[systemd-devel] [PATCH] WIP: conf-parser: allow config_parse_iec_off to parse percentages

[Lennart Poettering]

On Wed, 20.05.15 10:37, jsynacek at redhat.com (jsynacek at redhat.com) wrote:

> From: Jan Synacek <jsynacek at redhat.com>
> 
> Allow certain configuration options to be specified as percentages. For
> example, in journald.conf, SystemMaxUse= can now also be specified as 33%.
> 
> There is a slight "problem" with the patch. It parses option names to determine
> what filesystem it should use to get the available space from. This approach
> is probably not the rigth thing to do, but I couldn't think of a better one.
> Another approach that came to my mind was to use the highest bit of the off_t
> value returned by the parser to indicate if the value was a percentage, or
> a normal value. This would require to rewrite a significant amount of code
> which now counts on the values being definite (not percentages), and would,
> IMHO, be hardly any improvement at all.
> 
> What do you think? Is there a better way to implement this functionality? Is it
> a real problem to parse the option lvalues like that?

Yes, it's ugly! Also, it's problematic since disk sizes and space
change dynamically, hence evaluating this only when parsing is not
enough.

What about this: introduce a new type:

        typedef struct SizeParameter {
                uint64_t value;
                bool relative;
        } SizeParameter;

When .relative is false, then .value is an absolute value, otherwise
it's a relative value normalized to let's say 0x100000000 (so that
this value equals 100%, and values below it < 100% and above it >
100%).

Then add new helper calls:

     int size_parameter_from_string(const char *s, SizeParameter *ret);
     uint64 size_parameter_evaluate(SizeParameter *p, uint64_t base);


The latter should return .value as-is if p->relative is false, and
(base * .value) >> 32 otherwise.

THen, change the appropriate places to use SizeParameter instead of
simple uint64_t where necessary, and use size_parameter_evaluate()
with the data from statvfs when the actual value is required.

Lennart

-- 
Lennart Poettering, Red Hat