[QGIS-Developer] QGIS 3.44 projects compatibility with previous versions

[Stefanos Natsis]

I'm not sure I like the idea of adding the data there twice. It feels like
pollution that we inevitably need to deal with at some point!

I'd really rather defer the fix for ~3 months until QGIS 4.0, especially
since the bug it fixes has come unnoticed for sooo long :)

Best
Stefanos



On Thu, 12 Jun 2025 at 15:50, Denis Rouzaud <denis.rouzaud at gmail.com> wrote:

> just thinking that it will probably give the same error message!
> so maybe a v2 like:
>
> <properties>
>   <foo>
>     <bar type="QString">baz</bar>
>   </foo>
> </properties>
> <properties-v2>
>   <properties name="foo">
>     <properties name="bar" type="QString">baz</properties>
>   </properties>
> </properties-v2>
>
> Le jeu. 12 juin 2025 à 14:28, Denis Rouzaud <denis.rouzaud at gmail.com> a
> écrit :
>
>> Hi,
>>
>> I did something similar in the symbology area and I went for saving twice
>> the data.
>> We could probably leave the two at the same time in the same tag:
>>
>> <properties>
>>   <foo>
>>     <bar type="QString">baz</bar>
>>   </foo>
>>   <properties name="foo">
>>     <properties name="bar" type="QString">baz</properties>
>>   </properties>
>> </properties>
>>
>> Old QGIS would only save the old, new QGIS would save both and use the
>> new if it exists.
>>
>> Best of the 2 worlds?
>>
>> Cheers,
>> Denis
>>
>>
>>
>> Le jeu. 12 juin 2025 à 13:55, Stefanos Natsis via QGIS-Developer <
>> qgis-developer at lists.osgeo.org> a écrit :
>>
>>> Hi list
>>>
>>> During the last dev cycle, a change was introduced in the way that
>>> properties are stored in the .qgs xml file
>>> https://github.com/qgis/QGIS/pull/60855
>>>
>>> While this solved a real issue (data-driven properties could result in
>>> an invalid property name with a leading digit) in an elegant way and is
>>> technically backwards compatible (projects generated with older versions
>>> can be opened without issues), it has the side effect that projects stored
>>> with 3.44 will not properly load in older QGIS versions (stored properties
>>> are ignored and also project CRS is undefined). Plugins could also
>>> malfunction as they often use project properties for storing their
>>> project-related configurations.
>>>
>>> One potential solution would be to backport this to LTR, however from my
>>> experience a good percentage of the users use older QGIS versions and even
>>> those on LTR do not always upgrade as soon as a new release is out, so
>>> backporting could eventually make things worse!
>>>
>>> With QGIS 4.0 around the corner, imho, it would be better to defer this
>>> change for a couple more months and only include this kind-of-breaking
>>> change in QGIS 4.0
>>>
>>> I briefly chatted with Jurgen at the hackfest about this and while
>>> acknowledging the issue, was not super happy to revert, so I'm interested
>>> in gathering more opinions here.
>>>
>>> Best
>>> Stefanos
>>>
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>>
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