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Re: [Qemu-devel] [RFC v4 02/13] qapi: qobject_compare() helper
From: |
Eduardo Habkost |
Subject: |
Re: [Qemu-devel] [RFC v4 02/13] qapi: qobject_compare() helper |
Date: |
Tue, 15 Aug 2017 14:59:52 -0300 |
User-agent: |
Mutt/1.8.0 (2017-02-23) |
On Tue, Aug 15, 2017 at 11:16:57AM -0500, Eric Blake wrote:
> On 08/14/2017 04:57 PM, Eduardo Habkost wrote:
> > The helper function will be useful when writing support code to
> > deal with device slot information.
> >
> > TODO: documentation is incomplete and unclear, needs to be
> > improved.
> >
> > Signed-off-by: Eduardo Habkost <address@hidden>
> > ---
> > include/qapi/util.h | 39 +++++++++++++++++++++++++++++
> > qapi/qapi-util.c | 66
> > ++++++++++++++++++++++++++++++++++++++++++++++++++
> > tests/test-qapi-util.c | 53 ++++++++++++++++++++++++++++++++++++++++
> > 3 files changed, 158 insertions(+)
> >
>
> > +/**
> > + * qobject_compare:
> > + *
> > + * Compare the value of @a and @b.
> > + *
> > + * If @a and @b have the same type and the same value (see list
> > + * of supported types below), return 0.
> > + *
> > + * If @a and @b are both strings, return strcmp(a, b).
> > + *
> > + * If @a and @b are numbers, return a negative value if a < b,
> > + * and a positive value if a > b.
> > + *
> > + * Otherwise (if @a and @b are not the same, have different types,
> > + * are of an unsupported type, or are different), return a non-zero value.
>
> Is this number going to be commutative and distributive, in order to
> provide stable qsort()ing? That is, if comparing a and b gives a
> positive number, then comparing b and a should give a negative number;
> and if comparing a and b then b and c results in two positive numbers,
> then comparing a and c should also give a positive number. It is
> unclear from the documentation whether you are able to make this
> guarantee; and without it, it is unsafe to use this comparator in places
> that require stability.
No, I don't make that guarantee when the types don't match or in
the case of unsupported types. That's a limitation of this
implementation.
Guaranteeing it when types don't match should be easy.
Guaranteeing it in the case of QTYPE_QDICT looks a bit harder.
Probably it's easier to simply implement full dictionary
comparison.
>
> > + *
> > + * Note that this function doesn't support some types, and may
> > + * return false if the types are unsupported, or if the types don't
> > + * match exactly.
>
> How is a return of false (== 0, which also means equivalent) correct?
This is a documentation bug. Leftover from a version that
returned a boolean value (true for equal, false for not equal) in
the past.
>
> > + *
> > + * Supported types:
> > + * - QTYPE_QNULL
> > + * - QTYPE_QSTRING
> > + * - QTYPE_QBOOL
> > + * - QTYPE_QNUM (integers only)
> > + * - QTYPE_QLIST
> > + *
> > + * Unsupported (always return false):
> > + * - QTYPE_QNUM (non-integer values)
> > + * - QTYPE_QDICT
> > + *
> > + * TODO: rewrite documentation to be clearer.
> > + * TODO: support non-integer QTYPE_NUM values and QTYPE_QDICT.
>
> There's another patch series pending for qobject_is_equal(); should
> these two patches share approaches or even code?
>
> https://lists.gnu.org/archive/html/qemu-devel/2017-07/msg01134.html
> https://lists.gnu.org/archive/html/qemu-devel/2017-08/msg02459.html
I will take a look. Thanks!
--
Eduardo
- [Qemu-devel] [RFC v4 00/13] qmp: query-device-slots command, Eduardo Habkost, 2017/08/14
- [Qemu-devel] [RFC v4 01/13] qmp: Define query-device-slots command, Eduardo Habkost, 2017/08/14
- [Qemu-devel] [RFC v4 02/13] qapi: qobject_compare() helper, Eduardo Habkost, 2017/08/14
- [Qemu-devel] [RFC v4 03/13] qdev: Add BusClass::device_type field, Eduardo Habkost, 2017/08/14
- [Qemu-devel] [RFC v4 04/13] qdev: Slot info helpers, Eduardo Habkost, 2017/08/14
- [Qemu-devel] [RFC v4 05/13] query-device-slots: Collapse similar entries, Eduardo Habkost, 2017/08/14
- [Qemu-devel] [RFC v4 06/13] qdev core: generic enumerate_slots implementation, Eduardo Habkost, 2017/08/14
- [Qemu-devel] [RFC v4 07/13] qdev: Enumerate CPU slots on query-device-slots, Eduardo Habkost, 2017/08/14
- [Qemu-devel] [RFC v4 09/13] pci: pci_bus_has_pcie_upstream_port() function, Eduardo Habkost, 2017/08/14
- [Qemu-devel] [RFC v4 08/13] ide: enumerate_slots implementation, Eduardo Habkost, 2017/08/14