Use sqlite's WAL mode to avoid database is locked errors

[n8henrie]

Sending PR to help prompt discussion hopefully, likely still a WIP (due to my lack of in-depth knowledge in the area).

• Relevant issue: (sqlite3.OperationalError) database is locked #4780

Code:

• http://stackoverflow.com/a/23661501/1588795
• http://docs.sqlalchemy.org/en/rel_0_9/dialects/sqlite.html#foreign-key-support
• https://github.com/g2p/bedup/pull/86/files

Description:

Try using sqlite WAL mode to fix "database is locked" errors. Please see: #4780 (comment)

(sqlite3.OperationalError) database is locked is [a common error](https://github.com/home-assistant/home-assistant/issues?utf8=✓&q=is%3Aissue%20database%20is%20locked encountered by HomeAssistant newbies using devices like Raspberry Pi.

WAL mode helps facilitate simultaneous read and write for simple sqlite databases which seems to help alleviate this error. The major drawback to this mode seems to be its requirement for a relatively recent version of sqlite, which seems to be satisfied on a standard Raspbian Jessie / Python 3.4 setup:

$ python3.4 -c 'import sqlite3; print(sqlite3.sqlite_version)'
3.8.7.1

Related issue (if applicable): fixes #4780

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works.

[homeassistant]

Hi @n8henrie,

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Once you do that we will be able to review and accept this pull request.

Thanks!

[mention-bot]

@n8henrie, thanks for your PR! By analyzing the history of the files in this pull request, we identified @balloob, @rhooper and @kellerza to be potential reviewers.

[balloob]

Not sure what the impact of this setting is, but we should only set it when we're using SQLite as the engine

[n8henrie]

@balloob definitely.

The pragma command is specific to SQLite and is not compatible with any other SQL database engine.

source -- I don't know if that means this would produce errors with e.g. mysql or if it would just have no effect. Will try to find out.

[n8henrie]

Seems to work okay with postgres (at least with the latest commit that checks for sqlite).

[balloob]

Ok to merge when conflicts resolved.