Fix: make the global media lock reentrant within a thread

Every caller built its own FileLock on media.lock, so code already holding it could not call into code that takes it again without blocking itself. That made it impossible to take media.lock before opening a transaction in paths that also attach files or move files inside that transaction.

media_lock() hands out one FileLock instance per path with per-thread hold counts, so a thread re-enters while other threads and processes stay excluded. All media.lock call sites now use it.

src/documents/tasks.py

@@ -16,7 +16,6 @@ from django.db import models
 from django.db import transaction
 from django.db.models.signals import post_save
 from django.utils import timezone
-from filelock import FileLock
 
 from documents import sanity_checker
 from documents.barcodes import BarcodePlugin
@@ -42,6 +41,7 @@ from documents.double_sided import CollatePlugin
 from documents.file_handling import create_source_path_directory
 from documents.file_handling import generate_unique_filename
 from documents.matching import prefilter_documents_by_workflowtrigger
+from documents.media_lock import media_lock
 from documents.models import Correspondent
 from documents.models import CustomFieldInstance
 from documents.models import Document
@@ -482,7 +482,7 @@ def update_document_content_maybe_archive_file(
                         modified=timezone.now(),
                     )
 
-                with FileLock(settings.MEDIA_LOCK):
+                with media_lock():
                     if parser.get_archive_path():
                         create_source_path_directory(document.archive_path)
                         shutil.move(parser.get_archive_path(), document.archive_path)