Re: g_autoptr(Error)

[Markus Armbruster <armbru@redhat.com>]

Daniel P. Berrangé <berrange@redhat.com> writes:

> On Tue, Nov 25, 2025 at 08:40:07AM +0100, Markus Armbruster wrote:
>> g_autoptr(T) is quite useful when the object's extent matches the
>> function's.
>> 
>> This isn't the case for an Error object the function propagates to its
>> caller.  It is the case for an Error object the function reports or
>> handles itself.  However, the functions to report Error also free it.
>> 
>> Thus, g_autoptr(Error) is rarely applicable.  We have just three
>> instances out of >1100 local Error variables, all in migration code.
>> 
>> Two want to move the error to the MigrationState for later handling /
>> reporting.  Since migrate_set_error() doesn't move, but stores a copy,
>> the original needs to be freed, and g_autoptr() is correct there.  We
>> have 17 more that instead manually free with error_free() or
>> error_report_err() right after migrate_set_error().
>> 
>> We recently discussed storing a copy vs. move the original:
>> 
>>     From: Peter Xu <peterx@redhat.com>
>>     Subject: Re: [PATCH 0/3] migration: Error fixes and improvements
>>     Date: Mon, 17 Nov 2025 11:03:37 -0500
>>     Message-ID: <aRtHWbWcTh3OF2wY@x1.local>
>> 
>> The two g_autoptr() gave me pause when I investigated this topic, simply
>> because they deviate from the common pattern migrate_set_error(s, err)
>> followed by error_free() or error_report_err().
>> 
>> The third one became wrong when I cleaned up the reporting (missed in
>> the cleanup patch, fixed in the patch I'm replying to).  I suspect my
>> mistake escaped review for the same reason I made it: g_autoptr(Error)
>> is unusual and not visible in the patch hunk.
>> 
>> Would you like me to replace the two correct uses of g_autoptr(Error) by
>> more common usage?
>
> I had previously proposed g_autoptr(Error) a year or two back and you
> rejected it then, so I'm surprised to see that it got into the code,
> because it requires explicit opt-in via a G_DEFINE_AUTOPTR_CLEANUP_FUNC.
>
> Unfortunately it appears exactly that was added earlier this year in
>
>   commit 18eb55546a54e443d94a4c49286348176ad4b00a
>   Author: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
>   Date:   Tue Mar 4 23:03:35 2025 +0100
>
>     error: define g_autoptr() cleanup function for the Error type
>     
>     Automatic memory management helps avoid memory safety issues.
>     
>     Reviewed-by: Peter Xu <peterx@redhat.com>
>     Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
>     Link: https://lore.kernel.org/qemu-devel/a5843c5fa64d7e5239a4316092ec0ef0d10c2320.1741124640.git.maciej.szmigiero@oracle.com
>     Signed-off-by: Cédric Le Goater <clg@redhat.com>

I missed it.  Not he submitter's fault; it was cc'ed to me.

> When removing this usage, ensure that commit is reverted too, which
> will prevent anyone unwittingly re-introducing g_autoptr(Error)
> usage

Thanks for the pointer!