Fix: prevent overlapping mail-account processing runs

process_mail_accounts had no guard against a scheduled run still being
in progress when the next one fires (e.g. a large attachment batch
taking longer than the check interval). ProcessedMail dedup only
records a message once handling finishes, so an overlapping run could
still pick up the same not-yet-recorded message. Skip a run outright
if another MAIL_FETCH task is already PENDING/STARTED.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/paperless_mail/tasks.py

@@ -1,7 +1,9 @@
 import logging
 
+from celery import Task
 from celery import shared_task
 
+from documents.models import PaperlessTask
 from paperless_mail.mail import MailAccountHandler
 from paperless_mail.mail import MailError
 from paperless_mail.models import MailAccount
@@ -10,8 +12,27 @@ from paperless_mail.models import MailRule
 logger = logging.getLogger("paperless.mail.tasks")
 
 
-@shared_task
+@shared_task(bind=True)
-def process_mail_accounts(account_ids: list[int] | None = None) -> str:
+def process_mail_accounts(self: Task, account_ids: list[int] | None = None) -> str:
+    # A scheduled check can still be running (or queued) when the next one
+    # fires, e.g. a large attachment batch that takes longer to process than
+    # the check interval. ProcessedMail dedup only records a message once its
+    # handling has finished, so an overlapping run can still pick up the same
+    # not-yet-recorded message. Skip outright rather than race it.
+    other_mail_fetch_running = (
+        PaperlessTask.objects.filter(
+            task_type=PaperlessTask.TaskType.MAIL_FETCH,
+            status__in=[PaperlessTask.Status.PENDING, PaperlessTask.Status.STARTED],
+        )
+        .exclude(task_id=self.request.id)
+        .exists()
+    )
+    if other_mail_fetch_running:
+        logger.info(
+            "Mail account processing is already running; skipping this run.",
+        )
+        return "Skipped: mail account processing already in progress."
+
     total_new_documents = 0
     accounts = (
         MailAccount.objects.filter(pk__in=account_ids)

src/paperless_mail/tests/test_process_mail_accounts_overlap.py

@@ -0,0 +1,89 @@
+from unittest import mock
+
+import pytest
+
+from documents.models import PaperlessTask
+from paperless_mail import tasks
+from paperless_mail.tests.factories import MailAccountFactory
+from paperless_mail.tests.factories import MailRuleFactory
+
+
+@pytest.mark.django_db
+class TestProcessMailAccountsOverlap:
+    def test_skips_when_another_mail_fetch_task_is_running(self) -> None:
+        account = MailAccountFactory.create()
+        MailRuleFactory.create(account=account, enabled=True)
+
+        PaperlessTask.objects.create(
+            task_id="other-running-task",
+            task_type=PaperlessTask.TaskType.MAIL_FETCH,
+            trigger_source=PaperlessTask.TriggerSource.SCHEDULED,
+            status=PaperlessTask.Status.STARTED,
+        )
+
+        with mock.patch.object(
+            tasks.MailAccountHandler,
+            "handle_mail_account",
+        ) as mocked_handle:
+            result = tasks.process_mail_accounts()
+
+        mocked_handle.assert_not_called()
+        assert result == "Skipped: mail account processing already in progress."
+
+    def test_runs_when_no_other_mail_fetch_task_is_running(self) -> None:
+        account = MailAccountFactory.create()
+        MailRuleFactory.create(account=account, enabled=True)
+
+        with mock.patch.object(
+            tasks.MailAccountHandler,
+            "handle_mail_account",
+            return_value=0,
+        ) as mocked_handle:
+            result = tasks.process_mail_accounts()
+
+        mocked_handle.assert_called_once()
+        assert result == "No new documents were added."
+
+    def test_ignores_completed_mail_fetch_tasks(self) -> None:
+        account = MailAccountFactory.create()
+        MailRuleFactory.create(account=account, enabled=True)
+
+        PaperlessTask.objects.create(
+            task_id="finished-task",
+            task_type=PaperlessTask.TaskType.MAIL_FETCH,
+            trigger_source=PaperlessTask.TriggerSource.SCHEDULED,
+            status=PaperlessTask.Status.SUCCESS,
+        )
+
+        with mock.patch.object(
+            tasks.MailAccountHandler,
+            "handle_mail_account",
+            return_value=0,
+        ) as mocked_handle:
+            result = tasks.process_mail_accounts()
+
+        mocked_handle.assert_called_once()
+        assert result == "No new documents were added."
+
+    def test_does_not_skip_due_to_its_own_task_row(self) -> None:
+        account = MailAccountFactory.create()
+        MailRuleFactory.create(account=account, enabled=True)
+
+        PaperlessTask.objects.create(
+            task_id="self-task-id",
+            task_type=PaperlessTask.TaskType.MAIL_FETCH,
+            trigger_source=PaperlessTask.TriggerSource.SCHEDULED,
+            status=PaperlessTask.Status.STARTED,
+        )
+
+        with mock.patch.object(
+            tasks.MailAccountHandler,
+            "handle_mail_account",
+            return_value=0,
+        ) as mocked_handle:
+            result = tasks.process_mail_accounts.apply(
+                task_id="self-task-id",
+            ).result
+
+        mocked_handle.assert_called_once()
+        assert result == "No new documents were added."