make wms layer groups requestable

[tbonfort]

In the current layer grouping implementation, a layer with wms_layer_group=/g will only let its group "g" appear as requestable (i.e. with a <name>g</name> in capabilities) if there is also a LAYER in the mapfile with NAME=g.

I'd argue that this is unintuitive as when you are creating a hierarchy of layers you end up being obliged to define dummy/empty layers in order to get that hierarchy to be actually useful. e.g. see https://github.com/mapserver/mapserver/blob/rel-7-0-4/msautotest/wxs/wms_inspire.map#L257

The WMS spec (wms 1.1 - 7.1.5.4) does provision for the case where a group of layers appears but cannot be called if it has only a <title> defined but not a <name>. This is what we are implementing currently in the capabilities, but we are not respecting it in GetMap requests (we happily render the group even if it has no <name> defined).

In order to be coherent with the concept of layer grouping, be coherent with our actual behavior, and simplify writing grouped mapfiles, I propose we modify the current behavior and always add a <name> tag to group layers.

• patch to add <name> tag to group layers
• capabilities test
• request test
• documentation update
• HISTORY.TXT update
• migration guide update

[tbonfort]

cc @yjacolin

[tbonfort]

see also #5220

[yjacolin]

@tbonfort do you need a feedback from me?

[sdlime]

Is this one ready to merge?

[yjacolin]

If we can help in any way, let's us know.

[ebelo]

Hi, is this ready to merge? Can we do something so that it gets into master?

[yjacolin]

@tbonfort I am working on the doc parts of this feature. I created a new branch up-to-date with master and cherry-pick your 3 commits, added an item in HISTORY.TXT. Are you ok to close this PR and I will open a new one with a link to the doc. Thank for you answer.

[yjacolin]

MIGRATION_GUIDE.txt doesn't need to be updated.

[yjacolin]

following discussion in Mapserver-dev list, a new PR superseed this one: #5533