[docs] [issue25442] Shelve consistency issues

[Yanyan Jiang]

New submission from Yanyan Jiang:

I am currently working on the file system reliability issues. I have a disk driver that is able to simulate crash disk sites after injected power failures. This disk is totally compatible with the Linux block driver semantics (refer to  https://www.kernel.org/doc/Documentation/block/writeback_cache_control.txt), and may create many crash sites that pending blocks are partially flushed into the disk which is a common behavior of a commodity disk with write buffer.

Our automated tool confirms the corruptions could happen on a crash site at an unclean shutdown (Linux with default ext4 setting). We also found that there are some discussions on [Stackoverflow](http://stackoverflow.com/questions/4226580/prevent-python-shelve-corruption) concerning this issue. I am suggesting to explicitly remind the developers of such behaviors.

Suggested documentation enhancement
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As a minimal database library, `shelve` does not offer as strong ACID (atomicity, consistency, isolation and durability) guarantee as a database (like SQLite). On certain system configurations, a system crash would lead to a corrupted shelve file. If you are using shelve to persistent precious data like user's document, we suggest using the following steps to ensure data is not lost:

1. Create a copy of the file, say, the temporary.
2. Operate on a copy of the temporary file. Closing a shelve db implies data to be flushed to the disk.
3. Rename the temporary file to replace the original file. Renaming is carefully treated by a journaled filesystem to be atomic.

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assignee: docs at python
components: Documentation
messages: 253188
nosy: Yanyan Jiang, docs at python
priority: normal
severity: normal
status: open
title: Shelve consistency issues
type: enhancement
versions: Python 2.7, Python 3.2, Python 3.3, Python 3.4, Python 3.5, Python 3.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue25442>
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