wehub-resource-sync
40 lines
1.3 KiB
Markdown
40 lines
1.3 KiB
Markdown
# Session File Locking
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When saving session JSON, use exclusive file locking to prevent concurrent writes from corrupting data.
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## The Problem
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Never use bare `open("w") + json.dump()` — `open("w")` truncates the file before any lock can be acquired.
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## The Solution: `_locked_save_json`
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Open with `"r+"`, lock, then truncate inside the lock:
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```python
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def _locked_save_json(path, data, **dump_kwargs) -> None:
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"""Atomically write JSON with exclusive file locking."""
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try:
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f = open(path, "r+") # no truncation on open
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except FileNotFoundError:
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os.makedirs(os.path.dirname(os.path.abspath(path)), exist_ok=True)
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f = open(path, "w") # first save — file doesn't exist yet
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with f:
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_locked = False
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try:
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import fcntl
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fcntl.flock(f.fileno(), fcntl.LOCK_EX)
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_locked = True
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except (ImportError, OSError):
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pass # Windows / unsupported FS — proceed unlocked
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try:
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f.seek(0)
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f.truncate() # truncate INSIDE the lock
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json.dump(data, f, **dump_kwargs)
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f.flush()
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finally:
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if _locked:
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fcntl.flock(f.fileno(), fcntl.LOCK_UN)
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```
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Copy this pattern into `core/session.py` for all session saves.
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