A tag PATCH moved files from inside the tags.set() transaction while holding the document row lock. A concurrent save holding media.lock and waiting on that row deadlocked with it, and every later save on the instance queued behind media.lock until the webserver restarted.
FileMoveMiddleware opens a file move scope around every request, in sync or async mode, so each changed document is moved once after the view returns and never while a transaction holds row locks.
Each save of a document moved its files on the spot, so one logical change could move them several times, and moves could run inside another open transaction.
defer_file_moves() opens a ContextVar-backed scope. Inside it, move requests are recorded and the outermost scope moves each document once on exit, or on commit if a transaction is still open. A failing move is logged and does not stop the others. Outside any scope moves stay immediate.
The file move receiver refreshed the caller's own instance from the database, which wiped unsaved in-memory changes. It ran on every m2m_changed action including pre_add and pre_remove, wrote modified even when nothing moved, and its no-move branch skipped trashed documents.
The move body now lives in documents.file_moves.move_files_for_document. It skips documents without a filename before taking media.lock, then takes the lock with a 300 second timeout, loads a fresh instance by pk, writes only when a filename column changes, and returns the instance. The receiver ignores reverse and pre_* m2m actions and copies just the final filename columns back onto the caller's instance.
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.
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