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Re: [Qemu-devel] [PATCH] Makefile: simplify qapi-py definition with wild

From: Wei Yang
Subject: Re: [Qemu-devel] [PATCH] Makefile: simplify qapi-py definition with wildcard
Date: Wed, 22 May 2019 10:36:51 +0800
User-agent: Mutt/1.10.1 (2018-07-13)

On Tue, May 21, 2019 at 08:36:42PM -0500, Eric Blake wrote:
>On 5/21/19 7:53 PM, Wei Yang wrote:
>> On Tue, May 21, 2019 at 05:28:27PM +0200, Markus Armbruster wrote:
>>> Eric Blake <address@hidden> writes:
>>>> On 5/21/19 3:12 AM, Wei Yang wrote:
>>>>> All the python script in scripts/qapi is used to generate qapi code. Use
>>>>> wildcard to simplify it.
>>>>> Signed-off-by: Wei Yang <address@hidden>
>>>>> ---
>>>>>  Makefile | 8 +-------
>>>>>  1 file changed, 1 insertion(+), 7 deletions(-)
>>>> I'm not a fan of $(wildcard). It makes your tarball creation
>>>> non-deterministic - if there is a leftover file from development that is
>>>> no longer part of the build, wildcard will still pick it up.  Explicit
>>> Actually, in this case can "only" adds spurious prerequisites.
>> Hmm... which spurious prerequisites will be added? I may miss something here.
>If I touch the file scripts/qapi/foo.py, then that becomes a
>prerequisite, and will rebuild my files even though it doesn't need to.
>And if the definition of $(qapi-py) is ever applied to determining which
>files to include in a tarball, then my tarball will include foo.py, even
>though it is not essential to the tarball, while the next person, who
>does not have foo.py lying around in their directory, produces a
>different tarball.  Non-deterministic results are bad, hence our rule of
>thumb to avoid $(wildcard) when listing files in a Makefile.

Ok, so $(wildcard) should be avoided in Makefile. Will not try to do this next

>>>> lists are better.  I'm inclined to NACK this, but Markus has final say
>>>> since he maintains the qapi generator.
>>> I consider use of $(wildcard) for the purpose of collecting sources a
>>> lazy mistake.
>Which is another polite way of saying that this patch should not be applied.
>If you WANT to factor things for less typing, you could use something like:
>var=commands.py events.py introspect.py types.py visit.py common.py doc.py
>$(addprefix $(SRC_PATH)/scripts/qapi/,$(var))
>for some suitably-named var.  Or even factor out the .py suffix as well.
>That doesn't use the problematic $(wildcard), but still lets you get
>away with easier maintenance of adding a new file to the explicit list
>should we add a file in the future.
>Eric Blake, Principal Software Engineer
>Red Hat, Inc.           +1-919-301-3226
>Virtualization:  qemu.org | libvirt.org

Wei Yang
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