Subject: bug#32234: [PATCH 2/2] database: Serialize all database accesses in a thread.

From: Clément Lassieur <[email-masked]>
Date: Sun, 26 Aug 2018 16:07:12 +0200

Hi Ludovic,

Ludovic Courtès <[email-masked]> writes:

> Excellent, thanks for working on this!  This looks great to me, and I
> think the pros outweigh the cons.  Did you check on a big database how
> well it performs?

Yes, I didn't see any difference.  When I use Berlin's database, it
works well but crashes quickly for another reason (lack of disk space I
think, and /tmp being tmpfs).

> One comment:
>
>> -(define* (handle-build-event db event)
>> +(define* (handle-build-event event)
>>    "Handle EVENT, a build event sexp as produced by 'build-event-output-port',
>> -updating DB accordingly."
>> +updating the database accordingly."
>
> [...]
>
>> +(define %db-channel
>> +  (make-parameter #f))
>> +
>> +(define-syntax-rule (with-db-critical-section db exp ...)
>> +  "Evaluate EXP... in the critical section corresponding to %DB-CHANNEL.
>> +DB is bound to the argument of that critical section: the database
>> +connection."
>> +  (call-with-critical-section (%db-channel)
>> +                              (lambda (db) exp ...)))
>> +
>
> I think I find it nicer to keep the ‘db’ parameter everywhere (except
> that it’s now a channel instead of an actual database) rather than using
> this global variable.
>
> WDYT?

That 'db' parameter made sense before, because there were different
database connections: one per fiber.  But now that there is only one
global channel accessible from everywhere, I can't find any use for a
'db-channel' parameter.

Also, using two differents channels for the same database would be a
bug, it would break the serialization mechanism.

And I don't think using several databases (with one channel per
database) would make sense either.

WDYT?