Subject: Cuirass: The SQLite built in busy handler might block the Fibers scheduler
From: Clément Lassieur <[email-masked]>
To: [email-masked]
Date: Sat, 21 Jul 2018 11:57:57 +0200
List-Id: <debbugs-submit.debbugs.gnu.org>

Hi,

I'm trying to understand why Berlin web API times out[1].

I think the SQlite built in busy handler may block the Fibers scheduler.
We use "PRAGMA busy_timeout = 30000;", which is an alternative to
calling sqlite3_busy_timeout(), whose description[2] is:

    This routine sets a busy handler that sleeps for a specified amount
    of time when a table is locked. The handler will sleep multiple
    times until at least "ms" milliseconds of sleeping have
    accumulated. After at least "ms" milliseconds of sleeping, the
    handler returns 0 which causes sqlite3_step() to return SQLITE_BUSY.

To me this sounds like non-cooperative and non-resumable code.

A solution would be to set a custom handler through
sqlite3_busy_handler[3] that would be Fibers compatible, i.e. it would
let the scheduler schedule other fibers instead of just sleeping, using
Fibers 'sleep' procedure[4].

WDYT?
Clément

[1]: https://bugs.gnu.org/32233.
[2]: https://www.sqlite.org/c3ref/busy_timeout.html
[3]: https://www.sqlite.org/c3ref/busy_handler.html
[4]: https://github.com/wingo/fibers/wiki/Manual#user-content-index-sleep