Re: Proposal for Signal Detection Refactoring

I did some more reading.

On Mon, Sep 24, 2018 at 10:15 AM Chris Travers <chris(dot)travers(at)adjust(dot)com>
wrote:

> First, thanks for taking the time to write this. Its very helpful.
> Additional thoughts inline.
>
> On Mon, Sep 24, 2018 at 2:12 AM Michael Paquier <michael(at)paquier(dot)xyz>
> wrote:
>
>>
>> There could be value in refactoring things so as all the *Pending flags
>> of miscadmin.h get stored into one single volatile sig_atomic_t which
>> uses bit-wise markers, as that's at least 4 bytes because that's stored
>> as an int for most platforms and can be performed as an atomic operation
>> safely across signals (If my memory is right;) ). And this leaves a lot
>> of room for future flags.
>>
>
> Yeah I will look into this.
>

Ok so having looked into this a bit more....

It looks like to be strictly conforming, you can't just use a series of
flags because neither C 89 or 99 guarantee that sig_atomic_t is read/write
round-trip safe in signal handlers. In other words, if you read, are
pre-empted by another signal, and then write, you may clobber what the
other signal handler did to the variable. So you need atomics, which are
C11....

What I would suggest instead at least for an initial approach is:

1. A struct of volatile bools statically stored
2. macros for accessing/setting/clearing flags
3. Consistent use of these macros throughout the codebase.

To make your solution work it looks like we'd need C11 atomics which would
be nice and maybe at some point we decide to allow newer feature, or we
could wrap this itself in checks for C11 features and provide atomic flags
in the future. It seems that the above solution would strictly comply to
C89 and pose no concurrency issues.

--
> Best Regards,
> Chris Travers
> Head of Database
>
> Tel: +49 162 9037 210 | Skype: einhverfr | www.adjust.com
> Saarbrücker Straße 37a, 10405 Berlin
>
>

--
Best Regards,
Chris Travers
Head of Database

Tel: +49 162 9037 210 | Skype: einhverfr | www.adjust.com
Saarbrücker Straße 37a, 10405 Berlin