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ngclient: Be better with concurrent instances

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難度
5/5
預估耗時
一週以上
新手友好度
25/100
Issue 類型
功能
描述清晰度
需要釐清
活躍度
停滯
技術堆疊
python
領域
backend

研究方向

先閱讀 ngclient 的 Updater 流程,尤其是 refresh()、get_targetinfo() 和 download_target(),並追蹤 metadata 和 target 檔案在本機快取中的讀取或寫入位置。為跨程序鎖的建立、等待、過期鎖定和鎖定範圍定義並記錄並行策略;當這些未決問題都有達成共識的實作路徑時,該 issue 即完成。

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描述

Users may run multiple updaters at the same time -- it's not useful but it can happen with longer dependency chains like https://github.com/sigstore/sigstore-python/issues/1403: model-signing uses sigstore-python which uses python-tuf...

Currently we don't handle the potential conflicts WRT reading and writing metadata and target files to local cache that result from concurrent updaters. This is especially an issue with windows where concurrent file access leads to issues more often but it can be problematic in linux too -- this is why currently we just document Updater as "should be a singleton".

Possible improvement: lockfile

I think it makes sense to not try to solve this for just a single process but multiple real processes running updaters (something I can easily imagine happening in real life). In that case I think a lock file per repository would be best we can do:

  • check for lock file before reading/writing
  • create lock file, read/write, remove lock file

The reason this hasn't been done is that lockfiles are a pain to do especially cross platform.

Some open questions:

  • what to do when a lock file exists -- log a warning and wait (for how long?)? I don't usually like when libraries wait for things but maybe this works? What if the lock file is stale (let's say hours old)?
  • manually handling lockfiles in a way that works on windows is painful... but we've also done a lot of work to avoid unnecessary dependencies in this project. Using a dependency for this is likely the smaller evil
  • when exactly do we lock?
    • locking for lifetime of Updater probably has annoying side effects and is not expected by users
    • locking for duration of refresh() and get_targetinfo() seems logical at least at first glance. Locking for duration of download_target() is probably less important but could be done
Possible improvement: global state

If we are not interested in multiple processes but just multiple threads with updaters, we could always add some global state tracking... It sounds quite unappealing but possible. I feel like I'd rather advice python-tuf users to use ngclient Updater as a "per-repository-singleton"?

CC @spencerschrock, let me know if you have thoughts

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Python
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