a test that "verifies" that the file renaming lock works and no inconsistencies are created.

src/documents/consumer.py

@@ -206,6 +206,8 @@ class Consumer(LoggingMixin):
                             document.archive_checksum = hashlib.md5(
                                 f.read()).hexdigest()
 
+                # Don't save with the lock active. Saving will cause the file
+                # renaming logic to aquire the lock as well.
                 document.save()
 
                 # Delete the file only if it was successfully consumed

src/documents/tests/test_file_handling.py

@@ -1,5 +1,9 @@
 import datetime
+import hashlib
 import os
+import random
+import uuid
+from concurrent.futures.thread import ThreadPoolExecutor
 from pathlib import Path
 from unittest import mock
 
@@ -8,8 +12,10 @@ from django.db import DatabaseError
 from django.test import TestCase, override_settings
 
 from .utils import DirectoriesMixin
-from ..file_handling import generate_filename, create_source_path_directory, delete_empty_directories
+from ..file_handling import generate_filename, create_source_path_directory, delete_empty_directories, \
+    generate_unique_filename
 from ..models import Document, Correspondent
+from ..sanity_checker import check_sanity
 
 
 class TestFileHandling(DirectoriesMixin, TestCase):
@@ -546,3 +552,42 @@ class TestFilenameGeneration(TestCase):
     def test_date(self):
         doc = Document.objects.create(title="does not matter", created=datetime.datetime(2020,5,21, 7,36,51, 153), mime_type="application/pdf", pk=2, checksum="2")
         self.assertEqual(generate_filename(doc), "2020-05-21.pdf")
+
+
+def run():
+    doc = Document.objects.create(checksum=str(uuid.uuid4()), title=str(uuid.uuid4()), content="wow")
+    doc.filename = generate_unique_filename(doc, settings.ORIGINALS_DIR)
+    Path(doc.thumbnail_path).touch()
+    with open(doc.source_path, "w") as f:
+        f.write(str(uuid.uuid4()))
+    with open(doc.source_path, "rb") as f:
+        doc.checksum = hashlib.md5(f.read()).hexdigest()
+
+    with open(doc.archive_path, "w") as f:
+        f.write(str(uuid.uuid4()))
+    with open(doc.archive_path, "rb") as f:
+        doc.archive_checksum = hashlib.md5(f.read()).hexdigest()
+
+    doc.save()
+
+    for i in range(30):
+        doc.title = str(random.randrange(1, 5))
+        doc.save()
+
+
+class TestSuperMassive(DirectoriesMixin, TestCase):
+
+    @override_settings(PAPERLESS_FILENAME_FORMAT="{title}")
+    def test_super_massive(self):
+        # try to save as many documents in parallel as possible.
+        # try to make the system fail.
+
+        with ThreadPoolExecutor(max_workers=16) as executor:
+            results = [executor.submit(run) for i in range(16)]
+
+        for r in results:
+            if r.exception():
+                raise r.exception()
+
+        # nope, everything still good. Thank you, lockfiles.
+        self.assertEqual(len(check_sanity()), 0)