already imported will not be duplicated.'));
             }
-            onErrorRef.current(errorKey);
+
+            onErrorRef.current(parts.join(' '));
         }
     }, [importStatus?.state, t]);
 

src/frontend/src/features/layouts/components/main/header/authenticated.tsx

@@ -169,7 +169,7 @@ const ApplicationMenu = () => {
       }
       if (taskStatus.state === StatusEnum.FAILURE) {
         label = t("Import failed");
-        icon = <Icon name="error" type={IconType.OUTLINED} />;
+          icon = <Icon name="error" type={IconType.OUTLINED} style={{ color: "var(--c--contextuals--content--semantic--error--primary)" }} />;
       }
     }
 

src/frontend/src/features/providers/sent-box/queued-message.tsx

@@ -45,12 +45,26 @@ export const QueueMessage = ({ taskId, onSettled }: QueueMessageProps) => {
     }, []);
 
     useEffect(() => {
+        if (taskQuery.isError) {
+            toast.update(toastId, {
+                render: (
+                    <ToasterItem type="error">
+                        <span className="material-icons">error</span>
+                        <span>{t('The message could not be sent.')}</span>
+                    </ToasterItem>
+                ),
+                autoClose: QUEUED_MESSAGE_CLOSE_DELAY * 2,
+            });
+            onSettled?.();
+            return;
+        }
+
         const status_code = taskQuery?.data?.status;
-        
+
         if (!status_code) return;
 
         setRetryCount(retryCount => retryCount + 1);
-        
+
         const status = taskQuery.data!.data.status;
 
         if (status === StatusEnum.SUCCESS) {

src/frontend/src/hooks/use-import-task.ts

@@ -1,6 +1,9 @@
 import { StatusEnum, useTasksRetrieve } from "@/features/api/gen";
 import { TaskMetadata } from "@/features/controlled-modals/message-importer/step-loader";
-import { useEffect, useMemo, useState } from "react";
+import { useEffect, useMemo, useRef, useState } from "react";
+import { useTranslation } from "react-i18next";
+
+const MAX_POLL_ERRORS = 10;
 
 export function useImportTaskStatus(
   taskId: string | null,
@@ -9,7 +12,10 @@ export function useImportTaskStatus(
     enabled = true,
   }: { refetchInterval?: number; enabled?: boolean } = {}
 ) {
+  const { t } = useTranslation();
   const [queryEnabled, setQueryEnabled] = useState(enabled);
+  const [hasExhaustedRetries, setHasExhaustedRetries] = useState(false);
+  const errorCountRef = useRef(0);
   const taskQuery = useTasksRetrieve(taskId || "", {
     query: {
       enabled: Boolean(taskId) && queryEnabled === true,
@@ -36,20 +42,34 @@ export function useImportTaskStatus(
   }, [taskStatus, taskMetadata, hasKnownTotal]);
 
   useEffect(() => {
-    if (!enabled || taskStatus === StatusEnum.FAILURE || taskStatus === StatusEnum.SUCCESS) {
+    if (taskQuery.isError) {
+      errorCountRef.current += 1;
+      if (errorCountRef.current >= MAX_POLL_ERRORS) {
+        setHasExhaustedRetries(true);
+      }
+    } else if (taskQuery.data) {
+      errorCountRef.current = 0;
+    }
+  }, [taskQuery.dataUpdatedAt, taskQuery.errorUpdatedAt]);
+
+  useEffect(() => {
+    if (!enabled || taskStatus === StatusEnum.FAILURE || taskStatus === StatusEnum.SUCCESS || hasExhaustedRetries) {
       setQueryEnabled(false);
     } else if (enabled || taskStatus === StatusEnum.PROGRESS || taskStatus === StatusEnum.PENDING) {
       setQueryEnabled(true);
     }
-  }, [taskStatus, enabled]);
+  }, [taskStatus, enabled, hasExhaustedRetries]);
 
   if (!taskId) return null;
   return {
     progress: progress !== null ? Math.ceil(progress) : null,
-    state: taskQuery.data?.data.status,
+    state: hasExhaustedRetries ? StatusEnum.FAILURE : taskQuery.data?.data.status,
     loading: taskQuery.isPending || progress === null,
-    error: taskQuery.data?.data.error,
+    error: hasExhaustedRetries
+      ? t('An error occurred while importing messages.')
+      : taskQuery.data?.data.error,
     hasKnownTotal,
     currentMessage,
+    successCount: taskMetadata?.success_count ?? 0,
   };
 }