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Re: [Qemu-devel] [PATCH v22 03/25] improve some functions in qemu-option

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v22 03/25] improve some functions in qemu-option.c
Date: Mon, 10 Mar 2014 15:21:04 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.3.0

On 03/10/2014 02:29 PM, Eric Blake wrote:

>> +    opt = qemu_opt_find(opts, name);
>> +    if (opt) {
>> +        g_free((char *)opt->str);
> ...which means the cast is pointless here.
> Hmm.  This means that you are giving opt_set() the behavior of 'last
> version wins', by silently overwriting earlier versions.  If I'm
> understanding the existing code correctly, the previous behavior was
> that calling opt_set twice in a row on the same name would inject BOTH
> names into 'opts', but then subsequent lookups on opts would find the
> FIRST hit.  Doesn't that mean this is a semantic change:
> qemu -opt key=value1,key=value2
> would previously set key to value1, but now sets key to value2.

I've played with this a bit more, and now am more confused.  QemuOpts is
a LOT to comprehend.

Pre-patch, 'qemu-system-x86_64 -nodefaults -machine
type=none,type-noone' displayed a help message about unknown machine
type "noone", while swapping type=noone,type=none proceeded with the
'none' type.  So the last version silently won, which was not the
behavior I had predicted.

Post-patch, I get a compilation error (so how did you test your patch?):

qapi/opts-visitor.c: In function ‘opts_start_struct’:
qapi/opts-visitor.c:146:31: error: assignment discards ‘const’ qualifier
from pointer target type [-Werror]
         ov->fake_id_opt->name = "id";

If I press on in spite of that warning, then I get the same behavior
where the last type= still wins on behavior.  So I'm not sure how it all
worked, but at least behavior wise, my one test didn't uncover a regression.

Still, I'd feel a LOT better with a testsuite of what QemuOpts is
supposed to be able to do.  tests/test-opts-visitor.c was the only file
in tests/ that even mentions QemuOpts.

>> @@ -744,16 +777,24 @@ void qemu_opt_set_err(QemuOpts *opts, const char 
>> *name, const char *value,
>>  int qemu_opt_set_bool(QemuOpts *opts, const char *name, bool val)
>> +    opt = qemu_opt_find(opts, name);
>> +    if (opt) {
>> +        g_free((char *)opt->str);
> Another pointless cast.

Maybe not pointless, if you end up not removing the const in the struct
declaration due to the compile error; but that brings me back to my
earlier question - since the compiler error proves that we have places
that are assigning compile-time string constants into the name field, we
must NOT call g_free on those copies - how does your code distinguish
between a QemuOpt that is built up by mallocs, vs. one that is described
by compile-time constants?

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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